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The impact of biomass burning (BB) on the atmospheric burden of volatile organic compounds (VOCs) is highly uncertain. Here we apply the GEOS-Chem chemical transport model (CTM) to constrain BB emissions in the western USA at ∼ 25 km resolution. Across three BB emission inventories widely used in CTMs, the inventory–inventory comparison suggests that the totals of 14 modeled BB VOC emissions in the western USA agree with each other within 30 %–40 %. However, emissions for individual VOCs can differ by a factor of 1–5, driven by the regionally averaged emission ratios (ERs, reflecting both assigned ERs for specific biome and vegetation classifications) across the three inventories. We further evaluate GEOS-Chem simulations with aircraft observations made during WE-CAN (Western Wildfire Experiment for Cloud Chemistry, Aerosol Absorption and Nitrogen) and FIREX-AQ (Fire Influence on Regional to Global Environments and Air Quality) field campaigns. Despite being driven by different global BB inventories or applying various injection height assumptions, the model–observation comparison suggests that GEOS-Chem simulations underpredict observed vertical profiles by a factor of 3–7. The model shows small to no bias for most species in low-/no-smoke conditions. We thus attribute the negative model biases mostly to underestimated BB emissions in these inventories. Tripling BB emissions in the model reproduces observed vertical profiles for primary compounds, i.e., CO, propane, benzene, and toluene. However, it shows no to less significant improvements for oxygenated VOCs, particularly for formaldehyde, formic acid, acetic acid, and lumped ≥ C3 aldehydes, suggesting the model is missing secondary sources of these compounds in BB-impacted environments. The underestimation of primary BB emissions in inventories is likely attributable to underpredicted amounts of effective dry matter burned, rather than errors in fire detection, injection height, or ERs, as constrained by aircraft and ground measurements. We cannot rule out potential sub-grid uncertainties (i.e., not being able to fully resolve fire plumes) in the nested GEOS-Chem which could explain the negative model bias partially, though back-of-the-envelope calculation and evaluation using longer-term ground measurements help support the argument of the dry matter burned underestimation. The total ERs of the 14 BB VOCs implemented in GEOS-Chem only account for half of the total 161 measured VOCs (∼ 75 versus 150 ppb ppm−1). This reveals a significant amount of missing reactive organic carbon in widely used BB emission inventories. Considering both uncertainties in effective dry matter burned (× 3) and unmodeled VOCs (× 2), we infer that BB contributed to 10 % in 2019 and 45 % in 2018 (240 and 2040 Gg C) of the total VOC primary emission flux in the western USA during these two fire seasons, compared to only 1 %–10 % in the standard GEOS-Chem.

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, and 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. Surprisingly, the probability densities of species and reactivities averaged on a model scale (100 km) differ only slightly from the 2 km ATom 10 s 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 generally good agreement with the reactivity rates for O3 and CH4. Models distinctly underestimate O3 production below 2 km relative to the mid-troposphere, 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. This paper presents a corrected version of the paper published under the same authors and title (sans “corrected”) as https://doi.org/10.5194/acp-21-13729-2021.

This article provides an overview of the NASA Atmospheric Tomography (ATom) mission and a summary of selected scientific findings to date. ATom was an airborne measurements and modeling campaign aimed at characterizing the composition and chemistry of the troposphere over the most remote regions of the Pacific, Southern, Atlantic, and Arctic Oceans, and examining the impact of anthropogenic and natural emissions on a global scale. These remote regions dominate global chemical reactivity and are exceptionally important for global air quality and climate. ATom data provide the in situ measurements needed to understand the range of chemical species and their reactions, and to test satellite remote sensing observations and global models over large regions of the remote atmosphere. Lack of data in these regions, particularly over the oceans, has limited our understanding of how atmospheric composition is changing in response to shifting anthropogenic emissions and physical climate change. ATom was designed as a global-scale tomographic sampling mission with extensive geographic and seasonal coverage, tropospheric vertical profiling, and detailed speciation of reactive compounds and pollution tracers. ATom flew the NASA DC-8 research aircraft over four seasons to collect a comprehensive suite of measurements of gases, aerosols, and radical species from the remote troposphere and lower stratosphere on four global circuits from 2016 to 2018. Flights maintained near-continuous vertical profiling of 0.15–13-km altitudes on long meridional transects of the Pacific and Atlantic Ocean basins. Analysis and modeling of ATom data have led to the significant early findings highlighted here.

