Publications

Seasonal variation of the transport of black carbon aerosol from the Asian continent to the Arctic during the ARCTAS aircraft campaign

Extensive measurements of black carbon (BC) aerosol were conducted in and near the North American Arctic during the Arctic Research of the Composition of the Troposphere from Aircraft and Satellites (ARCTAS) aircraft campaign in April and June-July 2008. We identify the pathways and mechanisms of transport of BC to the Arctic from the Asian continent using these data. The concentration, transport efficiency, and measured altitude of BC over the North American Arctic were highly dependent on season and origin of air parcels, e.g., biomass burning (BB) in Russia (Russian BB) and anthropogenic (AN) in East Asia (Asian AN). Russian BB air was mainly measured in the middle troposphere and caused maximum BC concentrations at this altitude in spring. The median BC concentration and transport efficiency of the Russian BB air were 270 ng m -3 (at STP) and 80% in spring and 20 ng m-3 and 4% in summer, respectively. Asian AN air was measured most frequently in the upper troposphere, with median values of 20 ng m-3 and 13% in spring and 5 ng m-3 and 0.8% in summer. These distinct differences are explained by differences in the transport mechanisms and accumulated precipitation along trajectories (APT), which is a measure of wet removal processes during transport. The transport of Russian BB air to the Arctic was nearly isentropic with slow ascent (low APT), while Asian AN air underwent strong uplift associated with warm conveyor belts (high APT). The APT values in summer were much larger than those in spring due to the increase in humidity in summer. These results show that the impact of BC emitted from AN sources in East Asia on the Arctic was very limited in both spring and summer. The BB emissions in Russia in spring are demonstrated to be the most important sources of BC transported to the North American Arctic. Copyright 2011 by the American Geophysical Union.

Comment on acp-2021-782

<strong class="journal-contentHeaderColor">Abstract.</strong> <span id="page1550"/>Aerosol indirect radiative forcing (IRF), which characterizes how aerosols alter cloud formation and properties, is very sensitive to the preindustrial (PI) aerosol burden. Dimethyl sulfide (DMS), emitted from the ocean, is a dominant natural precursor of non-sea-salt sulfate in the PI and pristine present-day (PD) atmospheres. Here we revisit the atmospheric oxidation chemistry of DMS, particularly under pristine conditions, and its impact on aerosol IRF. Based on previous laboratory studies, we expand the simplified DMS oxidation scheme used in the Community Atmospheric Model version 6 with chemistry (CAM6-chem) to capture the OH-addition pathway and the H-abstraction pathway and the associated isomerization branch. These additional oxidation channels of DMS produce several stable intermediate compounds, e.g., methanesulfonic acid (MSA) and hydroperoxymethyl thioformate (HPMTF), delay the formation of sulfate, and, hence, alter the spatial distribution of sulfate aerosol and radiative impacts. The expanded scheme improves the agreement between modeled and observed concentrations of DMS, MSA, HPMTF, and sulfate over most marine regions, based on the NASA Atmospheric Tomography (ATom), the Aerosol and Cloud Experiments in the Eastern North Atlantic (ACE-ENA), and the Variability of the American Monsoon Systems (VAMOS) Ocean-Cloud-Atmosphere-Land Study Regional Experiment (VOCALS-REx) measurements. We find that the global HPMTF burden and the burden of sulfate produced from DMS oxidation are relatively insensitive to the assumed isomerization rate, but the burden of HPMTF is very sensitive to a potential additional cloud loss. We find that global sulfate burden under PI and PD emissions increase to 412 Gg S (<span class="inline-formula">+</span>29 %) and 582 Gg S (<span class="inline-formula">+</span>8.8 %), respectively, compared to the standard simplified DMS oxidation scheme. The resulting annual mean global PD direct radiative effect of DMS-derived sulfate alone is <span class="inline-formula">−</span>0.11 W m<span class="inline-formula">^−2</span>. The enhanced PI sulfate produced via the gas-phase chemistry updates alone dampens the aerosol IRF as anticipated (<span class="inline-formula">−</span>2.2 W m<span class="inline-formula">^−2</span> in standard versus <span class="inline-formula">−</span>1.7 W m<span class="inline-formula">^−2</span>, with updated gas-phase chemistry). However, high clouds in the tropics and low clouds in the Southern Ocean appear particularly sensitive to the additional aqueous-phase pathways, counteracting this change (<span class="inline-formula">−</span>2.3 W m<span class="inline-formula">^−2</span>). This study confirms the sensitivity of aerosol IRF to the PI aerosol loading and the need to better understand the processes controlling aerosol formation in the PI atmosphere and the cloud response to these changes.

