Consumer volatile chemical products (VCPs) are widely used indoors, yet their formulation-specific volatile organic compound (VOC) emissions remain poorly characterized. Here, we quantified formulation-resolved 119 VOC emission factors for four representative consumer VCPs, including cleaning products, air fresheners, hairsprays, and insecticides in Hong Kong, using dynamic chamber experiments and applied these experimentally derived emission factors to identify key species contributing to 1-h acute inhalation risk and secondary organic aerosol (SOA) formation potential. Oxygenated VOCs, monoterpenes, and liquefied petroleum gas-derived alkanes dominated emission profiles, with pronounced formulation-dependent variability. Emission factors of hairsprays and insecticides (350-641 g VOC kg-1 VCP) were significantly higher than those of cleaning products and air fresheners (3.8-52 g VOC kg-1 VCP), with nonwater-based formulations consistently showing emission factors 1.4-14 times higher than their water-based counterparts. Acetaldehyde and acrolein were identified as the primary drivers of 1-h acute inhalation risks, whereas monoterpenes and sesquiterpenes were identified as the key precursors driving SOA formation. Scenario analyses in Hong Kong indicated that consumer VCP use is unlikely to pose unacceptable 1-h acute inhalation risks under regulatory minimum living-condition standards. However, nonwater-based hairsprays may exceed acceptable 1-h acute inhalation risk thresholds in a typical indoor environment under poor ventilation due to elevated acetaldehyde emissions; increased ventilation substantially reduced these risks. Overall, this study provides formulation-resolved emission data and identifies key species contributing to acute inhalation risk and SOA formation potential from consumer VCPs.

Abstract. Asia accounts for approximately half of global anthropogenic emissions of major pollutants, yet emission inventories remain uncertain and ground-based monitoring is sparse across much of the region. To address these challenges, this study applies a multi-species satellite data assimilation framework to estimate emissions and concentrations of key chemical species during the NASA's ASIA-AQ campaign. The assimilation improves agreement with airborne observations for O3, NOx, CO, and CH2O, with the largest gains for CO (correlation increasing from 0.63–0.64 to 0.77) and CH2O (biases reduced by 41–70 %). Domain-wide, the optimized emissions show increases of 15 % for NOx and 9 % for CO, and a 52 % reduction in isoprene. Comparisons with multiple emission inventories reveal large discrepancies, with normalized standard deviation ranging from 11 % for NOx in mainland China to 68 % for CO in Taiwan. Over Thailand, the assimilation increases fire emissions from 3.6 Tg (GFASv1.2) to 8.3 Tg, while FINNv2.7 produces estimates roughly twice as high, highlighting persistent divergence among fire emission estimates. Source-receptor analysis reveals strong meteorological control on transboundary pollutant: long-range transport contributes up to 62 % of surface O3 in Manila during strong monsoon conditions, whereas Seoul exhibits local NOx-saturated chemistry. During a strong transport episode, Indian emissions account for 72–78 % of the free tropospheric O3 response over Taipei and Chiang Mai, and 24 % over Seoul, highlighting an overlooked transport pathway affecting Asian air quality. These results highlight the value of satellite data assimilation and the need for improved inventories and coordinated action on local and transboundary pollution.

