P. B. Shepson

Methane emissions from the U.S. oil and natural gas supply chain were estimated by using ground-based, facility-scale measurements and validated with aircraft observations in areas accounting for ~30% of U.S. gas production. When scaled up nationally, our facility-based estimate of 2015 supply chain emissions is 13 ± 2 teragrams per year, equivalent to 2.3% of gross U.S. gas production. This value is ~60% higher than the U.S. Environmental Protection Agency inventory estimate, likely because existing inventory methods miss emissions released during abnormal operating conditions. Methane emissions of this magnitude, per unit of natural gas consumed, produce radiative forcing over a 20-year time horizon comparable to the CO2 from natural gas combustion. Substantial emission reductions are feasible through rapid detection of the root causes of high emissions and deployment of less failure-prone systems.

Abstract. During springtime in the polar regions, unique photochemistry converts inert halide salt ions (e.g. Br−) into reactive halogen species (e.g. Br atoms and BrO) that deplete ozone in the boundary layer to near zero levels. Since their discovery in the late 1980s, research on ozone depletion events (ODEs) has made great advances; however many key processes remain poorly understood. In this article we review the history, chemistry, dependence on environmental conditions, and impacts of ODEs. This research has shown the central role of bromine photochemistry, but how salts are transported from the ocean and are oxidized to become reactive halogen species in the air is still not fully understood. Halogens other than bromine (chlorine and iodine) are also activated through incompletely understood mechanisms that are probably coupled to bromine chemistry. The main consequence of halogen activation is chemical destruction of ozone, which removes the primary precursor of atmospheric oxidation, and generation of reactive halogen atoms/oxides that become the primary oxidizing species. The different reactivity of halogens as compared to OH and ozone has broad impacts on atmospheric chemistry, including near complete removal and deposition of mercury, alteration of oxidation fates for organic gases, and export of bromine into the free troposphere. Recent changes in the climate of the Arctic and state of the Arctic sea ice cover are likely to have strong effects on halogen activation and ODEs; however, more research is needed to make meaningful predictions of these changes.

Abstract. It has been shown that sunlit snow and ice plays an important role in processing atmospheric species. Photochemical production of a variety of chemicals has recently been reported to occur in snow/ice and the release of these photochemically generated species may significantly impact the chemistry of the overlying atmosphere. Nitrogen oxide and oxidant precursor fluxes have been measured in a number of snow covered environments, where in some cases the emissions significantly impact the overlying boundary layer. For example, photochemical ozone production (such as that occurring in polluted mid-latitudes) of 3–4 ppbv/day has been observed at South Pole, due to high OH and NO levels present in a relatively shallow boundary layer. Field and laboratory experiments have determined that the origin of the observed NOx flux is the photochemistry of nitrate within the snowpack, however some details of the mechanism have not yet been elucidated. A variety of low molecular weight organic compounds have been shown to be emitted from sunlit snowpacks, the source of which has been proposed to be either direct or indirect photo-oxidation of natural organic materials present in the snow. Although myriad studies have observed active processing of species within irradiated snowpacks, the fundamental chemistry occurring remains poorly understood. Here we consider the nature of snow at a fundamental, physical level; photochemical processes within snow and the caveats needed for comparison to atmospheric photochemistry; our current understanding of nitrogen, oxidant, halogen and organic photochemistry within snow; the current limitations faced by the field and implications for the future.

The presence of snow greatly perturbs the composition of near-surface polar air, and the higher concentrations of hydroxyl radicals (OH) observed result in a greater oxidative capacity of the lower atmosphere. Emissions of nitrogen oxides, nitrous acid, light aldehydes, acetone, and molecular halogens have also been detected. Photolysis of nitrate ions contained in the snow appears to play an important role in creating these perturbations. OH formed in the snowpack can oxidize organic matter and halide ions in the snow, producing carbonyl compounds and halogens that are released to the atmosphere or incorporated into snow crystals. These reactions modify the composition of the snow, of the interstitial air, and of the overlying atmosphere. Reconstructing the composition of past atmospheres from ice-core analyses may therefore require complex corrections and modeling for reactive species.

Forest emissions of biogenic volatile organic compounds (BVOCs), such as isoprene and other terpenes, play a role in the production of tropospheric ozone and aerosols. In a northern Michigan forest, the direct measurement of total OH reactivity, which is the inverse of the OH lifetime, was significantly greater than expected. The difference between measured and expected OH reactivity, called the missing OH reactivity, increased with temperature, as did emission rates for terpenes and other BVOCs. These measurements are consistent with the hypothesis that unknown reactive BVOCs, perhaps terpenes, provide the missing OH reactivity.

Bromine atoms are believed to play a central role in the depletion of surface-level ozone in the Arctic at polar sunrise. Br2, BrCl, and HOBr have been hypothesized as bromine atom precursors, and there is evidence for chlorine atom precursors as well, but these species have not been measured directly. We report here measurements of Br2, BrCl, and Cl2 made using atmospheric pressure chemical ionization-mass spectrometry at Alert, Nunavut, Canada. In addition to Br2 at mixing ratios up to approximately 25 parts per trillion, BrCl was found at levels as high as approximately 35 parts per trillion. Molecular chlorine was not observed, implying that BrCl is the dominant source of chlorine atoms during polar sunrise, consistent with recent modeling studies. Similar formation of bromine compounds and tropospheric ozone destruction may also occur at mid-latitudes but may not be as apparent owing to more efficient mixing in the boundary layer.

NO x and NO y were determined in the interstitial air of surface snow and in ambient air at Summit, Greenland. NO x levels in interstitial air were 3 to >10 times those in ambient air, and were generally greater than ambient NO y levels. [NO y ] in interstitial air varied diurnally in a manner consistent with photochemical generation within the snowpack. These observations imply that photochemical reactions occurring within or upon the ice crystals of surface snow produced NO x from a N‐reservoir compound within the snow. Average [NO x ]:[HNO 3 ] and [NO x ]:[NO y ] ratios in ambient air above the snow were elevated relative to other remote sites, indicating that NO x release within the snowpack may have altered NO x levels in the overlying atmospheric boundary layer. We suggest that the observed release of NO x may have been initiated by photolysis of nitrate, present in relative abundance in surface snow at Summit. Such a process may affect levels of nitrate and other compounds in surface snow, the overlying atmosphere, and glacial ice, and its potential role in cirrus cloud chemistry should be investigated.

Significance We identified a significant regional flux of methane over a large area of shale gas wells in southwestern Pennsylvania in the Marcellus formation and further identified several pads with high methane emissions. These shale gas pads were identified as in the drilling process, a preproduction stage not previously associated with high methane emissions. This work emphasizes the need for top-down identification and component level and event driven measurements of methane leaks to properly inventory the combined methane emissions of natural gas extraction and combustion to better define the impacts of our nation’s increasing reliance on natural gas to meet our energy needs.

Speciated particle-phase organic nitrates (pONs) were quantified using online chemical ionization MS during June and July of 2013 in rural Alabama as part of the Southern Oxidant and Aerosol Study. A large fraction of pONs is highly functionalized, possessing between six and eight oxygen atoms within each carbon number group, and is not the common first generation alkyl nitrates previously reported. Using calibrations for isoprene hydroxynitrates and the measured molecular compositions, we estimate that pONs account for 3% and 8% of total submicrometer organic aerosol mass, on average, during the day and night, respectively. Each of the isoprene- and monoterpenes-derived groups exhibited a strong diel trend consistent with the emission patterns of likely biogenic hydrocarbon precursors. An observationally constrained diel box model can replicate the observed pON assuming that pONs (i) are produced in the gas phase and rapidly establish gas-particle equilibrium and (ii) have a short particle-phase lifetime (∼2-4 h). Such dynamic behavior has significant implications for the production and phase partitioning of pONs, organic aerosol mass, and reactive nitrogen speciation in a forested environment.

Abstract Based on a uniquely dense network of surface towers measuring continuously the atmospheric concentrations of greenhouse gases (GHGs), we developed the first comprehensive monitoring systems of CO 2 emissions at high resolution over the city of Indianapolis. The urban inversion evaluated over the 2012–2013 dormant season showed a statistically significant increase of about 20% (from 4.5 to 5.7 MtC ± 0.23 MtC) compared to the Hestia CO 2 emission estimate, a state‐of‐the‐art building‐level emission product. Spatial structures in prior emission errors, mostly undetermined, appeared to affect the spatial pattern in the inverse solution and the total carbon budget over the entire area by up to 15%, while the inverse solution remains fairly insensitive to the CO 2 boundary inflow and to the different prior emissions (i.e., ODIAC). Preceding the surface emission optimization, we improved the atmospheric simulations using a meteorological data assimilation system also informing our Bayesian inversion system through updated observations error variances. Finally, we estimated the uncertainties associated with undetermined parameters using an ensemble of inversions. The total CO 2 emissions based on the ensemble mean and quartiles (5.26–5.91 MtC) were statistically different compared to the prior total emissions (4.1 to 4.5 MtC). Considering the relatively small sensitivity to the different parameters, we conclude that atmospheric inversions are potentially able to constrain the carbon budget of the city, assuming sufficient data to measure the inflow of GHG over the city, but additional information on prior emission error structures are required to determine the spatial structures of urban emissions at high resolution.

Published estimates of methane emissions from atmospheric data (top-down approaches) exceed those from source-based inventories (bottom-up approaches), leading to conflicting claims about the climate implications of fuel switching from coal or petroleum to natural gas. Based on data from a coordinated campaign in the Barnett Shale oil and gas-producing region of Texas, we find that top-down and bottom-up estimates of both total and fossil methane emissions agree within statistical confidence intervals (relative differences are 10% for fossil methane and 0.1% for total methane). We reduced uncertainty in top-down estimates by using repeated mass balance measurements, as well as ethane as a fingerprint for source attribution. Similarly, our bottom-up estimate incorporates a more complete count of facilities than past inventories, which omitted a significant number of major sources, and more effectively accounts for the influence of large emission sources using a statistical estimator that integrates observations from multiple ground-based measurement datasets. Two percent of oil and gas facilities in the Barnett accounts for half of methane emissions at any given time, and high-emitting facilities appear to be spatiotemporally variable. Measured oil and gas methane emissions are 90% larger than estimates based on the US Environmental Protection Agency's Greenhouse Gas Inventory and correspond to 1.5% of natural gas production. This rate of methane loss increases the 20-y climate impacts of natural gas consumed in the region by roughly 50%.

Abstract. The role of ice in the formation of chemically active halogens in the environment requires a full understanding because of its role in atmospheric chemistry, including controlling the regional atmospheric oxidizing capacity in specific situations. In particular, ice and snow are important for facilitating multiphase oxidative chemistry and as media upon which marine algae live. This paper reviews the nature of environmental ice substrates that participate in halogen chemistry, describes the reactions that occur on such substrates, presents the field evidence for ice-mediated halogen activation, summarizes our best understanding of ice-halogen activation mechanisms, and describes the current state of modeling these processes at different scales. Given the rapid pace of developments in the field, this paper largely addresses advances made in the past five years, with emphasis given to the polar boundary layer. The integrative nature of this field is highlighted in the presentation of work from the molecular to the regional scale, with a focus on understanding fundamental processes. This is essential for developing realistic parameterizations and descriptions of these processes for inclusion in larger scale models that are used to determine their regional and global impacts.

We present estimates of regional methane (CH4) emissions from oil and natural gas operations in the Barnett Shale, Texas, using airborne atmospheric measurements. Using a mass balance approach on eight different flight days in March and October 2013, the total CH4 emissions for the region are estimated to be 76 ± 13 × 10(3) kg hr(-1) (equivalent to 0.66 ± 0.11 Tg CH4 yr(-1); 95% confidence interval (CI)). We estimate that 60 ± 11 × 10(3) kg CH4 hr(-1) (95% CI) are emitted by natural gas and oil operations, including production, processing, and distribution in the urban areas of Dallas and Fort Worth. This estimate agrees with the U.S. Environmental Protection Agency (EPA) estimate for nationwide CH4 emissions from the natural gas sector when scaled by natural gas production, but it is higher than emissions reported by the EDGAR inventory or by industry to EPA's Greenhouse Gas Reporting Program. This study is the first to show consistency between mass balance results on so many different days and in two different seasons, enabling better quantification of the related uncertainty. The Barnett is one of the largest production basins in the United States, with 8% of total U.S. natural gas production, and thus, our results represent a crucial step toward determining the greenhouse gas footprint of U.S. onshore natural gas production.

Abstract. Formation of organic nitrates (RONO2) during oxidation of biogenic volatile organic compounds (BVOCs: isoprene, monoterpenes) is a significant loss pathway for atmospheric nitrogen oxide radicals (NOx), but the chemistry of RONO2 formation and degradation remains uncertain. Here we implement a new BVOC oxidation mechanism (including updated isoprene chemistry, new monoterpene chemistry, and particle uptake of RONO2) in the GEOS-Chem global chemical transport model with ∼ 25 × 25 km2 resolution over North America. We evaluate the model using aircraft (SEAC4RS) and ground-based (SOAS) observations of NOx, BVOCs, and RONO2 from the Southeast US in summer 2013. The updated simulation successfully reproduces the concentrations of individual gas- and particle-phase RONO2 species measured during the campaigns. Gas-phase isoprene nitrates account for 25–50 % of observed RONO2 in surface air, and we find that another 10 % is contributed by gas-phase monoterpene nitrates. Observations in the free troposphere show an important contribution from long-lived nitrates derived from anthropogenic VOCs. During both campaigns, at least 10 % of observed boundary layer RONO2 were in the particle phase. We find that aerosol uptake followed by hydrolysis to HNO3 accounts for 60 % of simulated gas-phase RONO2 loss in the boundary layer. Other losses are 20 % by photolysis to recycle NOx and 15 % by dry deposition. RONO2 production accounts for 20 % of the net regional NOx sink in the Southeast US in summer, limited by the spatial segregation between BVOC and NOx emissions. This segregation implies that RONO2 production will remain a minor sink for NOx in the Southeast US in the future even as NOx emissions continue to decline.