Supplementary material to "Heterogeneity and Chemical Reactivity of the Remote Troposphere defined by Aircraft Measurements"

The ATom mission was designed to collect a multi-species, detailed chemical climatology that documents the patterns of physical and chemical heterogeneity throughout the remote troposphere. The work here requires a complete set of key species in each air parcel to initialize global 3D chemistry models to be able to calculate the CH4 and O3 reactivities over a 24 hour cycle. The ATom Modeling Data Stream (MDS) provides a semi-continuous set of 10 s air parcels with a full set of values for the key chemical reactants and conditions. We choose 10 s averages for our air parcels as a compromise to include most of the instruments, and because the 10 s merged data is a standard product Some of our core species are measured with gas chromatographs or flask samples with longer integrations times (30-90 sec), but these can be mapped onto the 10 s parcels with loss of the higher frequency variability found in the 10 s measurements. The frequent profiling of the DC-8 gives us both vertical and horizontal scales: the vertical extent of a 10 s parcel is 50 -110 m (55%-95% of all parcels, with <50% having near level flight) and the horizontal extent is typically 1.4 -2.5 km (10%-90% of all parcels). ATom completed its four deployments: ATom-1 starting 20160729 (YYYMMDD), ATom-2 starting 20170126, ATom-3 starting 20170928, and ATom-4 starting 20180424. ATom targets the remote troposphere by sampling over the middle of the Pacific and Atlantic Ocean basins. The DC-8 aircraft performed in situ profiling of the atmosphere from 0.2 km to 12 km along each flight segment as often as possible. Each deployment lasted about 4 weeks and contained 11 to 13 research flights (RF). Figure For convenience, we designate the RF across the 4 deployments as ATom flights (AF) 1 through 48. The MDS data reported here consisit of 149,133 air parcels over 4 deployments with a total of 48 research flights. AF 46 is a short ferry flight from Kangerlussuaq, Greenland to Bangor, Maine with many instruments turned off and no profiling, thus these 1,106 parcels contain only flight data (MDS variables 1:11) and no chemical data. ATom sampling of the troposphere is more uniform than most aircraft missions, but still contains some biases that can be adjusted by weighting each air parcel. Due to the typical profiling sequence (level at cruising attitude for 10 min, descent for 20 min, level flight about 160 m above the sea level for 5 min, and a 20-min climb back to cruising altitude) and to the occasional requirements of weather or air traffic control, the sampling is skewed towards the uppermost troposphere (P < 300 hPa) and, secondly, the marine boundary layer. We designate a weight for each MDS air parcel to achieve a more uniform sampling of the troposphere by mass: data are binned into 100 hPa-wide pressure bins and 10-wide latitude bins, and each point is assigned a weight equal to the inverse of the number of points in the bin times cosine of the latitude. There are very few measurements for pressures <200 hPa and so these points are included in the uppermost 200-300 hPa bin. This ATom-1 analysis has three study domains: Global includes all parcels (32,383) weighted as above; Pacific considers all measurements (11,486) over the Pacific Ocean from 54S to 60N (research flights RF 1,3,4,5,6); and the Atlantic,

Bacteria in the airways of patients with cystic fibrosis are genetically capable of producing VOCs in breath