he reactive chemistry of isoprene, which is the dominant hydrocarbon in biogenic emissions, has a controlling influence on the composition and cleansing capacity of the global atmosphere. Despite decades of research, isoprene continues to offer surprises in its atmospheric chemistry, particularly in environments with low-to-moderate levels of nitrogen oxides (NOₓ). Here, we probe the isoprene photochemical oxidation in this “intermediate-NOₓ” regime by examining the yield distributions of two major oxidation products, i.e., methacrolein and methyl vinyl ketone, using chamber experiments and aircraft measurements. Such a dataset provides strong constraints on the kinetics of the isoprene peroxy radical interconversion—a newly discovered mechanism that essentially governs the isoprene oxidation carbon flow. Insights from measurement-model comparisons further reveal an efficient operation of this mechanism across all the vegetated continents over the globe, constantly modulating the radical cycling and contributing to the formation of ozone and organic aerosols in the atmosphere.

22 pags., 10 figs., 3 tabs., Bromine released from the decomposition of short-lived brominated source gases contributes as a sink of ozone in the lower stratosphere. The two major contributors are CH2Br2 and CHBr3. In this study, we investigate the global seasonal distribution of these two substances, based on four High Altitude and Long Range Research Aircraft (HALO) missions, the HIAPER Pole-to-Pole Observations (HIPPO) mission, and the Atmospheric Tomography (ATom) mission. Observations of CH2Br2 in the free and upper troposphere indicate a pronounced seasonality in both hemispheres, with slightly larger mixing ratios in the Northern Hemisphere (NH). Compared to CH2Br2, CHBr3 in these regions shows larger variability and less clear seasonality, presenting larger mixing ratios in winter and autumn in NH midlatitudes to high latitudes. The lowermost stratosphere of SH and NH shows a very similar distribution of CH2Br2 in hemispheric spring with differences well below 0.1gppt, while the differences in hemispheric autumn are much larger with substantially smaller values in the SH than in the NH. This suggests that transport processes may be different in both hemispheric autumn seasons, which implies that the influx of tropospheric air ("flushing") into the NH lowermost stratosphere is more efficient than in the SH. The observations of CHBr3 support the suggestion, with a steeper vertical gradient in the upper troposphere and lower stratosphere in SH autumn than in NH autumn. However, the SH database is insufficient to quantify this difference. We further compare the observations to model estimates of TOMCAT (Toulouse Off-line Model of Chemistry And Transport) and CAM-Chem (Community Atmosphere Model with Chemistry, version 4), both using the same emission inventory of . The pronounced tropospheric seasonality of CH2Br2 in the SH is not reproduced by the models, presumably due to erroneous seasonal emissions or atmospheric photochemical decomposition efficiencies. In contrast, model simulations of CHBr3 show a pronounced seasonality in both hemispheres, which is not confirmed by observations. The distributions of both species in the lowermost stratosphere of the Northern and Southern hemispheres are overall well captured by the models with the exception of southern hemispheric autumn, where both models present a bias that maximizes in the lowest 40gK above the tropopause, with considerably lower mixing ratios in the observations. Thus, both models reproduce equivalent flushing in both hemispheres, which is not confirmed by the limited available observations. Our study emphasizes the need for more extensive observations in the SH to fully understand the impact of CH2Br2 and CHBr3 on lowermost-stratospheric ozone loss and to help constrain emissions., This research has been supported by the Bundesministerium für Bildung und Forschung (grant no. 01LG1908B), the Deutsche Forschungsgemeinschaft (grant nos. EN367/13-1, EN367/14-1, EN367/16-1, EN367/17-1), and DFG collaborative research program “The Tropopause Region in an Changing Atmosphere” TRR 301 – Project-ID 428312742, the National Center for Atmospheric Research (grant no. 1852977), the Consejo Nacional de Investigaciones Científicas y Técnicas (grant no. PICT 2019-2187), and the Natural Environment Research Council (grant no. NE/N014375/). This open-access publication was funded by the Goethe University Frankfurt.