Abstract Nine years ago, the Department of Interior’s Bureau of Ocean Energy Management (BOEM), the agency with air quality (AQ) jurisdiction over the outer continental shelf (OCS) of the U.S. Gulf Coast west of 87.5°W longitude, asked the National Aeronautics and Space Administration (NASA) to determine the feasibility of using satellite data to measure offshore emissions in a region of concentrated oil and natural gas (ONG) operations. To study this issue, NASA and BOEM conducted the May 2019 Satellite Coastal and Oceanic Atmospheric Pollution Experiment (SCOAPE) cruise in the Gulf. SCOAPE addressed both technological and scientific issues related to measuring nitrogen dioxide (NO 2 ; a common air pollutant), including contrasting nearshore and deep-water regimes. Given the April 2023 launch of the geostationary Tropospheric Emissions: Monitoring of Pollution (TEMPO) AQ satellite, a 2024 SCOAPE-II was conducted in the Gulf with both ship and aircraft measurements. We present an overview of the SCOAPE-II campaign, an analysis and validation of satellite-observed NO 2 , and evaluate measurements of methane from ship, aircraft, and satellite near ONG platforms. Our SCOAPE-II results are as follows: 1) Satellite NO 2 measurements (∼1330 local time) from the Tropospheric Monitoring Instrument (TROPOMI) are more accurate than TEMPO’s hourly scans (8.6% vs 23.6% mean absolute bias); a new version of the TEMPO data is currently being processed; 2) ship and aircraft measurements captured dozens of NO 2 and methane plumes from ONG operations, showing that they are persistent emitters; and 3) satellite measurements of methane failed to replicate ship and aircraft measurements, presenting ongoing challenges for operational emissions monitoring over the Gulf. Significance Statement The exploration, extraction, and processing of oil and natural gas (ONG) from deposits off the U.S. Gulf Coast generate detectable emissions affecting air quality (AQ). However, the lack of regular monitoring of these emissions makes quantifying their impact challenging. We conducted a ship- and aircraft-based campaign in 2024 in the Gulf to characterize surface AQ, validate satellite AQ and methane measurements, and evaluate ONG emissions inventories. Our measurements indicate that ONG platforms are persistent emitters of nitrogen oxides (NO x ) that affect AQ, and methane, a potent greenhouse gas. Satellite nitrogen dioxide (NO 2 ) measurements are generally of sufficient accuracy for characterizing Gulf AQ, but technological constraints preclude similar monitoring of ONG methane emissions from space.

Despite considerable reductions in mobile source emissions, annual average aerosol concentrations measured in Los Angeles using Federal Reference Methods (FRM) have not appreciably declined over the last decade. Here, we use submicron aerosol measurements and zero-dimensional modeling to quantify the impacts of these emissions reductions on aerosol formation in Pasadena, CA during the late spring and summer of 2022. Reductions in secondary organic aerosol (SOA) concentrations expected from reduced mobile source emissions appear to have been largely offset by increases in hydroxyl radical concentrations, an indirect effect of reduced nitrogen oxide (NOx) emissions. As a result, while the predicted contribution of mobile sources to the SOA burden has declined from ~50% in 2010 to only ~25% in 2022, concentrations of locally-formed SOA have remained relatively constant. In contrast, reductions in mobile source NOx emissions have likely reduced overnight production of nitric acid and ammonium nitrate (AN) aerosol. We provide indirect evidence that FRM measurements may have failed to capture the reduction in AN since 2010 due to evaporation of semi-volatile species from FRM filter samples. Our results suggest that given the effectiveness of historical regulatory efforts aimed at mobile sources, and on-road sources in particular, additional reductions in submicron aerosol concentrations in Los Angeles will likely require increased focus on abating emissions from non-road and area sources.

Wintertime photochemical air pollution in East Asia remains poorly constrained despite its impact on regional air quality. Sources and formation pathways of acyl peroxynitrates (PANs) and ozone (O-3), key photochemical products, are not well understood, hindering effective mitigation strategies. We investigate PANs and O-3 over South Korea using observations from the ASIA-AQ campaign (February-March 2024). PANs reached 5.5 ppbv, strongly correlating with formaldehyde and particulate matter, indicating active winter photochemistry. Median PANs were higher in the mid-southern peninsula (MS; 990 pptv) and Yellow Sea (1200 pptv) than the Seoul Metropolitan Area (840 pptv). Elevated homologue-to-acetyl peroxynitrate ratios over the MS, with enhanced acryloyl peroxynitrate, acrolein, and ethylene oxide, provided tracers for petrochemical emissions and their impacts. Acetaldehyde contributed 53 %-80 % of PAN production. Ethanol was a major precursor of acetaldehyde (similar to 50 %). Strong correlations ( r(2) > 0.8) of ethanol and halocarbons indicate industrial and solvent sources under-represented in inventories. Formaldehyde and C2+ aldehydes contributed similar to 30 % to ozone production. Low ozone production efficiency ( < 10) and radical termination dominated by nitric acid and PANs ( > 80 %) indicate VOC-limited conditions. The fractional PANs contribution to NOx loss increased with decreasing OH reactivity ratio of NO2 to aldehydes, suggesting spatial increases in ozone production following NOx reductions. These findings demonstrate that a comprehensive understanding of VOC oxidation, particularly oxygenates from industrial sources, is essential for representing winter photochemistry. PANs measurements provide critical constraints on oxidation processes and their implications for emission control.