Breath contains hundreds of volatile organic compounds (VOCs), the composition of which is altered in a wide variety of diseases. Bacteria are implicated in the formation of VOCs, but the biochemical mechanisms that lead to the formation of breath VOCs remain largely hypothetical. We hypothesized that bacterial DNA fragments in sputum of CF patients could be sequenced to identify whether the bacteria present were capable of producing VOCs found in the breath of these patients. Breath from seven patients with cystic fibrosis was sampled and analyzed by gas-chromatography and mass-spectrometry. Sputum samples were also collected and microbial DNA was isolated. Metagenomic sequencing was performed and the DNA fragments were compared to a reference database with genes that are linked to the metabolism of acetaldehyde, ethanol and methanol in the KEGG database. Bacteria in the genera Escherichia, Lactococcus, Pseudomonas, Rothia and Streptococcus were found to have the genetic potential to produce acetaldehyde and ethanol. Only DNA sequences from Lactococcus were implicated in the formation of acetaldehyde from acetate through aldehyde dehydrogenase family 9 member A1 (K00149). Escherichia was found to be genetically capable of producing ethanol in all patients, whilst there was considerable heterogeneity between patients for the other genera. The ethanol concentration in breath positively correlated with the amount of Escherichia found in sputum (Spearman rho = 0.85, P = 0.015). Rothia showed the most versatile genetic potential for producing methanol. To conclude, bacterial DNA fragments in sputum of CF patients can be linked to enzymes implicated in the production of ethanol, acetaldehyde and methanol, which are VOCs that are predictive of respiratory tract colonization and/or infection. This supports that the lung microbiome can produce VOCs directly.

Large‐scale ozone and aerosol distributions, air mass characteristics, and ozone fluxes over the western Pacific Ocean in late winter/early spring

Large‐scale measurements of ozone (O 3 ) and aerosol distributions were made from the NASA DC‐8 aircraft during the Transport and Chemical Evolution over the Pacific (TRACE‐P) field experiment conducted in February–April 2001. Remote measurements were made with an airborne lidar to provide O 3 and multiple‐wavelength aerosol backscatter profiles from near the surface to above the tropopause along the flight track. In situ measurements of O 3 , aerosols, and a wide range of trace gases were made onboard the DC‐8. Five‐day backward trajectories were used in conjunction with the O 3 and aerosol distributions on each flight to indicate the possible origin of observed air masses, such as from biomass burning regions, continental pollution, desert regions, and oceanic regions. Average latitudinal O 3 and aerosol scattering ratio distributions were derived from all flights west of 150°E, and these distributions showed the average latitude and altitude dependence of different dynamical and chemical processes in determining the atmospheric composition over the western Pacific. TRACE‐P (TP) showed an increase in the average latitudinal distributions of both O 3 and aerosols compared to PEM‐West B (PWB), which was conducted in February–March 1994. O 3 , aerosol, and potential vorticity levels were used to identify nine air mass types and quantify their frequency of occurrence as a function of altitude. This paper discusses the characteristics of the different air mass types encountered during TP and compares them to PWB. These results confirmed that most of the O 3 increase in TP was due to photochemistry. The average latitudinal eastward O 3 flux in the western Pacific during TP was found to peak near 32°N with a total average O 3 flux between 14 and 46°N of 5.2 Tg/day. The eastward total CO flux was calculated to be 2.2 Tg‐C/day with ∼6% estimated from Asia. The Asian flux of CO 2 and CH 4 was estimated at 4.9 and 0.06 Tg‐C/day.

The contribution of oceanic methyl iodide to stratospheric iodine

Abstract. We investigate the contribution of oceanic methyl iodide (CH3I) to the stratospheric iodine budget. Based on CH3I measurements from three tropical ship campaigns and the Lagrangian transport model FLEXPART, we provide a detailed analysis of CH3I transport from the ocean surface to the cold point in the upper tropical tropopause layer (TTL). While average oceanic emissions differ by less than 50% from campaign to campaign, the measurements show much stronger variations within each campaign. A positive correlation between the oceanic CH3I emissions and the efficiency of CH3I troposphere–stratosphere transport has been identified for some cruise sections. The mechanism of strong horizontal surface winds triggering large emissions on the one hand and being associated with tropical convective systems, such as developing typhoons, on the other hand, could explain the identified correlations. As a result of the simultaneous occurrence of large CH3I emissions and strong vertical uplift, localized maximum mixing ratios of 0.6 ppt CH3I at the cold point have been determined for observed peak emissions during the SHIVA (Stratospheric Ozone: Halogen Impacts in a Varying Atmosphere)-Sonne research vessel campaign in the coastal western Pacific. The other two campaigns give considerably smaller maxima of 0.1 ppt CH3I in the open western Pacific and 0.03 ppt in the coastal eastern Atlantic. In order to assess the representativeness of the large local mixing ratios, we use climatological emission scenarios to derive global upper air estimates of CH3I abundances. The model results are compared with available upper air measurements, including data from the recent ATTREX and HIPPO2 aircraft campaigns. In the eastern Pacific region, the location of the available measurement campaigns in the upper TTL, the comparisons give a good agreement, indicating that around 0.01 to 0.02 ppt of CH3I enter the stratosphere. However, other tropical regions that are subject to stronger convective activity show larger CH3I entrainment, e.g., 0.08 ppt in the western Pacific. Overall our model results give a tropical contribution of 0.04 ppt CH3I to the stratospheric iodine budget. The strong variations in the geographical distribution of CH3I entrainment suggest that currently available upper air measurements are not representative of global estimates and further campaigns will be necessary in order to better understand the CH3I contribution to stratospheric iodine.