Both snow manipulation experiments and ambient measurements during the Polar Sunrise Experiment 2000 at Alert (Alert2000) indicate intensive photochemical production of nitrous acid (HONO) in the snowpack. This process constitutes a major HONO source for the overlying atmospheric boundary layer in the Arctic during the springtime, and sustained concentrations of HONO high enough that upon photolysis they became the dominant hydroxyl radical (OH) source. This implies a much greater role for OH radicals in Arctic polar sunrise chemistry than previously believed. Although the observations were made in the high Arctic, this finding has a significant implication for the boundary layer atmospheric chemistry in Antarctica during sunlit seasons and in the mid to high latitudes of the Northern Hemisphere during the winter and spring seasons when approximately 50% of the land mass may be covered by snow.

Results are presented from the 2‐month Polar Sunrise Experiment 1988, which was undertaken to further investigate the cause of ozone destruction during spring in the lower Arctic atmosphere. A strong anticorrelation between the decrease in ozone and concurrent increase in bromine compounds collected on filters observed during a 21‐day period in 1986 was confirmed. It is shown that the reason for this observation is a meteorological modulation which alternately brings to the sampling location lower boundary layer air, depleted in ozone and enriched in filterable bromine, and free tropospheric air with abundant ozone and few bromine compounds. Several other compounds that may have a bearing on the chemical reactions leading to ozone depletion were measured. Bromoform, a potential source for Br atoms, was found to be present at levels between 1 and 10 parts per trillion by volume (pptv); good evidence was obtained that, as with filterable bromine, it had increased in ozone depleted boundary layer air. The mean NO 2 level was 85 pptv with an estimated uncertainty of a factor less than 2. However, this measurement of NO 2 may have been the sum of NO 2 +N 2 O 5 . The possibility that Br atoms are formed from an N 2 O 5 +NaBr interaction is therefore not ruled out. There was a good indication that apparent NO 2 was depleted during episodes of low ozone. An upper limit for the mixing ratio of formaldehyde was found to be 39 pptv, while acetaldehyde was observed at mixing ratios of ca. 65 pptv. Ethylene and acetylene were also found to be depleted concurrently with O 3 . These compounds do react with Br radicals in air (as do the aldehydes) and it is speculated that they may create a link with HO x . chemistry. Data on several other chemical compounds are also presented. Their participation in the O 3 depletion chemistry is less clear. The implications of the measurements with respect to the hypothesis that the ozone depletion are due to a BrO x ‐O 3 destruction cycle are explored.

Results from a tightly constrained photochemical point model for OH and HO 2 are compared to OH and HO 2 data collected during the Program for Research on Oxidants: Photochemistry, Emissions, and Transport (PROPHET) summer 1998 intensive campaign held in northern Michigan. The PROPHET campaign was located in a deciduous forest marked by relatively low NO x levels and high isoprene emissions. Detailed HO x budgets are presented. The model is generally unable to match the measured OH, with the observations 2.7 times greater than the model on average. The model HO 2 , however, is in good agreement with the measured HO 2 . Even with an additional postulated OH source from the ozonolysis of unmeasured terpenes, the measured OH is 1.5 times greater than the model; the model HO 2 with this added source is 15% to 30% higher than the measured HO 2 . Moreover, the HO 2 /OH ratios as modeled are 2.5 to 4 times higher than the measured ratios, indicating that the cycling between OH and HO 2 is poorly described by the model. We discuss possible reasons for the discrepancies.

We report the study of a liquidlike phase that is found in dilute NaCl aqueous solutions frozen at temperatures below the liquid-to-solid-phase transition temperatures of H2O and NaCl·2H2O. There is strong evidence that heterogeneous reactions of gases with halides in liquid layers on ice are the source of halogen radicals that destroy the lower tropospheric ozone, and a subeutectic brine phase is thus of particular relevance to discussions of atmospheric composition and its dependence on the chemistry of polar marine ice and snow. The fractions and concentrations of water and NaCl in this subeutectic quasi-liquid phase were measured by 1H and 23Na NMR spectroscopy, and the experimental results compared to predictions derived from an equilibrium thermodynamic analysis. The temperature dependence of the salt concentration is well-described by the equilibrium theory for temperature ranges where ideal solution behavior holds; for lower temperatures, where the observed salt concentration increases and deviati...

The quantification of greenhouse gas emissions requires high precision measurements made with high spatial resolution. Here we describe measurements of carbon dioxide (CO2) and methane (CH4) conducted using Purdue University's Airborne Laboratory for Atmospheric Research (ALAR), aimed at the quantification of the "footprints" for these greenhouse gases for Indianapolis, IN. A cavity ring-down spectrometer measured atmospheric concentrations, and flask samples were obtained at various points for comparison. Coupled with pressure, temperature, and model-derived horizontal winds, these measurements allow for flux estimation. Long horizontal transects were flown perpendicular to the wind downwind of the city. Emissions were calculated using the wind speed and the difference between the concentration in the plume and the background concentration. A kriging method is applied to interpolate the measured values to a vertical plane traced out by the flight pattern within the mixed layer. Results show the urban plume is clearly distinguishable in the downwind concentrations for most flights. Additionally, there is large variability in the measured day-to-day emissions fluxes as well as in the relative CH4 and CO2 fluxes. Uncertainties in the method are discussed, and its potential utilityin determining sector-based emission factors is shown.

Abstract The Indianapolis Flux Experiment (INFLUX) aims to develop and assess methods for quantifying urban greenhouse gas emissions. Here we use CO 2 , 14 CO 2 , and CO measurements from tall towers around Indianapolis, USA, to determine urban total CO 2 , the fossil fuel derived CO 2 component (CO 2 ff), and CO enhancements relative to background measurements. When a local background directly upwind of the urban area is used, the wintertime total CO 2 enhancement over Indianapolis can be entirely explained by urban CO 2 ff emissions. Conversely, when a continental background is used, CO 2 ff enhancements are larger and account for only half the total CO 2 enhancement, effectively representing the combined CO 2 ff enhancement from Indianapolis and the wider region. In summer, we find that diurnal variability in both background CO 2 mole fraction and covarying vertical mixing makes it difficult to use a simple upwind‐downwind difference for a reliable determination of total CO 2 urban enhancement. We use characteristic CO 2 ff source sector CO:CO 2 ff emission ratios to examine the contribution of the CO 2 ff source sectors to total CO 2 ff emissions. This method is strongly sensitive to the mobile sector, which produces most CO. We show that the inventory‐based emission product (“bottom up”) and atmospheric observations (“top down”) can be directly compared throughout the diurnal cycle using this ratio method. For Indianapolis, the top‐down observations are consistent with the bottom‐up Hestia data product emission sector patterns for most of the diurnal cycle but disagree during the nighttime hours. Further examination of both the top‐down and bottom‐up assumptions is needed to assess the exact cause of the discrepancy.

Organic nitrates are an important aerosol constituent in locations where biogenic hydrocarbon emissions mix with anthropogenic NOx sources. While regional and global chemical transport models may include a representation of organic aerosol from monoterpene reactions with nitrate radicals (the primary source of particle-phase organic nitrates in the Southeast United States), secondary organic aerosol (SOA) models can underestimate yields. Furthermore, SOA parametrizations do not explicitly take into account organic nitrate compounds produced in the gas phase. In this work, we developed a coupled gas and aerosol system to describe the formation and subsequent aerosol-phase partitioning of organic nitrates from isoprene and monoterpenes with a focus on the Southeast United States. The concentrations of organic aerosol and gas-phase organic nitrates were improved when particulate organic nitrates were assumed to undergo rapid (τ = 3 h) pseudohydrolysis resulting in nitric acid and nonvolatile secondary organic aerosol. In addition, up to 60% of less oxidized-oxygenated organic aerosol (LO-OOA) could be accounted for via organic nitrate mediated chemistry during the Southern Oxidants and Aerosol Study (SOAS). A 25% reduction in nitrogen oxide (NO + NO2) emissions was predicted to cause a 9% reduction in organic aerosol for June 2013 SOAS conditions at Centreville, Alabama.

Abstract. We performed an atmospheric inversion of the CO2 fluxes over Iowa and the surrounding states, from June to December 2007, at 20 km resolution and weekly timescale. Eight concentration towers were used to constrain the carbon balance in a 1000×1000 km2 domain in this agricultural region of the US upper midwest. The CO2 concentrations of the boundaries derived from CarbonTracker were adjusted to match direct observations from aircraft profiles around the domain. The regional carbon balance ends up with a sink of 183 Tg C±35 Tg C over the area for the period June–December, 2007. Potential bias from incorrect boundary conditions of about 0.55 ppm over the 7 months was corrected using mixing ratios from four different aircraft profile sites operated at a weekly time scale, acting as an additional source of uncertainty of 24 Tg C. We used two different prior flux estimates, the SiBCrop model and the inverse flux product from the CarbonTracker system. We show that inverse flux estimates using both priors converge to similar posterior estimates (20 Tg C difference), in our reference inversion, but some spatial structures from the prior fluxes remain in the posterior fluxes, revealing the importance of the prior flux resolution and distribution despite the large amount of atmospheric data available. The retrieved fluxes were compared to eddy flux towers in the corn and grassland areas, revealing an improvement in the seasonal cycles between the two compared to the prior fluxes, despite large absolute differences due to representation errors. The uncertainty of 34 Tg C (or 34 g C m2) was derived from the posterior uncertainty obtained with our reference inversion of about 25 to 30 Tg C and from sensitivity tests of the assumptions made in the inverse system, for a mean carbon balance over the region of −183 Tg C, slightly weaker than the reference. Because of the potential large bias (~24 Tg C in this case) due to choice of background conditions, proportional to the surface but not to the regional flux, this methodology seems limited to regions with a large signal (sink or source), unless additional observations can be used to constrain the boundary inflow.

Methane emissions from the oil and gas industry (O&G) and other sources in the Barnett Shale region were estimated by constructing a spatially resolved emission inventory. Eighteen source categories were estimated using multiple data sets, including new empirical measurements at regional O&G sites and a national study of gathering and processing facilities. Spatially referenced activity data were compiled from federal and state databases and combined with O&G facility emission factors calculated using Monte Carlo simulations that account for high emission sites representing the very upper portion, or fat-tail, in the observed emissions distributions. Total methane emissions in the 25-county Barnett Shale region in October 2013 were estimated to be 72,300 (63,400-82,400) kg CH4 h(-1). O&G emissions were estimated to be 46,200 (40,000-54,100) kg CH4 h(-1) with 19% of emissions from fat-tail sites representing less than 2% of sites. Our estimate of O&G emissions in the Barnett Shale region was higher than alternative inventories based on the United States Environmental Protection Agency (EPA) Greenhouse Gas Inventory, EPA Greenhouse Gas Reporting Program, and Emissions Database for Global Atmospheric Research by factors of 1.5, 2.7, and 4.3, respectively. Gathering compressor stations, which accounted for 40% of O&G emissions in our inventory, had the largest difference from emission estimates based on EPA data sources. Our inventory's higher O&G emission estimate was due primarily to its more comprehensive activity factors and inclusion of emissions from fat-tail sites.

Abstract [1] A measurement intensive was carried out in Barrow, Alaska, in spring 2009 as part of the Ocean-Atmosphere-Sea-Ice–Snowpack (OASIS) program. The central focus of this campaign was the role of halogen chemistry in the Arctic. A chemical ionization mass spectrometer (CIMS) performed in situ bromine oxide (BrO) measurements. In addition, a long path-differential optical absorption spectrometer (LP-DOAS) measured the average concentration of BrO along light paths of either 7.2 or 2.1 km. A comparison of the 1 min observations from both instruments is presented in this work. The two measurements were highly correlated and agreed within their uncertainties (R2 = 0.74, slope = 1.10, and intercept = −0.15 pptv). Better correlation was found (R2 = 0.85, slope = 1.04, and intercept = −0.11 pptv) for BrO observations at moderate wind speeds (>3 m s−1 and <8 m s−1) and low nitric oxide (NO) mixing ratios (<100 pptv). The improved agreement is likely due to the elimination of periods when the spatial distribution of BrO is inhomogeneous. The detection limit obtained for the CIMS was 2.6 pptv (3σ) for a 4 s integration period, and the estimated uncertainty was ∼30%. The detection limits for the LP-DOAS ranged from 0.7 to 5 pptv (3σ) depending on the level of ambient light and the chosen light path, and the estimated systematic error was 10%. The agreement between the CIMS and LP-DOAS is excellent and demonstrates the capability of both instruments to selectively and accurately measure BrO with high sensitivity.

Abstract. Urban environments are the primary contributors to global anthropogenic carbon emissions. Because much of the growth in CO2 emissions will originate from cities, there is a need to develop, assess, and improve measurement and modeling strategies for quantifying and monitoring greenhouse gas emissions from large urban centers. In this study the uncertainties in an aircraft-based mass balance approach for quantifying carbon dioxide and methane emissions from an urban environment, focusing on Indianapolis, IN, USA, are described. The relatively level terrain of Indianapolis facilitated the application of mean wind fields in the mass balance approach. We investigate the uncertainties in our aircraft-based mass balance approach by (1) assessing the sensitivity of the measured flux to important measurement and analysis parameters including wind speed, background CO2 and CH4, boundary layer depth, and interpolation technique, and (2) determining the flux at two or more downwind distances from a point or area source (with relatively large source strengths such as solid waste facilities and a power generating station) in rapid succession, assuming that the emission flux is constant. When we quantify the precision in the approach by comparing the estimated emissions derived from measurements at two or more downwind distances from an area or point source, we find that the minimum and maximum repeatability were 12 and 52%, with an average of 31%. We suggest that improvements in the experimental design can be achieved by careful determination of the background concentration, monitoring the evolution of the boundary layer through the measurement period, and increasing the number of downwind horizontal transect measurements at multiple altitudes within the boundary layer.