Fires emit a substantial amount of non-methane organic gases (NMOGs), the atmospheric oxidation of which can contribute to ozone and secondary particulate matter formation. However, the abundance and reactivity of these fire NMOGs are uncertain and historically not well constrained. In this work, we expand the representation of fire NMOGs in a global chemical transport model, GEOS-Chem. We update emission factors to Andreae (2019) and the chemical mechanism to include recent aromatic and ethene and ethyne model improvements (Bates et al., 2021; Kwon et al., 2021). We expand the representation of NMOGs by adding lumped furans to the model (including their fire emission and oxidation chemistry) and by adding fire emissions of nine species already included in the model, prioritized for their reactivity using data from the Fire Influence on Regional to Global Environments (FIREX) laboratory studies. Based on quantified emissions factors, we estimate that our improved representation captures 72 % of emitted, identified NMOG carbon mass and 49 % of OH reactivity from savanna and temperate forest fires, a substantial increase from the standard model (49 % of mass, 28 % of OH reactivity). We evaluate fire NMOGs in our model with observations from the Amazon Tall Tower Observatory (ATTO) in Brazil, Fire Influence on Regional to Global Environments and Air Quality (FIREX-AQ) and DC3 in the US, and Arctic Research of the Composition of the Troposphere from Aircraft and Satellites (ARCTAS) in boreal Canada. We show that NMOGs, including furan, are well simulated in the eastern US with some underestimates in the western US and that adding fire emissions improves our ability to simulate ethene in boreal Canada. We estimate that fires provide 15 % of annual mean simulated surface OH reactivity globally, as well as more than 75 % over fire source regions. Over continental regions about half of this simulated fire reactivity comes from NMOG species. We find that furans and ethene are important globally for reactivity, while phenol is more important at a local level in the boreal regions. This is the first global estimate of the impact of fire on atmospheric reactivity.

Formic acid (HCOOH) is an important component of atmospheric acidity but its budget is poorly understood, with prior observations implying substantial missing sources. Here we combine pole-to-pole airborne observations from the Atmospheric Tomography Mission (ATom) with chemical transport model (GEOS-Chem CTM) and back trajectory analyses to provide the first global in-situ characterization of HCOOH in the remote atmosphere. ATom reveals sub-100 ppt HCOOH concentrations over most of the remote oceans, punctuated by large enhancements associated with continental outflow. Enhancements correlate with known combustion tracers and trajectory-based fire influences. The GEOS-Chem model underpredicts these in-plume HCOOH enhancements, but elsewhere we find no broad indication of a missing HCOOH source in the background free troposphere. We conclude that missing non-fire HCOOH precursors inferred previously are predominantly short-lived. We find indications of a wet scavenging underestimate in the model consistent with a positive HCOOH bias in the tropical upper troposphere. Observations reveal episodic evidence of ocean HCOOH uptake, which is well-captured by GEOS-Chem; however, despite its strong seawater undersaturation HCOOH is not consistently depleted in the remote marine boundary layer. Over fifty fire and mixed plumes were intercepted during ATom with widely varying transit times and source regions. HCOOH:CO normalized excess mixing ratios in these plumes range from 3.4 to >50 ppt/ppb CO and are often over an order of magnitude higher than expected primary emission ratios. HCOOH is thus a major reactive organic carbon reservoir in the aged plumes sampled during ATom, implying important missing pathways for in-plume HCOOH production.