Controlling ozone (O3) in rapidly urbanizing megacities in Southeast and East Asia remains a challenge. O3 is a secondary pollutant formed through nonlinear photochemical reactions with its precursors: nitrogen oxides (NOx) and volatile organic compounds (VOCs). Observation-based quantification of precursor sensitivity remains scarce, limiting actionable O3 control. To address this, we leverage airborne observations from the NASA DC-8 during the ASIA-AQ campaign conducted in February and March 2024 across four Asian megacities: Metro Manila, the Seoul Metropolitan Area, the Tainan-Kaohsiung Metropolitan Area, and the Bangkok Metropolitan Region. These extensive measurements of various trace gases were used to constrain a zero-dimensional box model and estimate the net production rates of Ox (POx, Ox = O3 + NO2). Precursor sensitivity regimes were characterized for each megacity by generating isopleths of POx across varying levels of NOx and VOCs. The analysis revealed that Manila and Tainan-Kaohsiung exhibited predominantly NOx-sensitive conditions, favoring NOx reduction as an effective O3 mitigation strategy, while Bangkok showed a more mixed sensitivity, suggesting combined NOx and VOC reductions. In contrast, Seoul exhibited a primarily VOC-sensitive regime associated with its higher NOx conditions relative to the other cities, underscoring the importance of VOC-focused strategies. In addition, to quantitatively assess sensitivity transitions, we computed orthogonal distances from the isopleth transition boundaries for all four study areas. Diurnal analyses of these distances revealed a shift from more VOC-sensitive conditions in the morning toward more NOx-sensitive regimes in the afternoon. These findings provide critical insights for formulating effective, city-specific O3 control policies in urban environments.

Terpenoids play a significant role in the formation of tropospheric ozone and secondary organic aerosol. While terpenoids are largely attributed to biogenic sources, they are also widely used in consumer products that end up in the atmosphere. Terpenoid mixing ratios are reported here from samples collected during the Los Angeles (LA) Air Quality Campaign (LAAQC) in 2022 and were compared with data from three other campaigns in the LA Basin conducted between 2010 and 2021. Across all campaigns, differences in diurnal mixing ratios and composition suggest anthropogenic sources are predominant contributors to terpenoid mixing ratios in the evening to early morning (22:00-6:00 PDT), shifting to predominately biogenic sources in the afternoon (10:00-18:00 PDT). This manuscript presents the first evidence for a significant presence of anthropogenic terpenoids in the LA Basin and highlights the need for systematically studying anthropogenic and biogenic terpenoid emissions in urban areas.

Architectural paints represent an important source of volatile organic compounds (VOCs) in urban environments. The ongoing shift from solvent-based to water-based formulations can substantially alter VOC composition, emission factors, and associated secondary formation potentials; however, these effects remain poorly quantified and may introduce biases in current emission inventories. In this study, dynamic-chamber evaporation experiments were performed on selected architectural paint products spanning water-based and solvent-based formulations to obtain their VOC composition profiles and emission factors and to estimate their ozone formation potential (OFP) and secondary organic aerosol formation potential (SOAP). The selected water-based paints were dominated by oxygenated VOCs (OVOCs) and alkenes, with an average total emission factor of 56 g VOC kg-1 paint. In contrast, the solvent-based products contained abundant alkanes, OVOCs, and aromatics, resulting in a higher total emission factor of 228 g VOC kg-1 paint. For the water-based products, the emission-factor-derived OFP was 90 g O3 kg-1 paint, mainly contributed by acetaldehyde and limonene, while the SOAP was 0.8 g SOA kg-1 paint and was also dominated by limonene. In solvent-based products, the emission-factor-derived OFP and SOAP were 482 g O3 kg-1 paint and 5.3 g SOA kg-1 paint, respectively, with aromatics (e.g., toluene and ethylbenzene) and alkanes (e.g., n-octane) being the major contributors to both. Comparison between the mass-fraction- and emission-factor-based calculations showed distinct OFP and SOAP estimates, indicating that apportioning total emissions according to VOC composition may affect the accuracy of assessments of secondary pollution formation potentials. This study provides compound-specific and formulation-resolved emission factors for the selected architectural paint products and suggests that emission-factor-based characterization offers a more reliable assessment of their secondary formation potentials.

