Ultrafine particles impact climate and weather as a main source of cloud condensation nuclei. The vertical transport of boundary layer pollutants to the upper troposphere during convection drives new particle formation. During midlatitude summers, atmospheric convection occurs frequently. However, the extent to which convection influences upper tropospheric particle formation remains unclear. Herein, we report measurements of ultrafine particles in the upper troposphere during the Deep Convective Clouds and Chemistry (DC3) campaign. Across 21 flights tracking convective outflow, several flights exhibited elevated concentrations of particles with diameters of less than 100 nm (N 100 nm), often exceeding 1000 cm-3. One particular flight followed and intersected a plume downwind several times showing an ultrafine particle growth rate of 1.4 +/- 0.1 nm h-1 due to photochemical aging. Condensation of sulfuric acid can only explain a small fraction of this growth. Gas-phase measurements indicate that oxidized organic vapors likely contribute to particle growth in this region. Future measurements are needed to constrain these processes in atmospheric models, particularly of the molecular speciation of gas-phase precursors. Upper tropospheric ultrafine particles in this region may become more important as climate models project increasing frequency of atmospheric convection in the future.

Abstract The Unified Forecast System (UFS) is a community‐based Earth modeling system designed to support operational forecasts at the National Oceanic and Atmospheric Administration (NOAA), while also facilitating the integration of research advances from the broader scientific community. The Configurable ATmospheric Chemistry (CATChem) library and modeling component is being developed to include comprehensive chemical and aerosol processes for representing atmospheric composition through a flexible, easy‐to‐modify, and well‐documented infrastructure. Here CATChem version 1.0 (v1.0) is linked to the UFS High Resolution 3 configuration to create the Unified Forecast System with Chemistry (UFS‐Chem) v1.0. The configurability of UFS‐Chem enables its use for both research and operational applications, reducing time and effort for transitions to operations and enhancing collaboration with the research community. As a first step toward this goal, the gas‐phase chemistry from the Atmosphere Model version 4.1 (AM4.1), developed at NOAA Geophysical Fluid Dynamics Laboratory (GFDL), is incorporated into CATChem and linked to the UFS as the first UFS‐Chem configuration for global air quality applications. The simulated atmospheric compositions are generally consistent with those in GFDL‐AM4.1 and agree well with surface observations, aircraft measurements, and satellite retrievals (with biases mostly within 30%), demonstrating atmospheric chemistry is reasonably well represented in the model. This work documents model uncertainties and biases in UFS‐Chem v1.0 to help prioritize further improvements in emissions and process‐level representations. The new global configuration is shown to be robust in representing atmospheric composition and chemical processes and serves as a foundation for future development.

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.

Quantifying the abundance of volatile organic compounds (VOCs) and accurate representation of their oxidative processing in air quality models are essential for effective air quality management. The NASA ASIA-AQ (Airborne and Satellite Investigation of Asian Air Quality) campaign, conducted in February–March 2024, represents a collaborative international effort to investigate the drivers of air pollution—both local and transported—across four key regions: South Korea, Philippines, Taiwan, and Thailand. As part of this effort, more than 100 C1–C10 VOCs were measured using a suit of state-of-the-art in-situ instruments such as the NSF NCAR Trace Organic Gas Analyzer equipped with a high-resolution time-of-flight mass spectrometer (TOGA-TOF) and the UC Irvine Whole Air Sampler (WAS), all on-board the NASA DC-8 aircraft during its final mission. Here, we present the measured VOC speciated abundance and OH reactivity (OHR) in these four regions. Thailand, largely affected by regional biomass burning emissions during ASIA-AQ, had the highest VOC loading of all the sampled regions and non-methane VOCs (NMVOCs) contributed to approximately half of its total OHR. We complement the OH reactivity analysis with F0AM box modeling simulations to discuss the contribution of unmeasured species. Our results suggest that non-measured NMVOCs contribute up to 40% of OHR. Furthermore, we compare the OHR calculated results against the MUSICAv0 (Multi-Scale Infrastructure for Chemistry and Aerosols version 0) modeling results. Preliminary results suggest that MUSICAv0 broadly captured the total NMVOC OH reactivity but with differences noted, and that significant model/measurement discrepancies were found for specific compounds and compound classes in some regions. We further expand this analysis and quantify the sensitivity of the modeling results to the chemical mechanism complexity used in the simulations.