Polar halogen chemistry has long been known to be active, especially in spring, and has an important influence on the lifetime of some volatile organics, ozone, and mercury. Reactive chlorine and bromine species, produced from snow and aerosols, can have significant impacts on the oxidative capacity of the polar boundary layer. However, halogen production mechanisms from snow remain highly uncertain, making it challenging to include descriptions of halogen snow emissions in models and to understand the impact on atmospheric chemistry. In this work, we investigate the role of Arctic chlorine and bromine emissions from snow on boundary layer oxidation processes using a one-dimensional atmospheric chemistry and transport model (PACT-1D). We explore the impact of halogen snow emissions and boundary layer dynamics on atmospheric chemistry by modelling primary emissions of Cl2 and Br2 from snow, and heterogeneous recycling reactions on snow and aerosols. We present a two-day case study from the 2009 Ocean-Atmosphere-Sea Ice-Snowpack (OASIS) campaign at Utqiagvik, Alaska. The model reproduces both the diurnal cycle and high quantity of Cl2 measured, along with the observed concentrations of Br2, BrO, and HOBr. Due to a combination of chemical emissions, recycling, vertical mixing, and atmospheric chemistry, reactive chlorine is confined to the lowest 15 m of the atmosphere, whilst bromine impacts chemistry up to the boundary layer height. Following the inclusion of halogen emissions and recycling, HOx concentrations (HOx = OH+HO2) increase by as much as a factor of 30 at the surface at mid-day. Consequently, volatile organic compound (VOC) lifetimes are significantly reduced within a shallow layer near the surface, due to chlorine atoms from Cl2 snow emissions and increased HOx attributable to halogen chemistry.

Comprehensive aircraft measurements of volatile organic compounds (VOCs) covering the South Coast Air Basin (SoCAB) and San Joaquin Valley (SJV) of California were obtained in the summer of 2019. Combined with the CO, CH4, and NOx data, the total calculated gas-phase hydroxyl radical reactivity (cOHRTOTAL) was quantified to be 6.1 and 4.6 s−1 for the SoCAB and SJV, respectively. VOCs accounted for ∼ 60 %–70 % of the cOHRTOTAL in both basins. In particular, oxygenated VOCs (OVOCs) contributed >60 % of the cOHR of total VOCs (cOHRVOC) and the total observed VOC mixing ratio. Primary biogenic VOCs (BVOCs) represented a minor fraction (<2 %) of the total VOC mixing ratio but accounted for 21 % and 6 % of the cOHRVOC in the SoCAB and SJV, respectively. Furthermore, the contribution of BVOCs to the cOHRVOC increased with increasing cOHRVOC in the SoCAB, suggesting that BVOCs were important ozone precursors during high ozone episodes. Spatially, the trace gases were heterogeneously distributed in the SoCAB, with their mixing ratios and cOHR being significantly greater over the inland regions than the coast, while their levels were more evenly distributed in SJV. The results highlight that a better grasp of the emission rates and sources of OVOCs and BVOCs is essential for a predictive understanding of the ozone abundance and distribution in California.

Biomass burning emits ~34–41 Tg yr−1 of smoke aerosol to the atmosphere. Biomass burning aerosol directly influences the Earth’s climate by attenuation of solar and terrestrial radiation; however, its abundance and distribution on a global scale are poorly constrained, particularly after plumes dilute into the background remote troposphere and are subject to removal by clouds and precipitation. Here we report global-scale, airborne measurements of biomass burning aerosol in the remote troposphere. Measurements were taken during four series of seasonal flights over the Pacific and Atlantic Ocean basins, each with near pole-to-pole latitude coverage. We find that biomass burning particles in the remote troposphere are dilute but ubiquitous, accounting for one-quarter of the accumulation-mode aerosol number and one-fifth of the aerosol mass. Comparing our observations with a high-resolution global aerosol model, we find that the model overestimates biomass burning aerosol mass in the remote troposphere with a mean bias of >400%, largely due to insufficient wet removal by in-cloud precipitation. After updating the model’s aerosol removal scheme we find that, on a global scale, dilute smoke contributes as much as denser plumes to biomass burning’s scattering and absorption effects on the Earth’s radiation field. Aerosol particles produced by biomass burning are ubiquitous in the remote troposphere, according to global airborne measurements over remote ocean regions.