Initial success has been achieved in Hong Kong in controlling primary air pollutants, but ambient ozone levels kept increasing during the past three decades. Volatile organic compounds (VOCs) are important for mitigating ozone pollution as its major precursors. This study analyzed VOC characteristics of roadside, suburban, and rural sites in Hong Kong to investigate their compositions, concentrations, and source contributions. Here we show that the TVOC concentrations were 23.05 ± 13.24, 12.68 ± 15.36, and 5.16 ± 5.48 ppbv for roadside, suburban, and rural sites between May 2015 to June 2019, respectively. By using Positive Matrix Factorization (PMF) model, six sources were identified at the roadside site over five years: Liquefied petroleum gas (LPG) usage (33–46%), gasoline evaporation (8–31%), aged air mass (11–28%), gasoline exhaust (5–16%), diesel exhaust (2–16%) and fuel filling (7–9%). Similarly, six sources were distinguished at the suburban site, including LPG usage (30–33%), solvent usage (20–26%), diesel exhaust (14–26%), gasoline evaporation (8–16%), aged air mass (4–11%), and biogenic emissions (2–5%). At the rural site, four sources were identified, including aged air mass (33–51%), solvent usage (25–30%), vehicular emissions (11–28%), and biogenic emissions (6–12%). The analysis further revealed that fuel filling and LPG usage were the primary contributors to OFP and OH reactivity at the roadside site, while solvent usage and biogenic emissions accounted for almost half of OFP and OH reactivity at the suburban and rural sites, respectively. These findings highlight the importance of identifying and characterizing VOC sources at different sites to help policymakers develop targeted measures for pollution mitigation in specific areas.