New insight into the spatiotemporal variability and source apportionments ofC ₁ –C ₄ alkyl nitrates in Hong Kong

Abstract. C1–C4 alkyl nitrates (RONO2) were measured concurrently at a mountain site, Tai Mo Shan (TMS), and an urban site, Tsuen Wan (TW), at the base of the same mountain in Hong Kong from September to November 2010. Although the levels of parent hydrocarbons were much lower at TMS (p &lt; 0.05), similar alkyl nitrate levels were found at both sites regardless of the elevation difference, suggesting various source contributions of alkyl nitrates at the two sites. Prior to using a positive matrix factorization (PMF) model, the data at TW were divided into "meso" and "non-meso" scenarios for the investigation of source apportionments with the influence of mesoscale circulation and regional transport, respectively. Secondary formation was the prominent contributor of alkyl nitrates in the meso scenario (60 ± 2 %, 60.2 ± 1.2 pptv), followed by biomass burning and oceanic emissions, while biomass burning and secondary formation made comparable contributions to alkyl nitrates in the non-meso scenario, highlighting the strong emissions of biomass burning in the inland Pearl River delta (PRD) region. In contrast to TW, the alkyl nitrate levels measured at TMS mainly resulted from the photooxidation of the parent hydrocarbons at TW during mesoscale circulation, i.e., valley breezes, corresponding to 52–86 % of the alkyl nitrate levels at TMS. Furthermore, regional transport from the inland PRD region made significant contributions to the levels of alkyl nitrates (∼ 58–82 %) at TMS in the non-meso scenario, resulting in similar levels of alkyl nitrates observed at the two sites. The simulation of secondary formation pathways using a photochemical box model found that the reaction of alkyl peroxy radicals (RO2) with nitric oxide (NO) dominated the formation of RONO2 at both sites, and the formation of alkyl nitrates contributed negatively to O3 production, with average reduction rates of 4.1 and 4.7 pptv pptv−1 at TMS and TW, respectively.

Heterogeneity and chemical reactivity of the remote troposphere defined by aircraft measurements

Abstract. The NASA Atmospheric Tomography (ATom) mission built a photochemical climatology of air parcels based on in situ measurements with the NASA DC-8 aircraft along objectively planned profiling transects through the middle of the Pacific and Atlantic oceans. In this paper we present and analyze a data set of 10 s (2 km) merged and gap-filled observations of the key reactive species driving the chemical budgets of O3 and CH4 (O3, CH4, CO, H2O, HCHO, H2O2, CH3OOH, C2H6, higher alkanes, alkenes, aromatics, NOx, HNO3, HNO4, peroxyacetyl nitrate, other organic nitrates), consisting of 146 494 distinct air parcels from ATom deployments 1 through 4. Six models calculated the O3 and CH4 photochemical tendencies from this modeling data stream for ATom 1. We find that 80 %–90 % of the total reactivity lies in the top 50 % of the parcels and 25 %–35 % in the top 10 %, supporting previous model-only studies that tropospheric chemistry is driven by a fraction of all the air. In other words, accurate simulation of the least reactive 50 % of the troposphere is unimportant for global budgets. Surprisingly, the probability densities of species and reactivities averaged on a model scale (100 km) differ only slightly from the 2 km ATom data, indicating that much of the heterogeneity in tropospheric chemistry can be captured with current global chemistry models. Comparing the ATom reactivities over the tropical oceans with climatological statistics from six global chemistry models, we find excellent agreement with the loss of O3 and CH4 but sharp disagreement with production of O3. The models sharply underestimate O3 production below 4 km in both Pacific and Atlantic basins, and this can be traced to lower NOx levels than observed. Attaching photochemical reactivities to measurements of chemical species allows for a richer, yet more constrained-to-what-matters, set of metrics for model evaluation.