Inland sources of particulate chloride for atmospheric nitryl chloride (ClNO2) formation remain unknown and unquantified, hindering air quality assessments. Globally each winter, tens of millions of tons of road salt are spread on roadways for deicing. Here, we identify road salt aerosol as the primary chloride aerosol source, accounting for 80–100% of ClNO2 formation, at an inland urban area in the wintertime. This study provides experimental evidence of the connection between road salt and air quality through the production of this important reservoir for nitrogen oxides and chlorine radicals, which significantly impact atmospheric composition and pollutant fates. A numerical model was employed to quantify the contributions of chloride sources to ClNO2 production. The traditional method for simulating ClNO2 considers chloride to be homogeneously distributed across the atmospheric particle population; yet, we show that only a fraction of the particulate surface area contains chloride. Our new single-particle parametrization considers this heterogeneity, dramatically lowering overestimations of ClNO2 levels that have been routinely reported using the prevailing methods. The identification of road salt as a ClNO2 source links this common deicing practice to atmospheric composition and air quality in the urban wintertime environment., Road salt was identified as the primary source of chloride-containing aerosol in the wintertime urban atmosphere. This links deicing practices to air quality via the production of the trace gas ClNO2.

Wildfires are an important source of nitrous acid (HONO), a photolabile radical precursor, yet in situ measurements and quantification of primary HONO emissions from open wildfires have been scarce. We present airborne observations of HONO within wildfire plumes sampled during the Western Wildfire Experiment for Cloud chemistry, Aerosol absorption and Nitrogen (WE-CAN) campaign. ΔHONO/ΔCO close to the fire locations ranged from 0.7 to 17 pptv ppbv

20 pags., 11 figs. 1 tab. -- Open Access funded by Creative Commons Atribution Licence 4.0, Fluxes of halogenated volatile organic compounds (VOCs) over the Southern Ocean remain poorly understood, and few atmospheric measurements exist to constrain modeled emissions of these compounds. We present observations of CHBr, CHBr, CHI, CHClBr, CHBrCl, and CHBr during the O2=N Ratio and CO Airborne Southern Ocean (ORCAS) study and the second Atmospheric Tomography mission (ATom-2) in January and February of 2016 and 2017. Good model-measurement correlations were obtained between these observations and simulations from the Community Earth System Model (CESM) atmospheric component with chemistry (CAM-Chem) for CHBr, CHBr, CHI, and CHClBr but all showed significant differences in model: measurement ratios. The model: measurement comparison for CHBr was satisfactory and for CHBrCl the low levels present precluded us from making a complete assessment. Thereafter, we demonstrate two novel approaches to estimate halogenated VOC fluxes; the first approach takes advantage of the robust relationships that were found between airborne observations of O and CHBr, CHBr, and CHClBr. We use these linear regressions with O and modeled O distributions to infer a biological flux of halogenated VOCs. The second approach uses the Stochastic Time-Inverted Lagrangian Transport (STILT) particle dispersion model to explore the relationships between observed mixing ratios and the product of the upstream surface influence of sea ice, chl a, absorption due to detritus, and downward shortwave radiation at the surface, which in turn relate to various regional hypothesized sources of halogenated VOCs such as marine phytoplankton, phytoplankton in seaice brines, and decomposing organic matter in surface seawater. These relationships can help evaluate the likelihood of particular halogenated VOC sources and in the case of statistically significant correlations, such as was found for CHI, may be used to derive an estimated flux field. Our results are consistent with a biogenic regional source of CHBr and both nonbiological and biological sources of CHI over these regions., We would like to thank the ORCAS and ATom-2 science teams as well as the NCAR Research Aviation Facility and NASA DC-8 pilots, technicians, and mechanics for their support during the field campaigns. This work was made possible by grants from NSF Polar Programs (1501993, 1501997, 1501292, 1502301, 1543457), NSF Atmospheric Chemistry grants 1535364, 1623745, and 1623748, and NASA funding of the EVS2 Atmospheric Tomography (ATom) project, as well as the support of the NCAR Advanced Study Program (ASP) Postdoctoral Fellowship Program and computing support from Yellowstone, provided by NCAR’s Computational and Information Systems Laboratory. The National Center for Atmospheric Research is sponsored by the National Science Foundation. Financial support.