Understanding and quantifying the global methane (CH4) budget is important for assessing realistic pathways to mitigate climate change. CH4 is the second most important human-influenced greenhouse gas in terms of climate forcing after carbon dioxide (CO2), and both emissions and atmospheric concentrations of CH4 have continued to increase since 2007 after a temporary pause. The relative importance of CH4 emissions compared to those of CO2 for temperature change is related to its shorter atmospheric lifetime, stronger radiative effect, and acceleration in atmospheric growth rate over the past decade, the causes of which are still debated. Two major challenges in quantifying the factors responsible for the observed atmospheric growth rate arise from diverse, geographically overlapping CH4 sources and from the uncertain magnitude and temporal change in the destruction of CH4 by short-lived and highly variable hydroxyl radicals (OH). To address these challenges, we have established a consortium of multidisciplinary scientists under the umbrella of the Global Carbon Project to improve, synthesise, and update the global CH4 budget regularly and to stimulate new research on the methane cycle. Following Saunois et al. (2016, 2020), we present here the third version of the living review paper dedicated to the decadal CH4 budget, integrating results of top-down CH4 emission estimates (based on in situ and Greenhouse Gases Observing SATellite (GOSAT) atmospheric observations and an ensemble of atmospheric inverse-model results) and bottom-up estimates (based on process-based models for estimating land surface emissions and atmospheric chemistry, inventories of anthropogenic emissions, and data-driven extrapolations). We present a budget for the most recent 2010–2019 calendar decade (the latest period for which full data sets are available), for the previous decade of 2000–2009 and for the year 2020. The revision of the bottom-up budget in this 2025 edition benefits from important progress in estimating inland freshwater emissions, with better counting of emissions from lakes and ponds, reservoirs, and streams and rivers. This budget also reduces double counting across freshwater and wetland emissions and, for the first time, includes an estimate of the potential double counting that may exist (average of 23 Tg CH4 yr−1). Bottom-up approaches show that the combined wetland and inland freshwater emissions average 248 [159–369] Tg CH4 yr−1 for the 2010–2019 decade. Natural fluxes are perturbed by human activities through climate, eutrophication, and land use. In this budget, we also estimate, for the first time, this anthropogenic component contributing to wetland and inland freshwater emissions. Newly available gridded products also allowed us to derive an almost complete latitudinal and regional budget based on bottom-up approaches. For the 2010–2019 decade, global CH4 emissions are estimated by atmospheric inversions (top-down) to be 575 Tg CH4 yr−1 (range 553–586, corresponding to the minimum and maximum estimates of the model ensemble). Of this amount, 369 Tg CH4 yr−1 or ∼ 65 % is attributed to direct anthropogenic sources in the fossil, agriculture, and waste and anthropogenic biomass burning (range 350–391 Tg CH4 yr−1 or 63 %–68 %). For the 2000–2009 period, the atmospheric inversions give a slightly lower total emission than for 2010–2019, by 32 Tg CH4 yr−1 (range 9–40). The 2020 emission rate is the highest of the period and reaches 608 Tg CH4 yr−1 (range 581–627), which is 12 % higher than the average emissions in the 2000s. Since 2012, global direct anthropogenic CH4 emission trends have been tracking scenarios that assume no or minimal climate mitigation policies proposed by the Intergovernmental Panel on Climate Change (shared socio-economic pathways SSP5 and SSP3). Bottom-up methods suggest 16 % (94 Tg CH4 yr−1) larger global emissions (669 Tg CH4 yr−1, range 512–849) than top-down inversion methods for the 2010–2019 period. The discrepancy between the bottom-up and the top-down budgets has been greatly reduced compared to the previous differences (167 and 156 Tg CH4 yr−1 in Saunois et al. (2016, 2020) respectively), and for the first time uncertainties in bottom-up and top-down budgets overlap. Although differences have been reduced between inversions and bottom-up, the most important source of uncertainty in the global CH4 budget is still attributable to natural emissions, especially those from wetlands and inland freshwaters. The tropospheric loss of methane, as the main contributor to methane lifetime, has been estimated at 563 [510–663] Tg CH4 yr−1 based on chemistry–climate models. These values are slightly larger than for 2000–2009 due to the impact of the rise in atmospheric methane and remaining large uncertainty (∼ 25 %). The total sink of CH4 is estimated at 633 [507–796] Tg CH4 yr−1 by the bottom-up approaches and at 554 [550–567] Tg CH4 yr−1 by top-down approaches. However, most of the top-down models use the same OH distribution, which introduces less uncertainty to the global budget than is likely justified. For 2010–2019, agriculture and waste contributed an estimated 228 [213–242] Tg CH4 yr−1 in the top-down budget and 211 [195–231] Tg CH4 yr−1 in the bottom-up budget. Fossil fuel emissions contributed 115 [100–124] Tg CH4 yr−1 in the top-down budget and 120 [117–125] Tg CH4 yr−1 in the bottom-up budget. Biomass and biofuel burning contributed 27 [26–27] Tg CH4 yr−1 in the top-down budget and 28 [21–39] Tg CH4 yr−1 in the bottom-up budget. We identify five major priorities for improving the CH4 budget: (i) producing a global, high-resolution map of water-saturated soils and inundated areas emitting CH4 based on a robust classification of different types of emitting ecosystems; (ii) further development of process-based models for inland-water emissions; (iii) intensification of CH4 observations at local (e.g. FLUXNET-CH4 measurements, urban-scale monitoring, satellite imagery with pointing capabilities) to regional scales (surface networks and global remote sensing measurements from satellites) to constrain both bottom-up models and atmospheric inversions; (iv) improvements of transport models and the representation of photochemical sinks in top-down inversions; and (v) integration of 3D variational inversion systems using isotopic and/or co-emitted species such as ethane as well as information in the bottom-up inventories on anthropogenic super-emitters detected by remote sensing (mainly oil and gas sector but also coal, agriculture, and landfills) to improve source partitioning. The data presented here can be downloaded from https://doi.org/10.18160/GKQ9-2RHT (Martinez et al., 2024).