Characterization of errors in satellite-based HCHO ∕ NO ₂ tropospheric column ratios with respect to chemistry, column-to-PBL translation, spatial representation, and retrieval uncertainties

Abstract. The availability of formaldehyde (HCHO) (a proxy for volatile organic compound reactivity) and nitrogen dioxide (NO2) (a proxy for nitrogen oxides) tropospheric columns from ultraviolet–visible (UV–Vis) satellites has motivated many to use their ratios to gain some insights into the near-surface ozone sensitivity. Strong emphasis has been placed on the challenges that come with transforming what is being observed in the tropospheric column to what is actually in the planetary boundary layer (PBL) and near the surface; however, little attention has been paid to other sources of error such as chemistry, spatial representation, and retrieval uncertainties. Here we leverage a wide spectrum of tools and data to quantify those errors carefully. Concerning the chemistry error, a well-characterized box model constrained by more than 500 h of aircraft data from NASA's air quality campaigns is used to simulate the ratio of the chemical loss of HO2 + RO2 (LROx) to the chemical loss of NOx (LNOx). Subsequently, we challenge the predictive power of HCHO/NO2 ratios (FNRs), which are commonly applied in current research, in detecting the underlying ozone regimes by comparing them to LROx/LNOx. FNRs show a strongly linear (R2=0.94) relationship with LROx/LNOx, but only on the logarithmic scale. Following the baseline (i.e., ln(LROx/LNOx) = −1.0 ± 0.2) with the model and mechanism (CB06, r2) used for segregating NOx-sensitive from VOC-sensitive regimes, we observe a broad range of FNR thresholds ranging from 1 to 4. The transitioning ratios strictly follow a Gaussian distribution with a mean and standard deviation of 1.8 and 0.4, respectively. This implies that the FNR has an inherent 20 % standard error (1σ) resulting from not accurately describing the ROx–HOx cycle. We calculate high ozone production rates (PO3) dominated by large HCHO × NO2 concentration levels, a new proxy for the abundance of ozone precursors. The relationship between PO3 and HCHO × NO2 becomes more pronounced when moving towards NOx-sensitive regions due to nonlinear chemistry; our results indicate that there is fruitful information in the HCHO × NO2 metric that has not been utilized in ozone studies. The vast amount of vertical information on HCHO and NO2 concentrations from the air quality campaigns enables us to parameterize the vertical shapes of FNRs using a second-order rational function permitting an analytical solution for an altitude adjustment factor to partition the tropospheric columns into the PBL region. We propose a mathematical solution to the spatial representation error based on modeling isotropic semivariograms. Based on summertime-averaged data, the Ozone Monitoring Instrument (OMI) loses 12 % of its spatial information at its native resolution with respect to a high-resolution sensor like the TROPOspheric Monitoring Instrument (TROPOMI) (&gt; 5.5 × 3.5 km2). A pixel with a grid size of 216 km2 fails at capturing ∼ 65 % of the spatial information in FNRs at a 50 km length scale comparable to the size of a large urban center (e.g., Los Angeles). We ultimately leverage a large suite of in situ and ground-based remote sensing measurements to draw the error distributions of daily TROPOMI and OMI tropospheric NO2 and HCHO columns. At a 68 % confidence interval (1σ), errors pertaining to daily TROPOMI observations, either HCHO or tropospheric NO2 columns, should be above 1.2–1.5 × 1016 molec. cm−2 to attain a 20 %–30 % standard error in the ratio. This level of error is almost non-achievable with the OMI given its large error in HCHO. The satellite column retrieval error is the largest contributor to the total error (40 %–90 %) in the FNRs. Due to a stronger signal in cities, the total relative error (&lt; 50 %) tends to be mild, whereas areas with low vegetation and anthropogenic sources (e.g., the Rocky Mountains) are markedly uncertain (&gt; 100 %). Our study suggests that continuing development in the retrieval algorithm and sensor design and calibration is essential to be able to advance the application of FNRs beyond a qualitative metric.