Atmospheric aerosol acidity is an important characteristic of aqueous particles, which has been linked to the formation of secondary organic aerosol by catalyzing reactions of oxidized organic compounds that have partitioned to the particle phase. However, aerosol acidity is difficult to measure and traditionally estimated using indirect methods or assumptions based on composition. Ongoing disagreements between experiments and thermodynamic models of particle acidity necessitate improved fundamental understanding of pH and ion behavior in high ionic strength atmospheric particles. Herein, Raman microspectroscopy was used to determine the pH of individual particles (H2SO4+MgSO4) based on sulfate and bisulfate concentrations determined from νs(SO4(2-)) and νs(HSO4(-)), the acid dissociation constant, and activity coefficients from extended Debye-Hückel calculations. Shifts in pH and peak positions of νs(SO4(2-)) and νs(HSO4(-)) were observed as a function of relative humidity. These results indicate the potential for direct spectroscopic determination of pH in individual particles and the need to improve fundamental understanding of ion behavior in atmospheric particles.

This paper describes process-based estimation of CH4 emissions from sources in Indianapolis, IN and compares these with atmospheric inferences of whole city emissions. Emissions from the natural gas distribution system were estimated from measurements at metering and regulating stations and from pipeline leaks. Tracer methods and inverse plume modeling were used to estimate emissions from the major landfill and wastewater treatment plant. These direct source measurements informed the compilation of a methane emission inventory for the city equal to 29 Gg/yr (5% to 95% confidence limits, 15 to 54 Gg/yr). Emission estimates for the whole city based on an aircraft mass balance method and from inverse modeling of CH4 tower observations were 41 ± 12 Gg/yr and 81 ± 11 Gg/yr, respectively. Footprint modeling using 11 days of ethane/methane tower data indicated that landfills, wastewater treatment, wetlands, and other biological sources contribute 48% while natural gas usage and other fossil fuel sources contribute 52% of the city total. With the biogenic CH4 emissions omitted, the top-down estimates are 3.5–6.9 times the nonbiogenic city inventory. Mobile mapping of CH4 concentrations showed low level enhancement of CH4 throughout the city reflecting diffuse natural gas leakage and downstream usage as possible sources for the missing residual in the inventory.

We report measurements of methane (CH4) emission rates observed at eight different high-emitting point sources in the Barnett Shale, Texas, using aircraft-based methods performed as part of the Barnett Coordinated Campaign. We quantified CH4 emission rates from four gas processing plants, one compressor station, and three landfills during five flights conducted in October 2013. Results are compared to other aircraft- and surface-based measurements of the same facilities, and to estimates based on a national study of gathering and processing facilities emissions and 2013 annual average emissions reported to the U.S. EPA Greenhouse Gas Reporting Program (GHGRP). For the eight sources, CH4 emission measurements from the aircraft-based mass balance approach were a factor of 3.2-5.8 greater than the GHGRP-based estimates. Summed emissions totaled 7022 ± 2000 kg hr(-1), roughly 9% of the entire basin-wide CH4 emissions estimated from regional mass balance flights during the campaign. Emission measurements from five natural gas management facilities were 1.2-4.6 times larger than emissions based on the national study. Results from this study were used to represent "super-emitters" in a newly formulated Barnett Shale Inventory, demonstrating the importance of targeted sampling of "super-emitters" that may be missed by random sampling of a subset of the total.

The hydroxyl radical oxidation of α-pinene under high NOx conditions was studied in a photochemical reaction chamber to investigate organic nitrate (RONO2) production and fate between the gas and particle phases. We report an organic nitrate yield of 26 ± 7% from the oxidation of this monoterpene in the presence of nitric oxide (NO). However, the apparent organic nitrate yield was found to be highly dependent on both chamber relative humidity (RH) and seed aerosol acidity, likely as a result of particle phase hydrolysis. The particle phase loss of organic nitrates is believed to increase the gas to particle partitioning within the system, leading to decreased RONO2 yields in both the gas and particle phases at elevated RH and an apparent non-equilibrium partitioning mechanism. The hydrolysis of particle phase organic nitrates in this study, starting at low chamber relative humidity, implies that aerosol partitioning of organic nitrates may be an important sink for atmospheric NOx and may have a significant impact on regional air quality.

Bromine atoms play a central role in atmospheric reactive halogen chemistry, depleting ozone and elemental mercury, thereby enhancing deposition of toxic mercury, particularly in the Arctic near-surface troposphere. However, direct bromine atom measurements have been missing to date, due to the lack of analytical capability with sufficient sensitivity for ambient measurements. Here we present direct atmospheric bromine atom measurements, conducted in the springtime Arctic. Measured bromine atom levels reached 14 parts per trillion (ppt, pmol mol-1; 4.2 × 108 atoms per cm-3) and were up to 3-10 times higher than estimates using previous indirect measurements not considering the critical role of molecular bromine. Observed ozone and elemental mercury depletion rates are quantitatively explained by the measured bromine atoms, providing field validation of highly uncertain mercury chemistry. Following complete ozone depletion, elevated bromine concentrations are sustained by photochemical snowpack emissions of molecular bromine and nitrogen oxides, resulting in continued atmospheric mercury depletion. This study provides a breakthrough in quantitatively constraining bromine chemistry in the polar atmosphere, where this chemistry connects the rapidly changing surface to pollutant fate.

Aerosol phase state is critical for quantifying aerosol effects on climate and air quality. However, significant challenges remain in our ability to predict and quantify phase state during its evolution in the atmosphere. Herein, we demonstrate that aerosol phase (liquid, semisolid, solid) exhibits a diel cycle in a mixed forest environment, oscillating between a viscous, semisolid phase state at night and liquid phase state with phase separation during the day. The viscous nighttime particles existed despite higher relative humidity and were independently confirmed by bounce factor measurements and atomic force microscopy. High-resolution mass spectrometry shows the more viscous phase state at night is impacted by the formation of terpene-derived and higher molecular weight secondary organic aerosol (SOA) and smaller inorganic sulfate mass fractions. Larger daytime particulate sulfate mass fractions, as well as a predominance of lower molecular weight isoprene-derived SOA, lead to the liquid state of the daytime particles and phase separation after greater uptake of liquid water, despite the lower daytime relative humidity. The observed diel cycle of aerosol phase should provoke rethinking of the SOA atmospheric lifecycle, as it suggests diurnal variability in gas–particle partitioning and mixing time scales, which influence aerosol multiphase chemistry, lifetime, and climate impacts.

This paper describes the results of measurements of the branching ratio (k 2b /(k 2a + k 2b )) for formation of organic nitrates via peroxy radical reaction with NO, following the reaction of the OH radical with isoprene (2‐methyl‐l,3‐butadiene). The experiments were conducted in a 5 m 3 all‐Teflon photochemical reaction chamber, via the photolysis of isopropyl nitrite in the presence of isoprene and NO. The organic nitrate yield was determined from the measurement of the sum of all organic nitrate isomers, using an organic nitrate selective detector, as a function of isoprene consumed. Organic nitrates were sampled directly from the reaction chamber into a capillary Chromatographic column, followed by separation and quantitative pyrolytic conversion to NO 2 , which was detected through luminol chemiluminescence. In this manner, seven isomeric organic nitrates were observed, with a total yield of 4.4%. The structural features of the precursor peroxy radicals that influence the magnitude of the yield is discussed. Emission inventories for isoprene and NO lead to the conclusion that as much as 7% of NO emitted in the eastern United States in the summer months is lost from the atmosphere through the isoprene nitrate channel.

Diurnal measurements of hydroxyl and hydroperoxy radicals (OH and HO 2 ) made during the Program for Research on Oxidants: Photochemistry, Emissions, and Transport (PROPHET) summer intensive of 1998 indicate that these key components of gas phase atmospheric oxidation are sustained in significant amounts throughout the night in this northern forested region. Typical overnight levels of OH observed were 0.04 parts per trillion (pptv) (1.1 × 10 6 molecules/cm 3 ), while HO 2 concentrations ranged from 1 to 4 pptv. Results of diagnostic testing performed before, after, and during the deployment suggest little possibility of interferences in the measurements. Collocated measurements of the reactive biogenic hydrocarbon isoprene corroborate the observed levels of OH by exhibiting significant decays overnight above the forest canopy. The observed isoprene lifetimes ranged from 1.5 to 12 hours in the dark, and they correlate well to those expected from chemical oxidation by the measured OH abundances. Possible dark reactions that could generate such elevated levels of OH include the ozonolysis of extremely reactive biogenic terpenoids. However, in steady state models, which include this hypothetical production mechanism, HO 2 radicals are generated in greater quantities than were measured. Nonetheless, if the measurements are representative of the nocturnal boundary layer in midlatitude temperate forests, this observed nocturnal phenomenon might considerably alter our understanding of the diurnal pattern of atmospheric oxidation in such pristine, low‐NO x environments.

Measurements of formaldehyde, acetaldehyde, acetone and propionaldehyde concentrations were made at two rural sites in central Ontario. One site (at Egbert, Ont.) is located ≈60 km northwest of Toronto, while the other site (at Dorset, Ont.) is ≈150 km northeast of the Egbert site. Measurements were made using a modified version of a derivatization technique in which sample air is pumped through Teflon tubes packed with silica gel that is coated with 2,4-dinitrophenylhydrazine (DNPH). The product hydrazones were separated and quantified using HPLC. Quantitative determinations of formaldehyde, acetaldehyde and acetone were made for 49 and 47 samples at the Dorset and Egbert sites, respectively, between 25 July and 30 August 1988. The average concentrations determined at the Dorset site for formaldehyde, acetaldehyde, and acetone were 1.6, 0.46 and 1.8 ppb, respectively, and for the Egbert site the corresponding averages were 1.8, 0.57 and 1.6 ppb. A set of 10 samples from the Egbert site were analysed for propionaldehyde yielding an average concentration of 0.03 ppb. The formaldehyde measurements were compared with measurements made at the same time using Tunable Diode Laser Absorption Spectroscopy. The observed concentrations reported here are compared with previously reported measurements of these species and interpreted in terms of atmospheric variables (e.g. meteorology, concentrations of precursor hydrocarbons) influencing their concentrations.

A multiphase chemical box model of Arctic halogen chemistry has been developed using a PC‐based modeling program developed by Environment Canada called the Chemical Reactions Modeling System (CREAMS). The multiphase model contains 125 gas phase reactions, 19 photolysis reactions, and 16 aqueous reactions occurring in suspended aerosol particles and the quasi‐liquid component of snow. The model simulates mass transfer of species between the gas phase and particles, and between the gas phase and the snowpack. Model simulations were conducted for the Arctic for the period April 16 to April 24 at 245 K within a 400 m boundary layer. The complete model simulates halogen‐catalyzed ozone depletion within 5 days from the start of the model run, via known gas and heterogeneous phase activation mechanisms. A critically important model reaction is BrO + HCHO → HOBr + CHO, which has a substantial impact on gas phase HOBr, and subsequent condensed phase chemistry. When coupled with a necessary snowpack efflux of aldehydes, required to maintain the aldehyde concentrations at observed levels, the new BrO chemistry has a significant impact on the concentrations of gas phase bromine species, particle bromide, and chlorine atoms, through chemistry occurring in the snowpack. We also find that O 3 depletion cannot be simulated without the presence of heterogeneous halogen chemistry occurring in the snowpack and that the rate of O 3 depletion is limited by the mass transfer rate of HOBr to the snowpack.

It has been shown that sunlit snowpacks are photochemically active, producing a number of species that can significantly impact the overlying atmosphere. Here we investigate the origin of the flux of low molecular weight carbonyl compounds (formaldehyde and acetaldehyde) from sunlit snow obtained from both Arctic (Alert, Canada, and Summit, Greenland) and Antarctic (South Pole) sources. In addition, efforts to characterize the potential snow‐phase organic matter (SPOM) precursors were undertaken. Using chemically characterized SPOM, we find that formaldehyde and acetaldehyde are produced upon irradiation, and that production is enhanced with the addition of nitrate (a precursor to OH radicals). SPOM from both Alert and Summit is found to be derived from a variety of sources, including vascular plants. This indicates deposition of atmospheric particulate matter containing vascular tissue to high‐altitude Arctic snow. These findings open a potential window for a rich record of variations in terrestrial vegetation‐derived aerosol signals that could be preserved in ice cores.

We report measurements of rapidly photolyzable chlorine (Cl p ; e.g., Cl 2 And HOCl) and bromine (Br p ; e.g., Br 2 and HOBr) in the high Arctic using a newly developed photoactive halogen detector (PHD). Ground level ambient air was sampled daily from mid‐February through mid‐April in the Canadian Arctic at Alert, Northwest Territories (82.5°N, 62.3°W), as part of the Polar Sunrise Experiment (PSE) 1995. Concentrations of “total photolyzable chlorine” varied from &lt;9 to 100 pptv as Cl 2 and that of “total photolyzable bromine” from &lt;4 to 38 pptv as Br 2 . High concentration episodes of chlorine were observed only prior to sunrise (March 21), while high concentration episodes of bromine were measured throughout the study. The high concentrations of photolyzable chlorine and bromine prior to sunrise suggest a “dark” production mechanism that we assume yields Cl 2 and Br 2 . An inverse correlation of bromine with ozone is clearly present in one major ozone depletion episode at the end of March. A trajectory analysis, taken with the differences in measured levels of photolyzable chlorine and bromine after sunrise, imply different production mechanisms for these two types of species. A steady state analysis of the data for one ozone depletion episode suggests a [Br]/[Cl] ratio in the range 100–300. The high concentrations of photolyzable bromine after sunrise imply the existence of a precursor other than aerosol bromide.