California legislation mandates a 40% reduction in CH4 emissions from 2013 levels by 2030. Achieving this requires an accurate inventory of greenhouse gas sources, including urban CH4. This study determined summertime CH4 and C2-C5 alkane emissions from the Los Angeles (L.A.) Basin using airborne field campaign measurements from 2010, 2019, and 2023. Our analysis combined enhancement ratios of CH4 and alkanes versus CO from in situ airborne measurements with the California Air Resources Board's CO emissions inventory. By incorporating known alkane abundances from various sectors, we apportioned the emission sources. We found an annual decline in CH4 emissions from the L.A. Basin at a rate of -7.2 ± 5.8 Gg/year, consistent with literature and ground-site measurements from the Mount Wilson Observatory. The primary CH4 sources were natural gas (52-57% of total emissions) and CH4-dominant sources, like landfills and dairies (41-47%). We also observed an annual increase in ethane emissions of 0.13 ± 0.19 Gg/year, which correlates with increasing ethane abundance in pipeline natural gas and decreasing ethane prices. If this linearly decreasing CH4 emission trend were to continue, the L.A. Basin would be on track to reach the state's 2030 CH4 emission reduction goals.

Reactive halogens catalytically destroy O3 and therefore affect (1) stratospheric O3 depletion and (2) the oxidative capacity of the troposphere. Reactive halogens also partition into the aerosol phase, but what governs halogen-aerosol partitioning is poorly constrained in models. In this work, we present global-scale measurements of non-sea-salt aerosol (nSSA) bromine and iodine taken during the NASA Atmospheric Tomography Mission (ATom). Using the Particle Analysis by Laser Mass Spectrometry instrument, we found that bromine and iodine are present in 8 %–26 % (interquartile range, IQR) and 12 %–44 % (IQR) of accumulation-mode nSSA, respectively. Despite being commonly found in nSSA, the concentrations of bromine and iodine in nSSA were low but potentially important, at 0.11–0.57 pmol mol−1 (IQR) and 0.04–0.24 pmol mol−1 (IQR), respectively. In the troposphere, we find two distinct sources of bromine and iodine for nSSA: (1) a primary source from biomass burning and (2) a pervasive secondary source. In the stratosphere, nSSA bromine and iodine concentrations increased with increasing O3 concentrations; however, higher concentrations of stratospheric nSSA bromine and iodine were found in organic-rich particles that originated in the troposphere. Finally, we compared our ATom nSSA iodine measurements to the global chemical transport model GEOS-Chem (Goddard Earth Observing System); nSSA bromine concentrations could not be compared because they were not tracked in the model. We found that the model compared well to our ATom nSSA iodine measurements in the background atmosphere but not in the marine boundary layer, biomass burning plumes, or stratosphere.

China is currently the world's largest carbon emitter and striving to mitigate its emissions to meet the carbon neutrality goals. However, due to sparse monitoring networks and uneven economic development, significant uncertainties exist in the country's carbon emission statistics, particularly in urban areas. In order to gain a comprehensive understanding of carbon source-sink characteristics in one of the most thriving regions in China, the Greater Bay Area (GBA), one-year monitoring of CH4, CO2, and CO was conducted at a suburban coastal site in Hong Kong since February 2023. After filtering out local influences, regional data for CH4, CO2 and CO show distinct seasonal variations, with higher concentrations observed during the cold season (2038.0 f 63.8 ppbv, 443.0 f 12.6 ppmv and 213.3 f 64.5 ppbv for CH4, CO2 and CO, respectively) and lower concentrations during the warm season (1948.7 f 76.2 ppbv, 431.6 f 10.1 ppmv and 113.6 f 55.8 ppbv for CH4, CO2 and CO, respectively), influenced by variations in regional source and sink strength and long-distance transport driven by the Asiatic monsoon. Notably, CH4 exhibits a strong correlation with CO, while CO2 demonstrates a weaker correlation with the other two gases, suggesting that Hong Kong possesses unique regional-derived emission pattern compared to other Chinese cities. Back-trajectory and potential source contribution function analyses highlight central China as the primary source regions for CH4 and CO, and eastern and central China as the primary source regions for CO2. These insights are of utmost importance for formulating effective carbon neutrality policies in Hong Kong and surrounding regions in China.