Airborne observations from ACCLIP on 2 August 2022, combined with Lagrangian particle dispersion model back trajectories, reveal that SO2 mixing ratios at 14-16 km were enhanced by a factor of 4-6 in regions influenced by tropical cyclones (TCs). These enhancements are linked to rapid lofting of marine dimethyl sulfide (DMS) into the upper troposphere (UT). GEOS-Chem simulations indicated that on 31 July 2022, TC-scale circulation injected DMS into the UT within hours, with a mean flux of 9.4 kg hr-1 across 0.5-12 km and 8.4% of emissions penetrating above 12 km, consistent with observations of elevated DMS at the same altitudes. Because of its low solubility and longer UT lifetime (59.2 vs. 5.7 hr at the surface), DMS sustains SO2 production that is largely resistant to wet scavenging. This TC-driven pathway provides a significant natural SO2 source in the UT, with implications for aerosol-cloud-climate interactions.

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.

Abstract. Clouds facilitate the transformation of atmospheric gases and particles, yet the impact of cloud processing on organic compounds remains poorly understood due to the paucity of routine measurements within aqueous samples (e.g., cloud water and precipitation). This study presents seven years (2018–2024) of routine summertime measurements of three major Low Molecular Weight Organic Acids (LMWOA: formate, acetate and oxalate) in cloud water samples collected from the summit of Whiteface Mountain in the northeastern United States, including their contributions to Dissolved Organic Carbon (DOC), ion balance, and cloud water acidity, with critical evaluation of sample handling procedures to minimize volatilization, microbial degradation and contamination. Formate and acetate were the dominant monocarboxylic acids, exhibiting seasonal variability consistent with changes in biogenic emissions, whereas oxalate showed higher concentrations in smoke-impacted clouds. A growing fraction of samples exhibit surplus ammonium relative to sulfate and nitrate concentrations, which previous research hypothesized results from unmeasured organic acids, consistent with the positive relationship between surplus ammonium and LMWOA concentrations observed in the current study. An observed correlation between oxalate and DOC, with higher slope at higher ozone concentrations, supports enhanced in-cloud secondary production of oxalate under high oxidant levels. A comparison of "Aged" versus "Fresh" wildfire smoke-influenced samples suggest that aging can enhance ammonium, with heavier organic acids dominating the DOC pool and acidity, whereas fresh plumes were primarily influenced by directly emitted LMWOA. This study highlights the need for continued monitoring of the evolving cloud water chemistry to better understand the broader impacts on atmospheric chemistry.

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.

Formaldehyde (HCHO) is a ubiquitous atmospheric constituent, originating from primary emissions (natural and anthropogenic) and secondary production via the oxidation of volatile organic compounds (VOCs). In addition to being a regulated pollutant, HCHO is a key species used as a tracer of recent photochemical activity due to its short atmospheric lifetime and its role as a source of HOx radicals. Given its diverse sources and high spatial variability, HCHO is challenging to represent accurately in chemical transport models, often resulting in significant discrepancies with observations. Airborne in situ measurements of HCHO, especially when combined with VOC precursor data, offer valuable insights into its atmospheric distributions for evaluating models. Here, we present HCHO observations from the NSF NCAR Trace Organic Gas Analyzer with Time-of-Flight mass spectrometer (TOGA-TOF), deployed during the 2019 Fire Influence on Regional to Global Environments and Air Quality (FIREX-AQ) campaign. While most HCHO instruments target at most a few selected species for measurement, the TOGA-TOF employs a rapid gas chromatography-mass spectrometry (GC/MS) technique and provides discrete VOC measurements - including > 100 C-1-C-10 species - at a time resolution of less than 2 min. We compare TOGA-TOF HCHO data to measurements from three 1 Hz instruments aboard the NASA DC-8: the Compact Atmospheric Multi-species Spectrometer (CAMS), the In Situ Airborne Formaldehyde (ISAF) instrument, and a proton-transfer-reaction time-of-flight mass spectrometer (PTR-ToF-MS). The wide dynamic range of observed HCHO concentrations (from < 100 ppt to similar to 100 ppb) during FIREX-AQ enabled a robust intercomparison. TOGA-TOF HCHO agreed well with CAMS (slope = 1.1), with similar agreement with the PTR-ToF-MS, while larger discrepancies were observed with ISAF (slope = 1.5), likely due to differences in calibrations. Normalized excess mixing ratios (NEMRs) of HCHO relative to CO in wildfire plumes exhibited consistent trends with plume age across instruments. These findings highlight the TOGA-TOF's capability for highly sensitive and accurate airborne HCHO measurements.