As part of the Pacific '93 Oxidant Study that took place in the summer in the Lower Fraser Valley of British Columbia, we conducted measurements of isoprene, and its oxidation products methyl vinyl ketone (MVK) and methacrolein (MACR) at a surface site about 40 km east of the city of Vancouver. Hourly measurements were conducted between 16 July and 10 August 1993. The data indicated evidence for substantial contributions of isoprene chemistry to the production of ozone during oxidant episodes in this region. Maximum concentrations of isoprene, MVK, and MACR were 5.3, 2.0, and 1.0 ppb, resp., for 4 August. Analysis of the relationship between MVK and 03 during the oxidant episode period l–6 August led to an estimated contribution of isoprene chemistry of ozone production of ⩾ 13%. The average measured ratio of MVK/MACR was about 1.9–2.0 in the daytime, compared to the published relative yield of 1.4. Comparison of the MVK and MACR measurements to those of organic nitrates led to the conclusion that there is a significant non-photochemical source of MVK and MACR in this urban area.

The molecular halogens Br2, BrCl and Cl2 were monitored from 9 February to 13 March 2000 as part of the ALERT 2000 campaign to investigate the causes of ozone depletion at polar sunrise. The measurements were performed over the transition period from winter to spring in the high Arctic, at Alert, on northern Ellesmere Island in Nunavut, Canada. The measurement campaign for these species covered the period from 24-h darkness, at the beginning of the campaign, to several hours of direct sunlight per day at the end of the campaign. The halogen measurements were made by atmospheric pressure ionization tandem mass spectrometry, using multiple isotopes for each species, and reporting a 20-s average for each species every 2 min. Bromine was observed above the 0.2 ppt detection limit throughout the campaign at mixing ratios up to 27 ppt. BrCl was not observed above its 2 ppt detection limit until mid-way through the campaign, but was present almost continuously thereafter, and reached levels of 35 ppt. Molecular chlorine was not observed above its 2 ppt detection limit. During periods of ozone depletion, there was a very strong inverse relationship between O3 and Br2, and a moderately strong inverse relationship between O3 and BrCl. The slopes of linear regressions of Br2 and BrCl vs. O3 indicate ≈1 ppb decrease in O3 mixing ratio for every ppt of either of the molecular halogens. In some cases, O3 depletion events seemed to be triggered by bursts of the halogen species initiated by photochemical processes, even in very weak “twilight”. In other cases, ozone depletion observed at Alert appeared to result from transport of O3-depleted, halogen-enriched air from other locations.

Abstract The kinetics of ice growth on the secondary prismatic plane and the basal plane {0001} is studied by Molecular Dynamics simulations. The simulation system initially consists of a slab of ice in contact with a layer of water on one side, and vacuum on the other side. The remaining surface of the water layer is also facing vacuum. The time evolution of the system shows the crystallization of the liquid water and the evaporation of very few molecules at the free surfaces. The ice vapour interfaces are wet on both sides by identical thin layers of liquid water, strongly suggesting that the system has reached its equilibrium state. To analyse the results, we have developed a new method to discriminate whether a molecule belongs to the ice lattice or is in liquid state. Using this method to monitor the number of ice molecules as a function of time, we find that the freezing is much faster on the prismatic plane than on the basal plane. For the prismatic plane, irregularities in the surface of the solid phase are observed during the growing period contrasting with a smooth interface on the basal plane at all times. We studied three different temperatures and found that the rate of crystallization decreases with temperature for the prismatic plane, while no conclusive behaviour was found for the basal plane growth. Keywords: Molecular dynamics simulationsIce growthSupercooled water Acknowledgements It is a pleasure to dedicate this work to Professor Ben Widom whose immense wisdom, patience and teachings has inspired our work throughout the years. This work is partially supported by the National Science Foundation. We would like to thank Prof. Hiroki Nada for providing us with ice configurations and energy values during the initial stages of this work.

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTRing fragmentation reactions in the photooxidations of toluene and o-xyleneP. B. Shepson, E. O. Edney, and E. W. CorseCite this: J. Phys. Chem. 1984, 88, 18, 4122–4126Publication Date (Print):August 1, 1984Publication History Published online1 May 2002Published inissue 1 August 1984https://doi.org/10.1021/j150662a053RIGHTS & PERMISSIONSArticle Views280Altmetric-Citations85LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (491 KB) Get e-Alerts

Abstract. Oxidation of isoprene through reaction with NO3 radicals is a significant sink for isoprene that persists after dark. The main products of the reaction are multifunctional nitrates. These nitrates constitute a significant NOx sink in the nocturnal boundary layer and they likely play an important role in formation of secondary organic aerosol. Products of the isoprene+NO3 reaction will, in many locations, be abundant enough to affect nighttime radical chemistry and to persist into daytime where they may represent a source of NOx. Product formation in the isoprene + NO3 reaction was studied in a smog chamber at Purdue University. Isoprene nitrates and other hydrocarbon products were observed using Proton Transfer Reaction-Mass Spectrometry (PTR-MS) and reactive nitrogen products were observed using Thermal Dissociation–Laser Induced Fluorescence (TD-LIF). The organic nitrate yield is found to be 65±12% of which the majority was nitrooxy carbonyls and the combined yield of methacrolein and methyl vinyl ketone (MACR+MVK) is found to be ∼10%. PTR-MS measurements of nitrooxy carbonyls and TD-LIF measurements of total organic nitrates agreed well. The PTR-MS also observed a series of minor oxidation products which were tentatively identified and their yields quantified These other oxidation products are used as additional constraints on the reaction mechanism.

Abstract. Isoprene is an important atmospheric volatile organic compound involved in ozone production and NOx (NO+NO2) sequestration and transport. Isoprene reaction with OH in the presence of NO can form either isoprene hydroxy nitrates ("isoprene nitrates") or convert NO to NO2 which can photolyze to form ozone. While it has been shown that isoprene nitrate production can represent an important sink for NOx in forest impacted environments, there is little experimental knowledge of the relative importance of the individual isoprene nitrate isomers, each of which has a different fate and reactivity. In this work, we have identified the 8 individual isomers and determined their total and individual production yields. The overall yield of isoprene nitrates at atmospheric pressure and 295 K was found to be 0.070(+0.025/−0.015). Three isomers, representing nitrates resulting from OH addition to a terminal carbon, represent 90% of the total IN yield. We also determined the ozone rate constants for three of the isomers, and have calculated their atmospheric lifetimes, which range from ~1–2 h, making their oxidation products likely more important as atmospheric organic nitrates and sinks for nitrogen.

Abstract. Vegetation emits large quantities of biogenic volatile organic compounds (BVOC). At remote sites, these compounds are the dominant precursors to ozone and secondary organic aerosol (SOA) production, yet current field studies show that atmospheric models have difficulty in capturing the observed HOx cycle and concentrations of BVOC oxidation products. In this manuscript, we simulate BVOC chemistry within a forest canopy using a one-dimensional canopy-chemistry model (Canopy Atmospheric CHemistry Emission model; CACHE) for a mixed deciduous forest in northern Michigan during the CABINEX 2009 campaign. We find that the base-case model, using fully-parameterized mixing and the simplified biogenic chemistry of the Regional Atmospheric Chemistry Model (RACM), underestimates daytime in-canopy vertical mixing by 50–70% and by an order of magnitude at night, leading to discrepancies in the diurnal evolution of HOx, BVOC, and BVOC oxidation products. Implementing observed micrometeorological data from above and within the canopy substantially improves the diurnal cycle of modeled BVOC, particularly at the end of the day, and also improves the observation-model agreement for some BVOC oxidation products and OH reactivity. We compare the RACM mechanism to a version that includes the Mainz isoprene mechanism (RACM-MIM) to test the model sensitivity to enhanced isoprene degradation. RACM-MIM simulates higher concentrations of both primary BVOC (isoprene and monoterpenes) and oxidation products (HCHO, MACR+MVK) compared with RACM simulations. Additionally, the revised mechanism alters the OH concentrations and increases HO2. These changes generally improve agreement with HOx observations yet overestimate BVOC oxidation products, indicating that this isoprene mechanism does not improve the representation of local chemistry at the site. Overall, the revised mechanism yields smaller changes in BVOC and BVOC oxidation product concentrations and gradients than improving the parameterization of vertical mixing with observations, suggesting that uncertainties in vertical mixing parameterizations are an important component in understanding observed BVOC chemistry.

Inorganic bromine plays a critical role in ozone and mercury depletions events (ODEs and MDEs) in the Arctic marine boundary layer. Direct observations of bromine species other than bromine oxide (BrO) during ODEs are very limited. Here we report the first direct measurements of hypobromous acid (HOBr) as well as observations of BrO and molecular bromine (Br 2 ) by chemical ionization mass spectrometry at Barrow, Alaska in spring 2009 during the Ocean‐Atmospheric‐Sea Ice‐Snowpack (OASIS) campaign. Diurnal profiles of HOBr with maximum concentrations near local noon and no significant concentrations at night were observed. The measured average daytime HOBr mixing ratio was 10 pptv with a maximum value of 26 pptv. The observed HOBr was reasonably well correlated (R 2 = 0.57) with predictions from a simple steady state photochemical model constrained to observed BrO and HO 2 at wind speeds &lt;6 m s −1 . However, predicted HOBr levels were considerably higher than observations at higher wind speeds. This may be due to enhanced heterogeneous loss of HOBr on blowing snow coincident with higher wind speeds. BrO levels were also found to be higher at elevated wind speeds. Br 2 was observed in significant mixing ratios (maximum = 46 pptv; average = 13 pptv) at night and was strongly anti‐correlated with ozone. The diurnal speciation of observed gas phase inorganic bromine species can be predicted by a time‐dependent box model that includes efficient heterogeneous recycling of HOBr, hydrogen bromide (HBr), and bromine nitrate (BrONO 2 ) back to more reactive forms of bromine.

Abstract We report the CH4 emission flux from the city of Indianapolis, IN, the site of the Indianapolis Flux Experiment (INFLUX) project for developing, assessing, and improving top-down and bottom-up approaches for quantifying urban greenhouse gas emissions. Using an aircraft-based mass balance approach, we find that the average CH4 emission rate from five flight experiments in 2011 is 135 ± 58 (1σ) moles s-1 (7800 ± 3300 kg hr-1). The effective per capita CH4 emission rate for Indianapolis is 77 kg CH4 person-1 yr-1, a figure that is less than the national anthropogenic CH4 emission (∼91 kg CH4 person-1 yr-1) but considerably larger than the global figure (∼48 kg CH4 person-1 yr-1). We consistently observed elevated CH4 concentrations at specific coordinates along our flight transects downwind of the city. Inflight investigations as well as back trajectories using measured wind directions showed that the elevated concentrations originated from the southwest side of the city where a landfill and a natural gas transmission regulating station (TRS) are located. Street level mobile measurements downwind of the landfill and the TRS supported the results of aircraft-based data, and were used to quantify the relative contributions from the two sources. We find that the CH4 emission from the TRS was negligible relative to the landfill, which was responsible for 33 ± 10% of the citywide emission flux. A regression of propane versus methane from aircraft flask samples suggests that the remaining citywide CH4 emissions (∼67%) derive from the natural gas distribution system. We discuss the combination of surface mobile observations and aircraft city-wide flux measurements to determine the total flux and apportionment to important sources.

Mercury is a toxic environmental contaminant originating from both natural and anthropogenic sources. Gaseous elemental mercury (GEM) is relatively long lived in the midlatitudes and can be transported long distances in the atmosphere. In the Polar Regions, mercury can have a much shorter lifetime and is known to experience episodic depletions following polar sunrise in concert with ozone depletion events. A series of photochemically initiated reactions involving halogen radicals is believed to be the primary pathway responsible for converting elemental mercury to oxidized forms of reactive gaseous mercury (RGM) that are subsequently deposited to snow and ice surfaces. Using field measurements from the Ocean‐Atmosphere‐Sea Ice‐Snowpack (OASIS) 2009 field campaign of GEM, RGM, ozone, and a large suite of both inorganic halogen and volatile organic compounds, we calculated steady state Br and Cl atom concentrations and investigated the contribution of Br, BrO, Cl, ClO, O 3 , and OH to the observed decay of GEM for five cases of apparent first‐order decay. The results of this study indicate that Br and BrO are the dominant oxidizers for Arctic mercury depletion events, with Br having the greatest overall contribution to GEM decay. Ozone is likely the primary factor controlling the relative contribution of Br and BrO, as BrO is a product of the reaction of Br with ozone, and reaction with O 3 can be the largest Br atom sink. Cl was not found to be significant in any of the studied events; however, this result is highly dependent on the rate constant, for which there is a large range in the literature. Modeled 48 h back trajectories of the mercury depletion events studied here indicate significant time spent over sea ice–covered regions, where the concentration of halogen radicals is likely higher than those estimated using local‐scale chemical mole fractions.

Abstract. Isoprene hydroxynitrates (IN) are tracers of the photochemical oxidation of isoprene in high NOx environments. Production and loss of IN have a significant influence on the NOx cycle and tropospheric O3 chemistry. To better understand IN chemistry, a series of photochemical reaction chamber experiments was conducted to determine the IN yield from isoprene photooxidation at high NO concentrations (&gt; 100 ppt). By combining experimental data and calculated isomer distributions, a total IN yield of 9(+4/−3) % was derived. The result was applied in a zero-dimensional model to simulate production and loss of ambient IN observed in a temperate forest atmosphere, during the Southern Oxidant and Aerosol Study (SOAS) field campaign, from 27 May to 11 July 2013. The 9 % yield was consistent with the observed IN/(MVK+MACR) ratios observed during SOAS. By comparing field observations with model simulations, we identified NO as the limiting factor for ambient IN production during SOAS, but vertical mixing at dawn might also contribute (~ 27 %) to IN dynamics. A close examination of isoprene's oxidation products indicates that its oxidation transitioned from a high-NO dominant chemical regime in the morning into a low-NO dominant regime in the afternoon. A significant amount of IN produced in the morning high NO regime could be oxidized in the low NO regime, and a possible reaction scheme was proposed.