Simulating the Weekly Cycle of NO_<i>x</i>‐VOC‐HO_<i>x</i>‐O₃ Photochemical System in the South Coast of California During CalNex‐2010 Campaign

Abstract United States Environmental Protection Agency guidance on the use of photochemical models for assessing the efficacy of an emissions control strategy for ozone requires that modeling be used in a relative sense. Consequently, testing a modeling system's ability to predict changes in ozone resulting from emission changes is critical. We evaluate model simulations for precursor species (NO x , CO, and volatile organic compounds [VOCs]), radicals (OH and HO 2 ), a secondary pollutant (O 3 ), and the model response of these compounds to weekend/weekday emission changes during California Nexus study in 2010. The modeling system correctly simulated the broad spatial and temporal variation of NO x and O 3 in California South Coast. Although the model generally underpredicted the daytime mixing ratios of NO 2 at the surface and overpredicted the NO 2 column, the simulated weekend to weekday ratios are consistent with each other and match the observed ratios well. The modeling system exhibited reasonable performance in simulating the VOC compounds with fossil fuel origins but has larger bias in simulating certain species associated with noncombustion sources. The modeling system successfully captured the weekend changes of the enhancement ratios for various VOC species to CO and the relative changes of HO x , which are indicators of faster chemical processing on weekends. This work demonstrates satisfactory model performances for O 3 and most relevant chemical compounds with more robust performance in simulating weekend versus weekday changes. Improved planetary boundary layer height simulations, a better understanding of OH‐HO 2 cycling, continued improvement of emissions, especially urban biogenic emissions and emissions of oxygenated VOCs, are important for future model improvement.

Characterization of volatile organic compounds at a roadside environment in Hong Kong: An investigation of influences after air pollution control strategies

Photochemistry of HO_<i>x</i> in the upper troposphere at northern midlatitudes

The factors controlling the concentrations of HO x radicals (= OH + peroxy) in the upper troposphere (8–12 km) are examined using concurrent aircraft observations of OH, HO 2 , H 2 O 2 , CH 3 OOH, and CH 2 O made during the Subsonic Assessment Ozone and Nitrogen Oxide Experiment (SONEX) at northern midlatitudes in the fall. These observations, complemented by concurrent measurements of O 3 , H 2 O, NO, peroxyacetyl nitrate (PAN), HNO 3 , CH 4 , CO, acetone, hydrocarbons, actinic fluxes, and aerosols, allow a highly constrained mass balance analysis of HO x and of the larger chemical family HO y (= HO x + 2 H 2 O 2 + 2 CH 3 OOH + HNO 2 + HNO 4 ). Observations of OH and HO 2 are successfully simulated to within 40% by a diel steady state model constrained with observed H 2 O 2 and CH 3 OOH. The model captures 85% of the observed HO x variance, which is driven mainly by the concentrations of NO x (= NO + NO 2 ) and by the strength of the HO x primary sources. Exceptions to the good agreement between modeled and observed HO x are at sunrise and sunset, where the model is too low by factors of 2–5, and inside cirrus clouds, where the model is too high by factors of 1.2–2. Heterogeneous conversion of NO 2 to HONO on aerosols (γ NO2 = 10 −3 ) during the night followed by photolysis of HONO could explain part of the discrepancy at sunrise. Heterogeneous loss of HO 2 on ice crystals (γ ice_HO2 = 0.025) could explain the discrepancy in cirrus. Primary sources of HO x from O( 1 D )+H 2 O and acetone photolysis were of comparable magnitude during SONEX. The dominant sinks of HO y were OH+HO 2 (NO x &lt;50 parts per trillion by volume (pptv)) and OH+HNO 4 (NO x &gt;50 pptv). Observed H 2 O 2 concentrations are reproduced by model calculations to within 50% if one allows in the model for heterogeneous conversion of HO 2 to H 2 O 2 on aerosols (γ HO2 = 0.2). Observed CH 3 OOH concentrations are underestimated by a factor of 2 on average. Observed CH 2 O concentrations were usually below the 50 pptv detection limit, consistent with model results; however, frequent occurrences of high values in the observations (up to 350 pptv) are not captured by the model. These high values are correlated with high CH 3 OH and with cirrus clouds. Heterogeneous oxidation of CH 3 OH to CH 2 O on aerosols or ice crystals might provide an explanation (γ ice_CH3OH ∼ 0.01 would be needed).