Abstract. Most global atmospheric chemistry models represent ≥C4 alkanes using lumped surrogates, limiting both detailed simulation of their oxygenated products and evaluation against comprehensive observational datasets. We present MOZART-T3, which replaces the lumped BIGALK representation with a more explicit treatment of individual C4−C6 alkane species and resolves propane peroxy radical isomers, enabling more mechanistically consistent alkane chemistry in the Community Atmosphere Model with chemistry (CAM-chem). Global simulations demonstrate that T3 maintains similar total alkane burdens compared to previous mechanisms, while substantially altering oxygenated product budgets and distributions. Relative to MOZART-T1, T3 significantly reduces the global burden of methyl ethyl ketone (MEK) primarily through incorporation of more comprehensive n-butane oxidation chemistry, with additional contributions from increased photolysis rates and updated emission speciation. T3 introduces six additional C5−C6 ketone species that contribute ∼40% to global ketone sources but only ∼2% to the total burden due to their short lifetimes. The acetaldehyde burden decreases by 8−14% through compound-specific yields that replace the fixed yield in previous mechanisms. The choice of anthropogenic emission inventory drives larger variations in alkane burdens (∼24%) than does mechanism complexity (∼4%), but mechanism choice dominates for oxidation products. T3 enables evaluation of previously unrepresented species including individual alkanes, propanal, and peroxypropionyl nitrate, with generally improved simulation of oxygenated compounds, although the evaluation results vary temporally and spatially. While lumped approaches sufficiently represent global-scale major pollutant concentrations, T3’s detailed treatment enables more comprehensive evaluation and is expected to be more important for urban air quality applications using higher-resolution regional simulations.

Abstract Airborne observations from ACCLIP on 2 August 2022, combined with Lagrangian particle dispersion model back trajectories, reveal that SO 2 mixing ratios at 14–16 km were enhanced by a factor of 4–6 in regions influenced by tropical cyclones (TCs). These enhancements are linked to rapid lofting of marine dimethyl sulfide (DMS) into the upper troposphere (UT). GEOS‐Chem simulations indicated that on 31 July 2022, TC‐scale circulation injected DMS into the UT within hours, with a mean flux of 9.4 kg hr −1 across 0.5–12 km and 8.4% of emissions penetrating above 12 km, consistent with observations of elevated DMS at the same altitudes. Because of its low solubility and longer UT lifetime (59.2 vs. 5.7 hr at the surface), DMS sustains SO 2 production that is largely resistant to wet scavenging. This TC‐driven pathway provides a significant natural SO 2 source in the UT, with implications for aerosol–cloud–climate interactions.

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.