Abstract. Hydroxyl (OH) and hydroperoxyl (HO2) radicals are key species driving the oxidation of volatile organic compounds that can lead to the production of ozone and secondary organic aerosols. Previous measurements of these radicals in forest environments with high isoprene, low NOx conditions have shown serious discrepancies with modeled concentrations, bringing into question the current understanding of isoprene oxidation chemistry in these environments. During the summers of 2008 and 2009, OH and peroxy radical concentrations were measured using a laser-induced fluorescence instrument as part of the PROPHET (Program for Research on Oxidants: PHotochemistry, Emissions, and Transport) and CABINEX (Community Atmosphere-Biosphere INteractions EXperiment) campaigns at a forested site in northern Michigan. Supporting measurements of photolysis rates, volatile organic compounds, NOx (NO + NO2 and other inorganic species were used to constrain a zero-dimensional box model based on the Regional Atmospheric Chemistry Mechanism, modified to include the Mainz Isoprene Mechanism (RACM-MIM). The CABINEX model OH predictions were in good agreement with the measured OH concentrations, with an observed-to-modeled ratio near one (0.70 ± 0.31) for isoprene mixing ratios between 1–2 ppb on average. The measured peroxy radical concentrations, reflecting the sum of HO2 and isoprene-based peroxy radicals, were generally lower than predicted by the box model in both years.

The objective of the Indianapolis Flux Experiment (INFLUX) is to develop, evaluate and improve methods for measuring greenhouse gas (GHG) emissions from cities. INFLUX's scientific objectives are to quantify CO2 and CH4 emission rates at 1 km resolution with a 10% or better accuracy and precision, to determine whole-city emissions with similar skill, and to achieve high (weekly or finer) temporal resolution at both spatial resolutions. The experiment employs atmospheric GHG measurements from both towers and aircraft, atmospheric transport observations and models, and activity-based inventory products to quantify urban GHG emissions. Multiple, independent methods for estimating urban emissions are a central facet of our experimental design. INFLUX was initiated in 2010 and measurements and analyses are ongoing. To date we have quantified urban atmospheric GHG enhancements using aircraft and towers with measurements collected over multiple years, and have estimated whole-city CO2 and CH4 emissions using aircraft and tower GHG measurements, and inventory methods. Significant differences exist across methods; these differences have not yet been resolved; research to reduce uncertainties and reconcile these differences is underway. Sectorally- and spatially-resolved flux estimates, and detection of changes of fluxes over time, are also active research topics. Major challenges include developing methods for distinguishing anthropogenic from biogenic CO2 fluxes, improving our ability to interpret atmospheric GHG measurements close to urban GHG sources and across a broader range of atmospheric stability conditions, and quantifying uncertainties in inventory data products. INFLUX data and tools are intended to serve as an open resource and test bed for future investigations. Well-documented, public archival of data and methods is under development in support of this objective.

During springtime, the Arctic atmospheric boundary layer undergoes frequent rapid depletions in ozone and gaseous elemental mercury due to reactions with halogen atoms, influencing atmospheric composition and pollutant fate. Although bromine chemistry has been shown to initiate ozone depletion events, and it has long been hypothesized that iodine chemistry may contribute, no previous measurements of molecular iodine (I2) have been reported in the Arctic. Iodine chemistry also contributes to atmospheric new particle formation and therefore cloud properties and radiative forcing. Here we present Arctic atmospheric I2 and snowpack iodide (I-) measurements, which were conducted near Utqiaġvik, AK, in February 2014. Using chemical ionization mass spectrometry, I2 was observed in the atmosphere at mole ratios of 0.3-1.0 ppt, and in the snowpack interstitial air at mole ratios up to 22 ppt under natural sunlit conditions and up to 35 ppt when the snowpack surface was artificially irradiated, suggesting a photochemical production mechanism. Further, snow meltwater I- measurements showed enrichments of up to ∼1,900 times above the seawater ratio of I-/Na+, consistent with iodine activation and recycling. Modeling shows that observed I2 levels are able to significantly increase ozone depletion rates, while also producing iodine monoxide (IO) at levels recently observed in the Arctic. These results emphasize the significance of iodine chemistry and the role of snowpack photochemistry in Arctic atmospheric composition, and imply that I2 is likely a dominant source of iodine atoms in the Arctic.

Abstract The Southeast Atmosphere Studies (SAS), which included the Southern Oxidant and Aerosol Study (SOAS); the Southeast Nexus (SENEX) study; and the Nitrogen, Oxidants, Mercury and Aerosols: Distributions, Sources and Sinks (NOMADSS) study, was deployed in the field from 1 June to 15 July 2013 in the central and eastern United States, and it overlapped with and was complemented by the Studies of Emissions, Atmospheric Composition, Clouds and Climate Coupling by Regional Surveys (SEAC4RS) campaign. SAS investigated atmospheric chemistry and the associated air quality and climate-relevant particle properties. Coordinated measurements from six ground sites, four aircraft, tall towers, balloon-borne sondes, existing surface networks, and satellites provide in situ and remotely sensed data on trace-gas composition, aerosol physicochemical properties, and local and synoptic meteorology. Selected SAS findings indicate 1) dramatically reduced NOx concentrations have altered ozone production regimes; 2) indicators of “biogenic” secondary organic aerosol (SOA), once considered part of the natural background, were positively correlated with one or more indicators of anthropogenic pollution; and 3) liquid water dramatically impacted particle scattering while biogenic SOA did not. SAS findings suggest that atmosphere–biosphere interactions modulate ambient pollutant concentrations through complex mechanisms and feedbacks not yet adequately captured in atmospheric models. The SAS dataset, now publicly available, is a powerful constraint to develop predictive capability that enhances model representation of the response and subsequent impacts of changes in atmospheric composition to changes in emissions, chemistry, and meteorology.

Oxidation of bromide in aqueous environments initiates the formation of molecular halogen compounds, which is important for the global tropospheric ozone budget. In the aqueous bulk, oxidation of bromide by ozone involves a [Br•OOO-] complex as intermediate. Here we report liquid jet X-ray photoelectron spectroscopy measurements that provide direct experimental evidence for the ozonide and establish its propensity for the solution-vapour interface. Theoretical calculations support these findings, showing that water stabilizes the ozonide and lowers the energy of the transition state at neutral pH. Kinetic experiments confirm the dominance of the heterogeneous oxidation route established by this precursor at low, atmospherically relevant ozone concentrations. Taken together, our results provide a strong case of different reaction kinetics and mechanisms of reactions occurring at the aqueous phase-vapour interface compared with the bulk aqueous phase.Heterogeneous oxidation of bromide in atmospheric aqueous environments has long been suspected to be accelerated at the interface between aqueous solution and air. Here, the authors provide spectroscopic, kinetic and theoretical evidence for a rate limiting, surface active ozonide formed at the interface.

Approximately 150 billion cubic meters (BCM) of natural gas is flared and vented in the world annually, emitting greenhouse gases and other pollutants with no energy benefit. About 7 BCM per year is flared in the United States, and half is from North Dakota alone. There are few emission measurements from associated gas flares and limited black carbon (BC) emission factors have been previously reported from the field. Emission plumes from 26 individual flares in the Bakken formation in North Dakota were sampled. Methane, carbon dioxide, and BC were measured simultaneously, allowing the calculation of BC mass emission factors using the carbon balance method. Particle optical absorption was measured using a three-wavelength particle soot absorption photometer (PSAP) and BC particle number and mass concentrations were measured with a single particle soot photometer. The BC emission factors varied over 2 orders of magnitude, with an average and uncertainty range of 0.14 ± 0.12 g/kg hydrocarbons in associated gas and a median of 0.07 g/kg which represents a lower bound on these measurements. An estimation of the BC emission factor derived from PSAP absorption provides an upper bound at 3.1 g/kg. These results are lower than previous estimations and laboratory measurements. The BC mass absorption cross section was 16 ± 12 m2/g BC at 530 nm. The average absorption Ångström exponent was 1.2 ± 0.8, suggesting that most of the light absorbing aerosol measured was black carbon and the contribution of light absorbing organic carbon was small.

Multiphase reactions involving sea spray aerosol (SSA) impact trace gas budgets in coastal regions by acting as a reservoir for oxidized nitrogen and sulfur species, as well as being a source of halogen gases (HCl, ClNO2, etc.). Whereas most studies of multiphase reactions on SSA have focused on marine environments, far less is known about SSA transported inland. Herein, single-particle measurements of SSA are reported at a site >320 km from the Gulf of Mexico, with transport times of 7–68 h. Samples were collected during the Southern Oxidant and Aerosol Study (SOAS) in June–July 2013 near Centreville, Alabama. SSA was observed in 93% of 42 time periods analyzed. During two marine air mass periods, SSA represented significant number fractions of particles in the accumulation (0.2–1.0 μm, 11%) and coarse (1.0–10.0 μm, 35%) modes. Chloride content of SSA particles ranged from full to partial depletion, with 24% of SSA particles containing chloride (mole fraction of Cl/Na ≥ 0.1, 90% chloride depletion). Both the frequent observation of SSA at an inland site and the range of chloride depletion observed suggest that SSA may represent an underappreciated inland sink for NOx/SO2 oxidation products and a source of halogen gases.

Presently, there is high uncertainty in estimates of methane (CH4) emissions from natural gas-fired power plants (NGPP) and oil refineries, two major end users of natural gas. Therefore, we measured CH4 and CO2 emissions at three NGPPs and three refineries using an aircraft-based mass balance technique. Average CH4 emission rates (NGPPs: 140 ± 70 kg/h; refineries: 580 ± 220 kg/h, 95% CL) were larger than facility-reported estimates by factors of 21–120 (NGPPs) and 11–90 (refineries). At NGPPs, the percentage of unburned CH4 emitted from stacks (0.01–0.14%) was much lower than respective facility-scale losses (0.10–0.42%), and CH4 emissions from both NGPPs and refineries were more strongly correlated with enhanced H2O concentrations (R2avg = 0.65) than with CO2 (R2avg = 0.21), suggesting noncombustion-related equipment as potential CH4 sources. Additionally, calculated throughput-based emission factors (EF) derived from the NGPP measurements made in this study were, on average, a factor of 4.4 (stacks) and 42 (facility-scale) larger than industry-used EFs. Subsequently, throughput-based EFs for both the NGPPs and refineries were used to estimate total U.S. emissions from these facility-types. Results indicate that NGPPs and oil refineries may be large sources of CH4 emissions and could contribute significantly (0.61 ± 0.18 Tg CH4/yr, 95% CL) to U.S. emissions.

To effectively address climate change, aggressive mitigation policies need to be implemented to reduce greenhouse gas emissions. Anthropogenic carbon emissions are mostly generated from urban environments, where human activities are spatially concentrated. Improvements in uncertainty determinations and precision of measurement techniques are critical to permit accurate and precise tracking of emissions changes relative to the reduction targets. As part of the INFLUX project, we quantified carbon dioxide (CO2), carbon monoxide (CO) and methane (CH4) emission rates for the city of Indianapolis by averaging results from nine aircraft-based mass balance experiments performed in November-December 2014. Our goal was to assess the achievable precision of the aircraft-based mass balance method through averaging, assuming constant CO2, CH4 and CO emissions during a three-week field campaign in late fall. The averaging method leads to an emission rate of 14,600 mol/s for CO2, assumed to be largely fossil-derived for this period of the year, and 108 mol/s for CO. The relative standard error of the mean is 17% and 16%, for CO2 and CO, respectively, at the 95% confidence level (CL), i.e. a more than 2-fold improvement from the previous estimate of ~40% for single-flight measurements for Indianapolis. For CH4, the averaged emission rate is 67 mol/s, while the standard error of the mean at 95% CL is large, i.e. ±60%. Given the results for CO2 and CO for the same flight data, we conclude that this much larger scatter in the observed CH4 emission rate is most likely due to variability of CH4 emissions, suggesting that the assumption of constant daily emissions is not correct for CH4 sources. This work shows that repeated measurements using aircraft-based mass balance methods can yield sufficient precision of the mean to inform emissions reduction efforts by detecting changes over time in urban emissions.

Abstract. The production of atmospheric organic nitrates (RONO2) has a large impact on air quality and climate due to their contribution to secondary organic aerosol and influence on tropospheric ozone concentrations. Since organic nitrates control the fate of gas phase NOx (NO + NO2), a byproduct of anthropogenic combustion processes, their atmospheric production and reactivity is of great interest. While the atmospheric reactivity of many relevant organic nitrates is still uncertain, one significant reactive pathway, condensed phase hydrolysis, has recently been identified as a potential sink for organic nitrate species. The partitioning of gas phase organic nitrates to aerosol particles and subsequent hydrolysis likely removes the oxidized nitrogen from further atmospheric processing, due to large organic nitrate uptake to aerosols and proposed hydrolysis lifetimes, which may impact long-range transport of NOx, a tropospheric ozone precursor. Despite the atmospheric importance, the hydrolysis rates and reaction mechanisms for atmospherically derived organic nitrates are almost completely unknown, including those derived from α-pinene, a biogenic volatile organic compound (BVOC) that is one of the most significant precursors to biogenic secondary organic aerosol (BSOA). To better understand the chemistry that governs the fate of particle phase organic nitrates, the hydrolysis mechanism and rate constants were elucidated for several organic nitrates, including an α-pinene-derived organic nitrate (APN). A positive trend in hydrolysis rate constants was observed with increasing solution acidity for all organic nitrates studied, with the tertiary APN lifetime ranging from 8.3 min at acidic pH (0.25) to 8.8 h at neutral pH (6.9). Since ambient fine aerosol pH values are observed to be acidic, the reported lifetimes, which are much shorter than that of atmospheric fine aerosol, provide important insight into the fate of particle phase organic nitrates. Along with rate constant data, product identification confirms that a unimolecular specific acid-catalyzed mechanism is responsible for organic nitrate hydrolysis under acidic conditions. The free energies and enthalpies of the isobutyl nitrate hydrolysis intermediates and products were calculated using a hybrid density functional (ωB97X-V) to support the proposed mechanisms. These findings provide valuable information regarding the organic nitrate hydrolysis mechanism and its contribution to the fate of atmospheric NOx, aerosol phase processing, and BSOA composition.