Bromoform and dibromomethane measurements in the seacoast region of New Hampshire, 2002–2004

Atmospheric measurements of bromoform (CHBr 3 ) and dibromomethane (CH 2 Br 2 ) were conducted at two sites, Thompson Farm (TF) in Durham, New Hampshire (summer 2002–2004), and Appledore Island (AI), Maine (summer 2004). Elevated mixing ratios of CHBr 3 were frequently observed at both sites, with maxima of 37.9 parts per trillion by volume (pptv) and 47.4 pptv for TF and AI, respectively. Average mixing ratios of CHBr 3 and CH 2 Br 2 at TF for all three summers ranged from 5.3–6.3 and 1.3–2.3 pptv, respectively. The average mixing ratios of both gases were higher at AI during 2004, consistent with AI's proximity to sources of these bromocarbons. Strong negative vertical gradients in the atmosphere corroborated local sources of these gases at the surface. At AI, CHBr 3 and CH 2 Br 2 mixing ratios increased with wind speed via sea‐to‐air transfer from supersaturated coastal waters. Large enhancements of CHBr 3 and CH 2 Br 2 were observed at both sites from 10 to 14 August 2004, coinciding with the passage of Tropical Storm Bonnie. During this period, fluxes of CHBr 3 and CH 2 Br 2 were 52.4 ± 21.0 and 9.1 ± 3.1 nmol m −2 h −1 , respectively. The average fluxes of CHBr 3 and CH 2 Br 2 during nonevent periods were 18.9 ± 12.3 and 2.6 ± 1.9 nmol m −2 h −1 , respectively. Additionally, CHBr 3 and CH 2 Br 2 were used as marine tracers in case studies to (1) evaluate the impact of tropical storms on emissions and distributions of marine‐derived gases in the coastal region and (2) characterize the transport of air masses during pollution episodes in the northeastern United States.

Photochemically induced production of CH3Br, CH3I, C2H5I, ethene, and propene within surface snow at Summit, Greenland

Airborne measurements of cirrus‐activated C₂Cl₄ depletion in the upper troposphere with evidence against Cl reactions

Airborne whole air samples collected over the western Pacific in spring, 2001 showed depletion of tetrachloroethene (C 2 Cl 4 ) in every upper tropospheric (UT) air parcel that had interacted with large areas of cirrus less than three days upwind. The amount of C 2 Cl 4 depletion showed a negative correlation with time since the interaction, consistent with the C 2 Cl 4 ‐depleted air parcels mixing with the ambient air as they moved downwind of the cirrus. Ethane and C 2 Cl 4 both react relatively quickly with atomic chlorine (Cl) but ethane was not significantly depleted in these same air parcels, indicating that the C 2 Cl 4 depletion cannot be attributed to Cl chemistry alone. Based on the minimum ethane depletion that can be detected by our measurements, a daytime upper limit of roughly 3 × 10 4 atom Cl cm −3 is indirectly estimated for heterogeneous chlorine activation by cirrus clouds in the UT in tropical‐ and mid‐latitudes during spring.