Tropospheric ozone (O3) production from wildfires is highly uncertain; previous studies have identified both production and loss of O3 in fire-influenced air masses. To capture the total ozone production attributable to a smoke plume, we bridge the gap between near-field fire plume chemistry and aged smoke in the remote troposphere. Using airborne measurements from several major campaigns, we find that fire-ozone production increases with age, with a regime transition from NOx-saturated to NOx-limited conditions, showing that O3 production in well-aged plumes is largely controlled by nitrogen oxides (NOx). Observations in fresh smoke demonstrate that suppressed photochemistry reduces O3 production by ∼ 70 % in units of ppb Ox (O3 + NO2) per ppm CO in the near-field (age < 20 h). We demonstrate that anthropogenic NOx injection into VOC-rich fire plumes drives additional O3 production, sometimes exceeding 50 ppb above background. Using a box model, we explore the evolving sensitivity of O3 production to fire emissions and chemical parameters. We demonstrate the importance of aerosol-induced photochemical suppression over heterogeneous HO2 uptake, validate HONO's importance as an oxidant precursor, and confirm evolving NOx sensitivity. We evaluate GEOS-Chem's performance against these observations, finding the model captures fire-induced O3 enhancements at older ages but overestimates near-field enhancements, fails to capture the magnitude and variability of fire emissions, and does not capture the chemical regime transition. These discrepancies drive biases in normalized ozone production (ΔO3/ΔCO) across plume lifetime, though the model generally captures observed absolute O3 enhancements in fire plumes. GEOS-Chem attributes 2.4 % of the global tropospheric ozone burden and 3.1 % of surface ozone concentrations to fire emissions in 2020, with stronger impacts in regions of frequent burning.

Bromine activation (the production of Br in an elevated oxidation state) promotes ozone destruction and mercury removal in the global troposphere and commonly occurs in both springtime polar boundary layers, often accompanied by nearly complete ozone destruction. The chemistry and budget of active bromine compounds (e.g., Br 2 , BrCl , BrO , HOBr ) reflect the cycling of Br and affect its environmental impact. Cyanogen bromide ( BrCN ) has recently been measured by iodide ion high-resolution time-of-flight mass spectrometry ( I - CIMS), and trifluoro methoxide ion time-of-flight mass spectrometry ( CF(3)o---CIMS) during the NASA Atmospheric Tomography Mission second, third, and fourth deployments (NASA ATom), and could be a previously unquantified participant in active Br chemistry. BrCN mixing ratios ranged from below the detection limit (1.5 pptv ) up to as high as 36 pptv (10 s average) and enhancements were almost exclusively confined to the polar boundary layers in the Arctic winter and in both polar regions during spring and fall. The coincidence of BrCN with active Br chemistry (often observable BrO , BrCl and O 3 loss) and high CHBr3 / CH2 Br-2 ratios imply that much of the observed BrCN is from atmospheric Br chemistry rather than a biogenic source. Likely BrCN formation pathways involve the heterogeneous reactions of active Br ( Br 2 , HOBr ) with reduced nitrogen compounds, for example hydrogen cyanide ( HCN /CN- ), on snow, ice, or particle surfaces. Competitive reaction calculations of HOBr reactions with Cl - / Br - and HCN / CN - in solution, as well as box model calculations with bromine chemistry, confirm the viability of this formation channel and show a distinct pH dependence, with BrCN formation favored at higher pH values. Gas-phase loss processes of BrCN due to reaction with radical species are likely quite slow and photolysis is known to be relatively slow ( BrCN lifetime of similar to 4 months in midlatitude summer). These features, and the lack of BrCN enhancements above the polar boundary layer, imply that surface reactions must be the major loss processes. The fate of BrCN determines whether BrCN production fuels or terminates bromine activation. BrCN reactions with other halogens ( Br - , HOCl , HOBr ) may perpetuate the active Br cycle; however, preliminary laboratory experiments showed that BrCN did not react with aqueous bromide ion ( < 0.1 %) to reform Br 2 . Liquid-phase reactions of BrCN are more likely to convert Br to bromide ( Br - ) or form a C - Br bonded organic species, as these are the known condensed-phase reactions of BrCN and would therefore constitute a loss of atmospheric active Br. Thus, further study of the chemistry of BrCN will be important for diagnosing polar Br cycling.