Abstract Urban areas are responsible for a substantial fraction of anthropogenic emissions of greenhouse gases (GHGs) including methane (CH 4 ), with the second largest anthropogenic direct radiative forcing relative to carbon dioxide (CO 2 ). Quantification of urban CH 4 emissions is important for establishing GHG mitigation policies. Comparison of observation‐based and inventory‐based urban CH 4 emissions suggests possible improvements in estimating CH 4 source emissions in urban environments. In this study, we quantify CH 4 emissions from the Baltimore‐Washington area based on the mass balance aircraft flight experiments conducted in Winters 2015 and 2016. The field measurement‐based mean winter CH 4 emission rates from this area were 8.66 ± 4.17 kg/s in 2015 and 9.14 ± 4.49 kg/s in 2016, which are 2.8 times the 2012 average U.S. GHG Inventory‐based emission rate. The observed emission rate is 1.7 times that given in a population‐apportioned state of Maryland inventory. Methane emission rates inferred from carbon monoxide (CO) and CO 2 emission inventories and observed CH 4 /CO and CH 4 /CO 2 enhancement ratios are similar to those from the mass balance approach. The observed ethane‐to‐methane ratios, with a mean value of 3.3% in Winter 2015 and 4.3% in Winter 2016, indicate that the urban natural gas system could be responsible for ~40–60% of total CH 4 emissions from this area. Landfills also appear to be a major contributor, providing 25 ± 15% of the total emissions for the region. Our study suggests there are grounds to reexamine the CH 4 emissions estimates for the Baltimore‐Washington area and to conduct flights in other seasons.

Urban areas contribute approximately three-quarters of fossil fuel derived CO2 emissions, and many cities have enacted emissions mitigation plans. Evaluation of the effectiveness of mitigation efforts will require measurement of both the emission rate and its change over space and time. The relative performance of different emission estimation methods is a critical requirement to support mitigation efforts. Here we compare results of CO2 emissions estimation methods including an inventory-based method and two different top-down atmospheric measurement approaches implemented for the Indianapolis, Indiana, U.S.A. urban area in winter. By accounting for differences in spatial and temporal coverage, as well as trace gas species measured, we find agreement among the wintertime whole-city fossil fuel CO2 emission rate estimates to within 7%. This finding represents a major improvement over previous comparisons of urban-scale emissions, making urban CO2 flux estimates from this study consistent with local and global emission mitigation strategy needs. The complementary application of multiple scientifically driven emissions quantification methods enables and establishes this high level of confidence and demonstrates the strength of the joint implementation of rigorous inventory and atmospheric emissions monitoring approaches.

Levels of OH and peroxy radicals in the atmospheric boundary layer at Summit, Greenland, a location surrounded by snow from which HOx radical precursors are known to be emitted, were deduced using steady-state analyses applied to (OH+HO2+CH3O2), (OH+HO2), and OH–HO2 cycling. The results indicate that HOx levels at Summit are significantly increased over those that would result from O3 photolysis alone, as a result of elevated concentrations of HONO, HCHO, H2O2, and other compounds. Estimated midday levels of (HO2+CH3O2) reached 30–40pptv during two summer seasons. Calculated OH concentrations averaged between 05:00 and 20:00 (or 21:00) exceeded 4×106 molecules cm−3, comparable to (or higher than) levels expected in the tropical marine boundary layer. These findings imply rapid photochemical cycling within the boundary layer at Summit, as well as in the upper pore spaces of the surface snowpack. The photolysis rate constants and OH levels calculated here imply that gas-phase photochemistry plays a significant role in the budgets of NOx, HCHO, H2O2, HONO, and O3, compounds that are also directly affected by processes within the snowpack.

The role of formaldehyde in the atmospheric chemistry of the Arctic marine boundary layer has been studied during both polar day and night at Alert, Nunavut, Canada. Formaldehyde concentrations were determined during two separate field campaigns (PSE 1998 and ALERT2000) from polar night to the light period. The large differences in the predominant chemistry and transport issues in the dark and light periods are examined here. Formaldehyde concentrations during the dark period were found to be dependent on the transport of air masses to the Alert site. Three regimes were identified during the dark period, including background (free-tropospheric) air, transported polluted air from Eurasia, and halogen-processed air transported across the dark Arctic Ocean. In the light period, background formaldehyde levels were compared to a calculation of the steady-state formaldehyde concentrations under background and low-ozone conditions. We found that, for sunlit conditions, the ambient formaldehyde concentrations cannot be reproduced by known gas-phase chemistry. We suggest that snowpack photochemistry contributes to production and emission of formaldehyde in the light period, which could account for the high concentrations observed at Alert.

Measurements of an extensive range of nonmethane hydrocarbons (NMHCs) including alkanes, alkenes, and aromatics, and oxygenated volatile organic compounds (OVOCs) including alcohols, ketones, and aldehydes were conducted for several weeks during the summer of 1995 as part of the Southern Oxidants Study (SOS) at a rural experimental site (Youth, Inc.) 32 km southeast of Nashville, Tennessee, in the southeastern United States. These measurements were conducted to (1) determine the absolute magnitude and variability of oxygenated compounds found in a contemporary rural region; (2) assess the importance of the measured ambient levels of OVOCs on a photochemical reactivity basis relative to the more commonly determined NMHCs; and (3) to evaluate our ability to accurately measure oxygenates by the current techniques employed under a field study scenario. Several other physical (temperature, insolation, etc.), meteorological (wind velocity, wind direction, atmospheric structure, and boundary layer height), and chemical (criterion pollutants, NO x , SO 2 , CO, O 3 , etc.) parameters were measured concurrently with the NMHC and OVOC measurements. During the study period, OVOCs were consistently the dominant compounds present, and methanol and acetone had the highest mixing ratios. Although OVOCs made up the majority of the volatile organic compound component on a mass basis, a substantial sink for OH was isoprene and its immediate oxidation products, methacrolein and methyl vinyl ketone. In combination with CO and formaldehyde, these compounds comprised about 85% of the observed OH reactivity at the site. Acetaldehyde and methanol were responsible for an additional 10%, with the NMHCs and remaining OVOCs making up the final 5% of the measured OH reactivity at the site. These observed patterns reinforce recent studies which find OVOCs to be an important component of the rural troposphere.

Recent measurements of reactive chemical species in snow and firn at polar sites have served to underscore the importance of air–snow transfer processes in understanding changes in atmospheric chemistry. In this paper we present the first quantitative assessment of the impact of physical processes in the snow on air–snow chemical exchange of ozone. Measurements of snow properties, interstitial ozone concentrations, and an ozone kinetic depletion experiment results are presented along with two-dimensional model results of the diffusion and ventilation processes affecting gas exchange at Alert, Nunavut, Canada. The Arctic snowpack at Alert will allow rapid exchange of gases with the atmosphere. Even under natural ventilation conditions with moderate winds, the entire pack is exposed to air movement and therefore available for chemical exchange processes. Both measurements and model results indicate that ozone undergoes rapid depletion in the top centimeters of the snow—approximately within the top 5 cm under diffusion alone, and in the top 10 cm or less during ventilation. Due to the higher permeability of the snowpack on the sea ice site as compared to the terrestrial site, it is possible that chemical exchange processes could be even more rapid over the sea ice in the greater Arctic than at the terrestrial site. A quantitative discussion of complications that arise from current firn air sampling techniques is presented and possible improvements for future measurements are suggested.

Isoprene and its oxidation products methyl vinyl ketone (MVK) and methacrolein (MACR) were measured over a 4 week period in July of 1995 at a rural/forest site near Nashville, Tennessee, as part of the 1995 Southern Oxidants Study (SOS) field intensive. High nighttime isoprene mixing ratios, measured during a 3 day period of stagnant high pressure, are reported. These high nighttime isoprene events are interpreted as a result of continuing emission of isoprene into a developing shallow nocturnal boundary layer in the early evening, followed by advective transport under the inversion to the measurement site. During some evenings, there is very rapid decay of isoprene just after sunset. These events occurred when the product [O 3 ]·[NO 2 ] was relatively large, consistent with loss via reaction with NO 3 . A chemical box model showed that isoprene decays were consistent with the NO 3 mechanism but only for relatively high NO x conditions. This study indicates that nighttime processing of isoprene can be important for forested regions susceptible to high‐NO x transport events. We also find that this nighttime NO 3 chemistry can lead to conditions where, at least at the surface, a significant fraction of the NO y is in the form of organic nitrates that are products of the NO 3 ‐isoprene reaction and that the NO 3 ‐isoprene reaction can be the dominant NO 3 sink.

Experiments were conducted during the ALERT 2000 field campaign aimed at understanding the role of air–snow interactions in carbonyl compound chemistry and the associated ozone depletion in the atmospheric boundary layer. Under sunlit conditions, we find that formaldehyde, acetaldehyde and acetone exhibit a significant diel cycle with average ambient air concentrations of 166, 53 and 385 ppt, respectively. A box model of Arctic surface layer chemistry was used to understand the diel behavior of carbonyl compound concentrations at Alert, Nunavut, Canada, with a focus on the chemical and physical processes that affect carbonyl compounds. Results of the study showed that the measured carbonyl compound concentrations can only be simulated when a radiation-dependent snowpack source term (possibly photochemistry) and a temperature-dependent sink (physical uptake on snow grains) of carbonyl compounds were added to the model. We are able to simulate the concentration and amplitude of the observed diel cycle, but not the phase of the cycle. These results help confirm the importance of snowpack chemistry and physical processes with respect to carbonyl compound concentrations in the Arctic surface boundary layer, and reveal weakness in the details of our understanding.

The oxidation of atmospheric alkenes by OH radicals results in small yields of β-hydroxy alkyl nitrates that can then provide a vehicle for the ultimate removal of NOx from the atmosphere. Although rainout may be an efficient mechanism for the removal of these species from the atmosphere, the Henry's law constants for these species are largely unknown. In this work, the Henry's law constants for β-hydroxy alkyl nitrates that are produced from the atmospheric oxidation of small alkenes in the presence of NO have been determined, over the temperature range 279−304 K. The compounds investigated were 2-nitrooxyethanol, 1-nitrooxy-2-propanol, 2-nitrooxy-1-propanol, 2-nitrooxy-3-butanol, 1-nitrooxy-2-butanol, and 2-nitrooxy-1-butanol. At 298 K, the measured Henry's law constants were 38 800, 10 900, 4500, 10 100, 5800, and 6 000 M/atm, respectively. From estimates of the rates of removal of these species from the lower troposphere by wet and dry deposition, OH radical reaction, and photolysis, we find that wet deposition accounts for between 26 and 60% of the removal rate, on average. Calculated atmospheric lifetimes for these species are all on the order of 2−3 days, which is long enough for long-range transport of these species to be possible. For hydroxy nitrates that retain a CC functionality, such as the isoprene nitrates, reaction with OH is expected to be more important than wet deposition.

Positive matrix factorization (PMF) was applied to air quality and temperature data collected as part of the Program for Research on Oxidants: Photochemistry, Emissions, and Transport 1997 summer measurement campaign. Unlike more conventional methods of factor analysis such as principal component analysis, PMF produces non-negative factors, aiding factor interpretation, and utilizes error estimates of the data matrix. This work uses PMF as a means of source identification and apportionment, important steps in the development of air pollution control strategies. Measurements of carbon monoxide, particulate matter, peroxyactyl nitrate (PAN), isoprene, temperature, and ozone were taken from a 31 m tower in rural northern Michigan and analyzed in this study. PMF resulted in three physically interpretable factors: an isoprene-dominated factor, a local source factor, and a long-range transport factor. The isoprene-dominated and local source factors exhibited strong and weak diurnal signals, respectively. Factor strengths for the long-range transport factor were relatively high during periods of south and southwesterly flow. The average contribution of the three factors was determined for each pollutant, enabling the modeled matrix to be compared to the data matrix. Good agreement between the fitted and data matrix was achieved for all parameters with the exception of coarse particulate matter. The PMF model explained at least 75% of variation for all species analyzed.

Acetaldehyde (CH3CHO) and acetone (CH3C(O)CH3) concentrations in ambient air, in snowpack air, and bulk snow were determined at Alert, Nunavut, Canada, as a part of the Polar Sunrise Experiment (PSE): ALERT 2000. During the period of continuous sunlight, vertical profiles of ambient and snowpack air exhibited large concentration gradients through the top ∼10 cm of the snowpack, implying a flux of carbonyl compounds from the surface to the atmosphere. From vertical profile and eddy diffusivity measurements made simultaneously on 22 April, acetaldehyde and acetone fluxes of 4.2(±2.1)×108 and 6.2(±4.2)×108 molecules cm−2 s−1 were derived, respectively. For this day, the sources and sinks of CH3CHO from gas phase chemistry were estimated. The result showed that the snowpack flux of CH3CHO to the atmosphere was as large as the calculated CH3CHO loss rate from known atmospheric gas phase reactions, and at least 40 times larger (in the surface layer) than the volumetric rate of acetaldehyde produced from the assumed main atmospheric gas phase reaction, i.e. reaction of ethane with hydroxyl radicals. In addition, acetaldehyde bulk snow phase measurements showed that acetaldehyde was produced in or on the snow phase, likely from a photochemical origin. The time series for the observed CH3C(O)CH3, ozone (O3), and propane during PSE 1995, PSE 1998, and ALERT 2000 showed a consistent anti-correlation between acetone and O3 and between acetone and propane. However, our data and model simulations showed that the acetone increase during ozone depletion events cannot be explained by gas phase chemistry involving propane oxidation. These results suggest that the snowpack is a significant source of acetaldehyde and acetone to the Arctic boundary layer.