Space‐based formaldehyde measurements as constraints on volatile organic compound emissions in east and south Asia and implications for ozone

We use a continuous 6‐year record (1996–2001) of GOME satellite measurements of formaldehyde (HCHO) columns over east and south Asia to improve regional emission estimates of reactive nonmethane volatile organic compounds (NMVOCs), including isoprene, alkenes, HCHO, and xylenes. Mean monthly HCHO observations are compared to simulated HCHO columns from the GEOS‐Chem chemical transport model using state‐of‐science, “bottom‐up” emission inventories from Streets et al. (2003a) for anthropogenic and biomass burning emissions and Guenther et al. (2006) for biogenic emissions (MEGAN). We find that wintertime GOME observations can diagnose anthropogenic reactive NMVOC emissions from China, leading to an estimate 25% higher than Streets et al. (2003a). We attribute the difference to vehicular emissions. The biomass burning source for east and south Asia is almost 5 times the estimate of Streets et al. (2003a). GOME reveals a large source from agricultural burning in the North China Plain in June missing from current inventories. This source may reflect a recent trend toward in‐field burning of crop residues as the need for biofuels diminishes. Biogenic isoprene emission in east and south Asia derived from GOME is 56 ± 30 Tg yr −1 , similar to 52 Tg yr −1 from MEGAN. We find, however, that MEGAN underestimates emissions in China and overestimates emissions in the tropics. The higher Chinese biogenic and biomass burning emissions revealed by GOME have important implications for ozone pollution. We find 5 to 20 ppb seasonal increases in surface ozone in GEOS‐Chem for central and northern China when using GOME‐derived versus bottom‐up emissions. Our methodology can be adapted for other regions of the world to provide top‐down constraints on NMVOC emissions where multiple emission source types overlap in space and time.

Secondary Organic Aerosol Production across Worldwide Megacities and Its Impact on Mortality

Observations of atmospheric oxidation and ozone production in South Korea

South Korea routinely experiences poor air quality with ozone and small particles exceeding air quality standards. To build a better understanding of this problem, in 2016, the KORea-United States cooperative Air Quality (KORUS-AQ) study collected surface and airborne measurements of many chemical species, including the reactive gases hydroxyl (OH) and hydroperpoxyl (HO2). Several different results are reported here. First, OH and HO2 measured on the NASA DC-8 agree to within uncertainties with values calculated by two different box models, both in statistical comparisons and as a function of altitude from the surface to 8 km. These comparisons show substantial scatter, likely due to both variability in instrument performance and the difficulty in interpolating measurements made with frequencies different from those of the model time step. Second, OH and HO2 calculated by a model including HO2 uptake on aerosol particles in the chemical mechanism are inconsistent with observations. Third, in the planetary boundary layer over both ocean and land, measured and model-calculated OH reactivity are sometimes different, and this missing OH reactivity, which is as much as ∼4 s−1, increased from April to June and originated primarily from the Korean peninsula. Fourth, repeated missed approaches at the Seoul Air Base during several days show that the changes in the sum of ozone and nitrogen dioxide are consistent with ozone production rates calculated from HO2 either observed or modeled by the Langley Research Center model.

Statistical inference of OH concentrations and air mass dilution rates from successive observations of nonmethane hydrocarbons in single air masses

Bayesian inference has been used to determine rigorous estimates of hydroxyl radical concentrations ( ) and air mass dilution rates ( K ) averaged following air masses between linked observations of nonmethane hydrocarbons (NMHCs) spanning the North Atlantic during the Intercontinental Transport and Chemical Transformation (ITCT)‐Lagrangian‐2K4 experiment. The Bayesian technique obtains a refined (posterior) distribution of a parameter given data related to the parameter through a model and prior beliefs about the parameter distribution. Here, the model describes hydrocarbon loss through OH reaction and mixing with a background concentration at rate K . The Lagrangian experiment provides direct observations of hydrocarbons at two time points, removing assumptions regarding composition or sources upstream of a single observation. The estimates are sharpened by using many hydrocarbons with different reactivities and accounting for their variability and measurement uncertainty. A novel technique is used to construct prior background distributions of many species, described by variation of a single parameter α . This exploits the high correlation of species, related by the first principal component of many NMHC samples. The Bayesian method obtains posterior estimates of , K and α following each air mass. Median values are typically between 0.5 and 2.0 × 10 6 molecules cm −3 , but are elevated to between 2.5 and 3.5 × 10 6 molecules cm −3 , in low‐level pollution. A comparison of estimates from absolute NMHC concentrations and NMHC ratios assuming zero background (the “photochemical clock” method) shows similar distributions but reveals systematic high bias in the estimates from ratios. Estimates of K are ∼0.1 day −1 but show more sensitivity to the prior distribution assumed.