The ozone air quality standard is regularly surpassed in the Los Angeles air basin, and efforts to mitigate ozone production have targeted emissions of precursor volatile organic compounds (VOCs), especially from mobile sources. In order to assess how VOC concentrations, emissions, and chemistry have changed over the past decade, VOCs were measured in this study using a Vocus-2R proton-transfer reaction time-of-flight mass spectrometer in Pasadena, California, downwind of Los Angeles, in summer 2022. Relative to 2010, ambient concentrations of aromatic hydrocarbons have declined at a similar rate as carbon monoxide, suggesting reduced overall emissions from mobile sources. However, the ambient concentrations of oxygenated VOCs have remained similar or increased, suggesting a greater relative importance of oxidation products and other emission sources, such as volatile chemical products whose emissions are largely unregulated. Relative to 2010, the range of measured VOCs was expanded, including higher aromatics and additional volatile chemical products, allowing a better understanding of a wider range of emission sources. Emission ratios relative to carbon monoxide were estimated and compared with 2010 emission ratios. Average measured ozone concentrations were generally comparable between 2022 and 2010; however, at the same temperature, daytime ozone concentrations were lower in 2022 than 2010. Faster photochemistry was observed in 2022, with average hydroxyl radical exposure being similar to 68% higher during midday (statistically significant at 95% confidence), although this difference reduces to similar to 35% when comparing observations at ambient temperatures of 25-30 degrees C only. Future trends in temperature are important in predicting ozone production.

This study shows that volatile and semi-volatile organic gases are emitted during light and heavy braking. These emissions will remain as we transition to zero exhaust emission vehicles and must be included in air pollution and climate assessments.

Proton-transfer-reaction time-of-flight mass spectrometry (PTR-ToF-MS) is a technique commonly used to measure ambient volatile organic compounds (VOCs) in urban, rural, and remote environments. PTR-ToF-MS is known to produce artifacts from ion fragmentation, which complicates the interpretation and quantification of key atmospheric VOCs. This study evaluates the extent to which fragmentation and other ionization processes impact urban measurements of the PTR-ToF-MS ions typically assigned to isoprene (m/z 69, C5H8H+), acetaldehyde (m/z 45, CH3CHO+), and benzene (m/z 79, C6H6H+). Interferences from fragmentation are identified using gas chromatography (GC) pre-separation, and the impact of these interferences is quantified using ground-based and airborne measurements in a number of US cities, including Las Vegas, Los Angeles, New York City, and Detroit. In urban regions with low biogenic isoprene emissions (e.g., Las Vegas), fragmentation from higher-carbon aldehydes and cycloalkanes emitted from anthropogenic sources may contribute to m/z 69 by as much as 50 % during the day, while the majority of the signal at m/z 69 is attributed to fragmentation during the night. Interferences are a higher fraction of m/z 69 during airborne studies, which likely results from differences in the reactivity between isoprene and the interfering species along with the subsequent changes to the VOC mixture at higher altitudes. For other PTR masses, including m/z 45 and m/z 79, interferences are observed due to fragmentation and O2+ ionization of VOCs typically used in solvents, which are becoming a more important source of anthropogenic VOCs in urban areas. We present methods to correct these interferences, which provide better agreement with GC measurements of isomer-specific molecules. These observations show the utility of deploying GC pre-separation for the interpretation PTR-ToF-MS spectra.