In this paper we introduce results obtained from the Program for Research on Oxidants: PHotochemistry, Emissions, and Transport (PROPHET) program that is being conducted at the University of Michigan Biological Station in northern Michigan. PROPHET is an independent consortium of individually funded scientists whose mutual interests and varied experiences have created a synergistic collaboration focused on studies of atmospheric chemical and meteorological processes linked to tropospheric ozone. Since 1997, the PROPHET science team has combined expertise to characterize the important atmospheric issues in this region and to begin to push the limits of our knowledge of the links between the biosphere and the atmosphere. The opportunity to conduct research in the physical context of the Biological Station enables this team to interact with a tremendous range of activities related to forest and ecosystem health and uniquely positions PROPHET to make contributions to the emerging field of biosphere‐atmosphere interactions.

A series of measurements of PAN and ozone was conducted during summer at three rural sites in Canada: Egbert and Dorset, Ontario, and Kejimkujik, Nova Scotia. For nights when a stable surface inversion layer forms, ozone and PAN at the surface are found to undergo first order decay, assumed to be due only to dry deposition. Analysis of the measurement data leads to determination of the relative dry deposition velocities. For all three sites, we find that V d (O 3 )/V d (PAN) = 0.42±0.19, at night. This ratio is roughly a factor of 5–6 times smaller than previously assumed. This smaller relative deposition velocity ratio can have a significant impact on model estimations of PAN concentrations near the surface. We estimate that for these sites, the PAN deposition velocity is at least 0.5 cm/s, and may be greater during daytime. This can have a significant impact on the tropospheric lifetime of PAN.

The yields of hydroxy nitrates from the reaction of selected C2−C6 alkenes with OH in the presence of NO were measured at 296 ± 3 K in a 9600 L photochemical smog chamber. Hydroxyl radicals were produced from the photolysis of isopropyl nitrite in the presence of NO. The loss of the alkene was followed using gas chromatography. The hydroxy nitrate products were determined using a combination of capillary chromatography and an organic nitrate specific chemiluminescence detector. The yield of hydroxy nitrates was observed to increase with the size of the precursor alkene as follows: ethene (0.86%), propene (1.5%), 1-butene (2.5%), cis-2-butene (3.4%), and 1-hexene (5.5%). Previous studies involving the production of alkyl nitrates from alkanes show a similar trend, but the yields reported here are a factor of 2−3 lower than for the corresponding simple alkylperoxy radical. The impact of a β-hydroxy group on the nitrate yield is examined using an ab initio molecular orbital study. It indicates that a hydrogen-bonded peroxy nitrite intermediate is formed, which results in a decrease in D0(O−O) for the peroxy linkage of about 8 kJ/mol. This would be expected to effectively decrease the organic nitrate yield. The implications of these findings for the organic nitrate path as an atmospheric NOx removal mechanism are discussed.

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTPreparation, analysis, and atmospheric production of multifunctional organic nitratesKayambu Muthuramu, Paul B. Shepson, and Jason M. O'BrienCite this: Environ. Sci. Technol. 1993, 27, 6, 1117–1124Publication Date (Print):June 1, 1993Publication History Published online1 May 2002Published inissue 1 June 1993https://doi.org/10.1021/es00043a010RIGHTS & PERMISSIONSArticle Views318Altmetric-Citations73LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (982 KB) Get e-Alerts Get e-Alerts

Dissolved inorganic ions (NH 4 + , Ca 2+ , Mg 2+ , K + , H + , NO 3 − , and SO 4 2− ) and organic nitrogen (DON) were measured in cloud water samples collected over the northern lower peninsula of Michigan. Within a given cloud field, several altitudes were sampled to examine changes in concentration and speciation with altitude. Several samples were analyzed for bacterial content and activity. Convective cumulus (cumulus congestus) were more concentrated than fair weather cumulus (cumulus humilis) for all major ions and DON, with the cloudy air DON concentrations in convective cumulus being twice as large as for fair weather cumulus, and for all other ions, the droplets were 4–6 times more concentrated. The molar average distribution of nitrogen in the cloud water was 43 (±10, 1 σ )% ammonium, 39 (±7)% nitrate and 18 (±11)% DON. High concentrations of bacteria were observed in the clouds with an average concentration of 2.9 × 10 5 (±1.0 × 10 5 , 1 σ ) bacteria m −3 of cloudy air but which contributed less than 1% of the nitrogen in the cloud water. In addition, nitrifying bacteria were identified, indicating bacterial processing of nitrogen in the cloud water may occur. Air mass origin and altitude influence observed cloud water concentrations, with the exception of DON. The correlation of ammonium and sulfate, and calcium and nitrate suggest that ammonium sulfate and calcium nitrate aerosol may be important sources of these ions.

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTProduction of organic nitrates from hydroxide and nitrate reaction with propylenePaul B. Shepson, E. O. Edney, T. E. Kleindienst, J. H. Pittman, and G. R. NamieCite this: Environ. Sci. Technol. 1985, 19, 9, 849–854Publication Date (Print):September 1, 1985Publication History Published online1 May 2002Published inissue 1 September 1985https://doi.org/10.1021/es00139a014RIGHTS & PERMISSIONSArticle Views196Altmetric-Citations73LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (790 KB) Get e-Alerts Get e-Alerts

Here we present the first HONO vertical profiles in the atmospheric boundary layer (BL) and the lower free troposphere (FT) over a forested region in northern Michigan and the neighboring Great Lakes, measured from a small aircraft in summer of 2007. The HONO mixing ratios ranged from 4 to 17 pptv in the FT and from 8 to 74 pptv in the BL. The HONO distribution pattern was strongly influenced by the air column stability, i.e., strong negative HONO gradients existed in stable BL in the morning hours, whereas the HONO distribution was relatively uniform in the unstable and well‐mixed BL in the afternoons. The ground surface was a major source of HONO in the lower BL. The presence of substantial daytime HONO in the FT (∼8 pptv) and in the upper BL (25 pptv) suggests that a significant in situ source of HONO exists in the air column.

Abstract Although the ability to measure vertical eddy fluxes of gases from aircraft platforms represents an important capability to obtain spatially resolved data, accurate and reliable determination of the turbulent vertical velocity presents a great challenge. A nine-hole hemispherical probe known as the “Best Air Turbulence Probe” (often abbreviated as the “BAT Probe”) is frequently used in aircraft-based flux studies to sense the airflow angles and velocity relative to the aircraft. Instruments such as inertial navigation and global positioning systems allow the measured airflow to be converted into the three-dimensional wind velocity relative to the earth’s surface by taking into account the aircraft’s velocity and orientation. Calibration of the aircraft system has previously been performed primarily through in-flight experiments, where calibration coefficients were determined by performing various flight maneuvers. However, a rigorous test of the BAT Probe in a wind tunnel has not been previously undertaken. The authors summarize the results of a complement of low-speed wind tunnel tests and in-flight calibrations for the aircraft–BAT Probe combination. Two key factors are addressed in this paper: The first is the correction of systematic error arising from airflow measurements with a noncalibrated BAT Probe. The second is the instrumental precision in measuring the vertical component of wind from the integrated aircraft-based wind measurement system. The wind tunnel calibration allows one to ascertain the extent to which the BAT Probe airflow measurements depart from a commonly used theoretical potential flow model and to correct for systematic errors that would be present if only the potential flow model were used. The precision in the determined vertical winds was estimated by propagating the precision of the BAT Probe data (determined from the wind tunnel study) and the inertial measurement precision (determined from in-flight tests). The precision of the vertical wind measurement for spatial scales larger than approximately 2 m is independent of aircraft flight speed over the range of airspeeds studied, and the 1σ precision is approximately 0.03 m s−1.

Abstract. Systems have been developed and deployed at a North Michigan forested site to measure ambient HONO and vertical HONO flux. The modified HONO measurement technique is based on aqueous scrubbing of HONO using a coil sampler, followed by azo dye derivatization and detection using a long-path absorption photometer (LPAP). A Na2CO3-coated denuder is used to generate "zero HONO" air for background correction. The lower detection limit of the method, defined by 3 times of the standard deviation of the signal, is 1 pptv for 1-min averages, with an overall uncertainty of ±(1 + 0.05 [HONO]) pptv. The HONO flux measurement technique has been developed based on the relaxed eddy accumulation approach, deploying a 3-D sonic anemometer and two HONO measurement systems. The overall uncertainty is estimated to be within ±(8 × 10−8 + 0.15 FHONO) mol m−2 h−1, with a 20-min averaged data point per 30 min. Ambient HONO and vertical HONO flux were measured simultaneously at the PROPHET site from 17 July to 7 August 2008. The forest canopy was found to be a net HONO source, with a mean upward flux of 0.37 × 10−6 moles m−2 h−1. The HONO flux reached a maximal mean of ~0.7 × 10−6 moles m−2 h−1 around solar noon, contributing a major fraction to the HONO source strength required to sustain the observed ambient concentration of ~70 pptv. There were no significant correlations between [NOx] and daytime HONO flux and between JNO2 × [NO2] and HONO flux, suggesting that NOx was not an important precursor responsible for HONO daytime production on the forest canopy surface in this low-NOx rural environment. Evidence supports the hypothesis that photolysis of HNO3 deposited on the forest canopy surface is a major daytime HONO source.

The behavior of lower atmospheric ozone and ozone exchanges at the snow surface were studied using a suite of platforms during the Ocean‐Atmosphere‐Sea Ice‐Snow (OASIS) Spring 2009 experiment at an inland, coastal site east of Barrow, Alaska. A major objective was to investigate if and how much chemistry at the snow surface at the site contributes to springtime ozone depletion events (ODEs). Between March 8 and April 16, seven ODEs, with atmospheric ozone dropping below 1.0 ppbv, were observed. The depth of the ozone‐depleted layer was variable, extending from the surface to ∼200–800 m. ODEs most commonly occurred during low wind speed conditions with flow coming from the Arctic Ocean. Two high‐sensitivity ozone chemiluminescence instruments were used to accurately define the remaining sub‐ppbv ozone levels during ODEs. These measurements showed variable residual ODE ozone levels ranging between 0.010 and 0.100 ppbv. During the most extended ODE, when ozone remained below 1.0 ppbv for over 78 h, these measurements showed a modest ozone recovery or production in the early afternoon hours, resulting in increases in the ozone mixing ratio of 0.100 to 0.800 ppbv. The comparison between high‐sensitivity ozone measurements and BrO measured by longpath differential absorption spectroscopy (DOAS) during ODEs indicated that at low ozone levels formation of BrO is controlled by the amount of available ozone. Measurements of ozone in air drawn from below the snow surface showed depleted ozone in the snowpack, with levels consistently remaining &lt;6 ppbv independent of above‐surface ambient air concentrations. The snowpack was always a sink of ozone. Ozone deposition velocities determined from ozone surface flux measurements by eddy covariance were on the order of 0.01 cm s −1 , which is of similar magnitude as ozone uptake rates found over snow at other polar sites that are not subjected to ODEs. The results from these multiple platform measurements unequivocally show that snow‐atmosphere chemical exchanges of ozone at the measurement site do not exhibit a major contribution to ozone removal from the boundary layer and the formation of ODE.

Flaring to dispose of natural gas has increased in the United States and is typically assumed to be 98% efficient, accounting for both incomplete combustion and venting during unintentional flame termination. However, no in situ measurements of flare emissions have been reported. We used an aircraft platform to sample 10 flares in North Dakota and 1 flare in Pennsylvania, measuring CO2, CH4, and meteorological data. Destruction removal efficiency (DRE) was calculated by assuming a flare natural gas input composition of 60-100% CH4. In all cases flares were >99.80 efficient at the 25% quartile. Crosswinds up to 15 m/s were observed, but did not significantly adversely affect efficiency. During analysis unidentified peaks of CH4, most likely from unknown venting practices, appeared much larger in magnitude than emissions from flaring practices. Our analysis suggests 98% efficiency for nonsputtering flares is a conservative estimate for incomplete combustion and that the unidentified venting is a greater contributor to CH4 emissions.

Cloudwater and below-cloud atmospheric particle samples were collected onboard a research aircraft during the Southern Oxidant and Aerosol Study (SOAS) over a forested region of Alabama in June 2013. The organic molecular composition of the samples was studied to gain insights into the aqueous-phase processing of organic compounds within cloud droplets. High resolution mass spectrometry (HRMS) with nanospray desorption electrospray ionization (nano-DESI) and direct infusion electrospray ionization (ESI) were utilized to compare the organic composition of the particle and cloudwater samples, respectively. Isoprene and monoterpene-derived organosulfates and oligomers were identified in both the particles and cloudwater, showing the significant influence of biogenic volatile organic compound oxidation above the forested region. While the average O:C ratios of the organic compounds were similar between the atmospheric particle and cloudwater samples, the chemical composition of these samples was quite different. Specifically, hydrolysis of organosulfates and formation of nitrogen-containing compounds were observed for the cloudwater when compared to the atmospheric particle samples, demonstrating that cloud processing changes the composition of organic aerosol.

Secondary organic aerosol formation via aqueous processing, particularly from the oxidation of biogenic volatile organic compounds, is hypothesized to contribute significantly to the global aerosol burden. In this study, electrospray ionization coupled with mass spectrometry (ESI-MS) was utilized to detect organosulfates and oligomers in cloud water collected in July above the Missouri Ozarks, an environment significantly influenced by isoprene oxidation. Community Multiscale Air Quality (CMAQ) modeling suggested that the aerosol at cloud height was characterized by high water, sulfate, and biogenic secondary organic aerosol content, conducive to aqueous-phase processing and organosulfate formation. CMAQ modeling also suggested the presence of gas-phase organic peroxides and nitrates, which can partition into the particle-phase and form organosulfates. Several potential organosulfates from isoprene, monoterpene, and sesquiterpene oxidation were detected in the cloud water. In particular, the ubiquitous organosulfate C5H12O7S (detected by ESI-MS at m/z −215), derived from isoprene epoxydiols, was detected. These results highlight the role of aqueous-phase reactions in biogenic SOA formation and cloud processes in isoprene oxidation-influenced regions.