22 pags., 11 figs., 2 tabs. -- Open Access funded by Creative Commons Atribution Licence 4.0, Halogenated very short lived substances (VSLS) affect the ozone budget in the atmosphere. Brominated VSLS are naturally emitted from the ocean, and current oceanic emission inventories vary dramatically. We present a new global oceanic emission inventory of Br‐VSLS (bromoform and dibromomethane), considering the physical forcing in the ocean and the atmosphere, as well as the ocean biogeochemistry control. A data‐oriented machine‐learning emulator was developed to couple the air‐sea exchange with the ocean biogeochemistry. The predicted surface seawater concentrations and the surface atmospheric mixing ratios of Br‐VSLS are evaluated with long‐term, global‐scale observations; and the predicted vertical distributions of Br‐VSLS are compared to the global airborne observations in both boreal summer and winter. The global marine emissions of bromoform and dibromomethane are estimated to be 385 and 54 Gg Br per year, respectively. The new oceanic emission inventory of Br‐VSLS is more skillful than the widely used top‐down approaches for representing the seasonal/spatial variations and the annual means of atmospheric concentrations. The new approach improves the model predictability for the coupled Earth system model and can be used as a basis for investigating the past and future ocean emissions and feedbacks under climate change. This model framework can be used to calculate the bidirectional oceanic fluxes for other compounds of interest., S.‐Y. W. is supported by the NCAR Advanced Study Program (ASP) Postdoctoral Fellowship. The CESM project is supported primarily by the National Science Foundation (NSF). This material is based upon work supported by NCAR, which is a major facility sponsored by the NSF under Cooperative Agreement 1852977. The computing and data storage resources, including the Cheyenne supercomputer (doi:10.5065/D6RX99HX), were provided by the Computational and Information Systems Laboratory (CISL) at NCAR. The Atmospheric Tomography Mission (ATom) is funded by the Earth Science Project Office at NASA (NNX15AJ23G). The NOAA ground‐based measurements were supported in part by NOAA's Atmospheric Chemistry, Carbon Cycle and Climate Program of its Climate Program Office. NASA's ATom mission was funded as a NASA Earth Science Venture ‐ suborbital mission. We thank NASA ESPO, the NASA DC‐8 crew, and the ATom Science Team for their exceptional professionalism in support of this mission. Franziska Ziska (GEOMAR, Germany) and colleagues are acknowledged for compiling the HalOcAt data set. Tomás Sherwen (University of York, UK) and Britton Stephens (NCAR, USA) are acknowledged for helpful discussions. CAM‐chem is a component of the NCAR CESM which is publicly available on the project website (http://www.cesm. ucar.edu/). The HalOcAt data set used for the training of the machine‐learning emulator is available in Ziska et al. (doi: https://doi.org/10.5194/acp‐13‐8915‐ 2013). The NCAR CESM Large Ensemble data set is available from the project website (http://www.cesm.ucar. edu/projects/community‐projects/ LENS/). The NCAR TOGA, UCI WAS, and NOAA PFP measurements during the NASA ATom campaign is available from Wofsy et al. (2018).