Abstract. The return of sunlight in the polar spring leads to the production of reactive halogen species from the surface snowpack, significantly altering the chemical composition of the Arctic near-surface atmosphere and the fate of long-range transported pollutants, including mercury. Recent work has shown the initial production of reactive bromine at the Arctic surface snowpack; however, we have limited knowledge of the vertical extent of this chemistry, as well as the lifetime and possible transport of reactive bromine aloft. Here, we present bromine monoxide (BrO) and aerosol particle measurements obtained during the March 2012 BRomine Ozone Mercury EXperiment (BROMEX) near Utqiaġvik (Barrow), AK. The airborne differential optical absorption spectroscopy (DOAS) measurements provided an unprecedented level of spatial resolution, over 2 orders of magnitude greater than satellite observations and with vertical resolution unable to be achieved by satellite methods, for BrO in the Arctic. This novel method provided quantitative identification of a BrO plume, between 500 m and 1 km aloft, moving at the speed of the air mass. Concurrent aerosol particle measurements suggest that this lofted reactive bromine plume was transported and maintained at elevated levels through heterogeneous reactions on colocated supermicron aerosol particles, independent of surface snowpack bromine chemistry. This chemical transport mechanism explains the large spatial extents often observed for reactive bromine chemistry, which impacts atmospheric composition and pollutant fate across the Arctic region, beyond areas of initial snowpack halogen production. The possibility of BrO enhancements disconnected from the surface potentially contributes to sustaining BrO in the free troposphere and must also be considered in the interpretation of satellite BrO column observations, particularly in the context of the rapidly changing Arctic sea ice and snowpack.

Atmospheric bromine and chlorine atoms have a significant influence on the pathways of atmospheric chemical species processing. The photolysis of molecular halogens and subsequent reactions with ozone, mercury, and hydrocarbons are common occurrences in the Arctic boundary layer during spring, following polar sunrise. While it was recently determined that Br2 is released from the sunlit surface snowpack, the source(s) and mechanisms of Cl2 and BrCl production have remained unknown. Current efforts to model Arctic atmospheric composition are limited by the lack of knowledge of the sources and emission rates of these species. Here, we present the first simultaneous direct measurements of Br2, Cl2, and BrCl in snowpack interstitial air, as well as the first measured emission rates of Br2 and Cl2 out of the snowpack into the atmosphere. Using chemical ionization mass spectrometry, Br2, Cl2, and BrCl were observed to be produced within the tundra surface snowpack near Utqiaġvik, AK, during Feb 2014, following both artificial and natural irradiation, consistent with a photolytic production mechanism. Maximum Cl2 and Br2 fluxes from the snowpack to the overlying atmosphere were quantified and reached maxima at mid-day during peak radiation. In-snowpack Br2 and BrCl production was enhanced, with Cl2 production reduced, at air temperatures below the eutectic point for the formation of NaCl·2H2O, suggesting limited chloride availability, as compared to production at air temperatures above this eutectic point. These new observations improves the ability of the community to simulate Arctic boundary layer composition and pollutant fate.

Abstract A Halo Photonics Stream Line XR Doppler lidar has been deployed for the Indianapolis Flux Experiment (INFLUX) to measure profiles of the mean horizontal wind and the mixing layer height for quantification of greenhouse gas emissions from the urban area. To measure the mixing layer height continuously and autonomously, a novel composite fuzzy logic approach has been developed that combines information from various scan types, including conical and vertical-slice scans and zenith stares, to determine a unified measurement of the mixing height and its uncertainty. The composite approach uses the strengths of each measurement strategy to overcome the limitations of others so that a complete representation of turbulent mixing is made in the lowest km, depending on clouds and aerosol distribution. Additionally, submeso nonturbulent motions are identified from zenith stares and removed from the analysis, as these motions can lead to an overestimate of the mixing height. The mixing height is compared with in situ profile measurements from a research aircraft for validation. To demonstrate the utility of the measurements, statistics of the mixing height and its diurnal and annual variability for 2016 are also presented. The annual cycle is clearly captured, with the largest and smallest afternoon mixing heights observed at the summer and winter solstices, respectively. The diurnal cycle of the mixing layer is affected by the mean wind, growing slower in the morning and decaying more rapidly in the evening with lighter winds.

Methane emissions from oil and gas facilities can exhibit operation-dependent temporal variability; however, this variability has yet to be fully characterized. A field campaign was conducted in June 2014 in the Eagle Ford basin, Texas, to examine spatiotemporal variability of methane emissions using four methods. Clusters of methane-emitting sources were estimated from 14 aerial surveys of two (“East” or “West”) 35 × 35 km grids, two aircraft-based mass balance methods measured emissions repeatedly at five gathering facilities and three flares, and emitting equipment source-types were identified via helicopter-based infrared camera at 13 production and gathering facilities. Significant daily variability was observed in the location, number (East: 44 ± 20% relative standard deviation (RSD), N = 7; West: 37 ± 30% RSD, N = 7), and emission rates (36% of repeat measurements deviate from mean emissions by at least ±50%) of clusters of emitting sources. Emission rates of high emitters varied from 150–250 to 880–1470 kg/h and regional aggregate emissions of large sources (>15 kg/h) varied up to a factor of ∼3 between surveys. The aircraft-based mass balance results revealed comparable variability. Equipment source-type changed between surveys and alterations in operational-mode significantly influenced emissions. Results indicate that understanding temporal emission variability will promote improved mitigation strategies and additional analysis is needed to fully characterize its causes.

Organic aerosol (OA) is a complex mixture of compounds with diverse elemental and structural features, and its composition affects its health and environmental impacts. A detailed speciation of the functional group distribution in OA is important for constraining atmospheric reaction pathways and products, evaluating chemical mechanisms and models, and understanding OA impacts. We used high-resolution tandem mass spectrometry to perform a nontargeted analysis of OA functional groups from three diverse ambient sites across times of day and seasons. We observed a range of oxygen-, nitrogen-, and/or sulfur-containing functional groups, including oxygenates such as hydroxyls (29–69%) and carboxylic acids (19–59%), that dominated the functional group distribution and that may participate in hydrogen bonding and thus impact the chemical and physical properties of OA (percentages indicate average ion abundance contributions across campaigns). We also observed esters (7–39%) and ethers (13–42%) that suggest the importance of oligomerization. On average, organonitrates represented only 12% of identified nitrogen-containing groups and organosulfates represented 21% of identified sulfur-containing groups, while we observed many other nitrogen- and/or sulfur-containing structures that were important contributors to OA composition (e.g., amines, imines, nitrophenols, and sulfides). Most compounds (81%) were multifunctional and likely multigenerational oxidation products, which typically contained two to five functional groups in total.

Gas‐phase formaldehyde (HCHO) was measured at a mixed deciduous/coniferous forest site as a part of the PROPHET 1998 summer field intensive. For the measurement period of July 11 through August 20, 1998, formaldehyde mixing ratios ranged from 0.5 to 12 ppb at a height ∼10 m above the forest canopy, with the highest concentrations observed in southeasterly air masses. Concentrations varied on average from a mid‐afternoon maximum influenced by photochemical production of 4.0 ppb, to a late night minimum of 2.2 ppb, probably resulting from dry depositional loss. An analysis of local HCHO sources revealed that isoprene was the most important of the measured formaldehyde precursors, contributing, on average, 82% of the calculated midday HCHO production rate. We calculate that the nighttime HCHO dry deposition velocity is 2.6 times that of ozone, or approximately 0.65 cm/s. In the daytime, photolysis, dry deposition, and reaction with hydroxyl radical (OH) are roughly equally important as loss processes. Explicit calculations of HCHO chemical behavior highlighted the probable importance of transport and surface deposition to understanding the diel behavior of formaldehyde.

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTLaboratory and Field Investigation of the DNPH Cartridge Technique for the Measurement of Atmospheric Carbonyl CompoundsAnna-Pearl. Sirju and Paul B. ShepsonCite this: Environ. Sci. Technol. 1995, 29, 2, 384–392Publication Date (Print):February 1, 1995Publication History Published online1 May 2002Published inissue 1 February 1995https://doi.org/10.1021/es00002a014RIGHTS & PERMISSIONSArticle Views501Altmetric-Citations67LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (2 MB) Get e-Alerts Get e-Alerts

Oxygenated hydrocarbons, including for the first time alcohols, in the atmosphere and snow-pack interstitial air were measured at Alert, Nunavut, Canada from 15 February to 5 May 2000. Unexpectedly high concentrations of oxygenated hydrocarbons were observed. Acetone, acetaldehyde and methanol represent about 90% of all oxygenated hydrocarbons measured in this work, and together with formaldehyde their total concentration was higher than the sum of measured NMHCs. During sunlit hours, concentrations in the snow-pack interstitial air were higher than those measured in the gas-phase, implying a positive flux from the snow-pack to the Arctic boundary layer. Fluxes of acetaldehyde, acetone and methanol at that time were estimated to be 26, 7.5 and 3.2×108 molecules cm−2 s−1, respectively. These rates would deplete the local snow of acetaldehyde and acetone in about 2 days if degassing was driving the flux. Additional evidence suggests that photochemical production in the snow-pack could explain these fluxes, especially for acetaldehyde. Diel variations were observed at Alert after polar sunrise in the snow-pack interstitial air and in ambient air. During decreasing O3 conditions, positive correlation with acetaldehyde was observed which is interpreted as implying local Br driven chemistry, but acetone mixing ratios showed a strong negative correlation.

Measurements of alkyl and multifunctional organic nitrates were conducted at a rural site in Ontario during periods with varying levels of photochemical activity. On August 6 and August 21–23, 1992, a total of 17 organic nitrates (12 C 3 ‐C 6 alkyl nitrates, 4 C 2 ‐C 4 hydroxynitrates and 1,2‐dinitrooxybutane) were quantitatively determined in atmospheric samples. The sum of the organic nitrate concentrations was found to be correlated with ozone and ranged from 12 to 140 parts per trillion (volume). The total concentration of organic nitrates measured contributed from 0.5 to 3.0% to the total odd nitrogen species. On average, the alkyl nitrates represented 82%, the hydroxynitrates 16%, and the dinitrate 2% of the total measured organic nitrates. Unidentified organic nitrate peaks determined from an organic nitrate selective detector were found to contribute an additional 0.25% to the total odd nitrogen budget. The distribution of alkyl and hydroxy nitrates measured was found to be reasonably consistent with computed relative production rates, for photochemically active air masses. Although the inclusion of the multifunctional organic nitrates does not significantly change the measured contribution of these species to NO y , it is shown that the isoprene nitrates may contribute as much as the combined contribution of all the measured organic nitrates.

During photochemical air pollution episodes in the Lower Fraser Valley (LFV) near Vancouver, BC, daytime upvalley flows carried polluted air, with high ozone (03) concentrations, into tributary valleys to the north of the LFV. Nighttime flows out of the valleys had low 03 concentrations, according to surface measurements, and also had low aerosol concentrations, as measured by a scanning Doppler lidar. Analysis of lidar scan data showed that the flows were highly complex, that the relatively clean flow was confined to the lower levels (lowest ∼ 500 m) of the valley, and that regions of strongest outflow were also the regions of “cleanest” air. Measurements of NO2 concentrations well above background levels in the outflow indicate that it was formerly polluted air from which 03 and aerosols had been removed. Possible removal mechanisms were found to be dry deposition in the katabatic (downslope) flows down the valley sidewalls, in agreement with a previous study in a Swiss valley, or fast chemical reactions with NO and N03. Nearly horizontal lidar scans showed that the valley exit flows penetrated into the LFV, where they merged with the downvalley/land-breeze system along the Fraser River.

The isoprene atmospheric oxidation products methyl vinyl ketone (MVK) and methacrolein (MACR), along with CO, were measured in Toronto, Ontario in the winter and summer of 1996. These carbonyl compounds were highly correlated with CO in the winter, indicating that they are produced in significant amounts by automotive sources. Regression of the observed MVK and MACR concentrations against CO leads to emission factors relative to CO of 1.4 (±0.3) × 10 −4 and 7.3 (±1.6) × 10 −5 (mole/mole), respectively. Emission inventories for CO and isoprene allow us to estimate that, for Toronto in the summer, as much as 48 (±30)% of the MVK and 36 (±23)% of the MACR input into the atmosphere is derived from mobile source emissions. This source also has a strong influence on the MVK/MACR ratio, which in this environment remains relatively constant, with values typically ranging between 2.0–2.5. The impact of this is that MVK and MACR mixing ratios are not unambiguous indicators for isoprene chemistry and its impact on ozone production for urban environments.

Measurements of HCHO, CH 3 CHO, and CH 3 C(O)CH 3 were made at the Narwhal ice floe camp in the Lincoln Sea at 84°N latitude from April 10 to 24, 1994. During the period April 13 to 18, O 3 was below the detection limit of the measurement (i.e., &lt;1 ppb), and the average HCHO, CH 3 CHO, and CH 3 C(O)CH 3 concentrations were 193, 93, and 1730 ppt, respectively. A box model of the chemistry involved in the surface O 3 depletion shows that the majority of BrO x termination reactions occur via Br atom reaction with the aldehydes. The reaction of Br atoms with CH 3 CHO is shown to be very effective in removing NO x from the Arctic marine boundary layer (MBL), via formation of peroxyacetyl nitrate (PAN). This denitrification of the surface layer has a significant impact on the radical chemistry. In particular, the model indicates that the observed levels of HCHO and CH 3 CHO cannot be reproduced if, as discussed in recent reports of Arctic ozone chemistry at sunrise, both Br atom and Cl atom chemistry occur simultaneously (at estimated concentrations of 1 × 10 4 and 1 × 10 7 atoms/cm 3 , respectively). However, if only chlorine atoms are present (at 1 × 10 4 atoms/cm 3 ), reasonable steady state CH 3 CHO levels (∼80 ppt), but rather low HCHO levels (∼50 ppt) are produced. The model HCHO levels for chlorine‐atom‐only chemistry are as much as a factor of 10 lower than those observed (by these authors and others) in the Arctic MBL at sunrise. Model simulations show that the ratio CH 3 C(O)CH 3 /C 2 H 5 CHO could be a useful indicator of the relative importance of Br atom and Cl atom chemistry.