A new configuration of the Community Earth System Model (CESM)/Community Atmosphere Model with full chemistry (CAM-chem) supporting the capability of horizontal mesh refinement through the use of the spectral element (SE) dynamical core is developed and called CESM/CAM-chem-SE. Horizontal mesh refinement in CESM/CAM-chem-SE is unique and novel in that pollutants such as ozone are accurately represented at human exposure relevant scales while also directly including global feedbacks. CESM/CAM-chem-SE with mesh refinement down to ∼14 km over the conterminous US (CONUS) is the beginning of the Multi-Scale Infrastructure for Chemistry and Aerosols (MUSICAv0). Here, MUSICAv0 is evaluated and used to better understand how horizontal resolution and chemical complexity impact ozone and ozone precursors over CONUS as compared to measurements from five aircraft campaigns, which occurred in 2013. This field campaign analysis demonstrates the importance of using finer horizontal resolution to accurately simulate ozone precursors such as nitrogen oxides and carbon monoxide. In general, the impact of using more complex chemistry on ozone and other oxidation products is more pronounced when using finer horizontal resolution where a larger number of chemical regimes are resolved. Large model biases for ozone near the surface remain in the Southeast US as compared to the aircraft observations even with updated chemistry and finer horizontal resolution. This suggests a need for adding the capability of replacing sections of global emission inventories with regional inventories, increasing the vertical resolution in the planetary boundary layer, and reducing model biases in meteorological variables such as temperature and clouds.

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 as well as 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 VAMOS Ocean-Cloud-Atmosphere-Land Study Regional Experiment (VOCALS-REx) measurements. We find that the global HPMTF burden, as well as 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 (+29 %) and 582 Gg-S (+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 −0.11 W m−2. The enhanced PI sulfate produced via the gas-phase chemistry updates alone dampens the aerosol IRF as anticipated (−2.2 W m−2 in standard versus −1.7 W m−2 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 (−2.3 W m−2). This study confirms the sensitivity of aerosol IRF to the PI aerosol loading, as well as the need to better understand the processes controlling aerosol formation in the PI atmosphere and the cloud response to these changes.

Wildland firefighters are exposed to smoke containing particulate matter (PM) and volatile organic compounds (VOCs) while suppressing wildfires. From 2015–2017, the US Forest Service conducted a field study collecting breathing zone measurements of PM(4) (particulate matter with aerodynamic diameter ≤4 μm) on wildland firefighters from different crew types and while performing various fire suppression tasks on wildfires. Emission ratios of VOC (parts per billion; ppb): PM(1) (particulate matter with aerodynamic diameter ≤1 μm; mg/m(3)) were calculated using data from a separate field study conducted in summer 2018, the Western Wildfire Experiment for Cloud Chemistry, Aerosol Absorption, and Nitrogen (WE-CAN) Campaign. These emission ratios were used to estimate wildland firefighter exposure to acrolein, benzene and formaldehyde. Results of this field sampling campaign reported exposure to PM(4) and VOC varied across wildland firefighter crew type and job task. Type 1 crews had greater exposures to both PM(4) and VOCs than Type 2 or Type 2 Initial Attack crews, and wildland firefighters performing direct suppression had statistically higher exposures than those performing staging and other tasks (mean differences=0.82 and 0.75 mg/m(3); 95% confidence intervals=0.38–1.26 mg/m(3) and 0.41–1.08 mg/m(3), respectively). Of the 81 personal exposure samples collected, 19% of measured PM(4) exposures exceeded the recommended National Wildland Fire Coordinating Group occupational exposure limit (0.7 mg/m(3)). Wildland fire management should continue to find strategies to reduce smoke exposures for wildland firefighters.