Formaldehyde (HCHO) in the atmosphere is an intermediate product from the oxidation of methane and non-methane volatile organic compounds. In remote marine regions, HCHO variability is closely related to atmospheric oxidation capacity, and modeled HCHO in these regions is usually added as a global satellite HCHO background. Thus, it is important to understand and validate the levels of satellite HCHO over the remote oceans. Here we intercompare three satellite retrievals of total HCHO columns from the Ozone Monitoring Instrument Smithsonian Astrophysical Observatory (OMI SAO (v004)) algorithm, Ozone Mapping and Profiler Suite on Suomi National Polar-orbiting Partnership Smithsonian Astrophysical Observatory (OMPS-NPP SAO) algorithm, and Ozone Monitoring Instrument Belgian Institute for Space Aeronomy (OMI BIRA) algorithm and validate them against in situ observations from the NASA Atmospheric Tomography Mission (ATom) mission. All retrievals are correlated with ATom-integrated columns over remote oceans, with OMI SAO (v004) showing the best agreement. This is also reflected in the mean bias (MB) for OMI SAO (−0.73 ± 0.87) × 1015 molec. cm−2, OMPS SAO (−0.76 ± 0.88) × 1015 molec. cm−2, and OMI BIRA (−1.40 ± 1.11) × 1015 molec. cm−2. We recommend the OMI-SAO (v004) retrieval for remote-ocean atmosphere studies. Three satellite HCHO retrievals and in situ ATom columns all generally captured the spatial and seasonal distributions of HCHO in the remote-ocean atmosphere. Retrieval bias varies by latitude and season, but a persistent low bias is found in all products at high latitudes, and the general low bias is most severe for the OMI BIRA product. Examination of retrieval components reveals that slant column corrections have a larger impact on the retrievals over remote marine regions, while AMFs play a smaller role. This study informs us that the potential latitude-dependent biases in the retrievals require further investigation for improvement and should be considered when using marine HCHO satellite data, and vertical profiles from in situ instruments are crucial for validating satellite retrievals.

We present global airborne observations of acetyl peroxynitrate (CH3C(O)OONO2, PAN) in the remote troposphere from the Atmospheric Tomography (ATom) campaign. These observations show that biomass burning is the dominant source of PAN in the Southern Hemisphere (SH). In the Northern Hemisphere, anthropogenic emissions from Asia and Europe also contribute significantly to PAN over the Pacific and Atlantic Oceans. Model simulations underestimate PAN in the lower troposphere, in part, due to the underestimation of local production driven by acetaldehyde oxidation and beta NO2 ${\beta }_{{\text{NO}}_{2}}$ (the ratio of acetyl peroxy radicals reacting with NO2 relative to other pathways). The significant impacts of biomass burning evident in the ATom PAN observations suggest that improving model treatment of plume transport and the conversion of NOx to PAN in biomass burning plumes is a viable focus for better simulating PAN. Global observations of PAN provide a benchmark for the evaluation of satellite observations and model simulations of PAN.

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.

This study presents a comprehensive data set tracking the trajectory of highly oxidized molecules (HOMs) produced from the ozonolysis of monoterpenes across the gas to particle conversion process. A particular focus was to examine the ultimate fate of HOMs in suspended particles after the completion of gas phase chemistry. The observed dynamics of HOMs, therefore, provides direct insights into the role of condensed-phase chemistry, if any, in modifying their molecular composition. Individual SOA-bound HOMs exhibited remarkably diverse behaviors, and even certain isomers of identical elemental composition appeared to undergo different transformations. A small group of C8-10 and C20 HOMs decayed rapidly in hydrated particles, on a time scale akin to those of organic hydroperoxides. On the other hand, a subset of C12-20 dimers was found to continuously grow in abundance, and the presence of water substantially enhanced their growth rates. This rising trend was well captured by box model simulations that incorporate the kinetics of condensed-phase chemistry, specifically the peroxyhemiacetal formation pathway. The majority of HOMs remained structurally stable across all hydrous and photolytic conditions investigated in this study. This persistence highlights the potential of HOMs as a widespread and sustained source of cloud condensation nuclei in the atmosphere.

The Asian Summer Monsoon (ASM) has garnered attention in recent years for its impacts on the composition of the upper troposphere and lower stratosphere (UTLS) via deep convection. A recent observational effort into this mechanism, the Asian Summer Monsoon Chemical and CLimate Impact Project (ACCLIP), sampled the composition of the ASM UTLS over the northwestern Pacific region during boreal summer 2022 using two airborne platforms. In this work, we integrate Lagrangian trajectory modeling with convective cloud top observations to diagnose ASM convective transport which contributed to ACCLIP airborne observations. This diagnostic is applied to explore the properties of convective transport associated with prominent ASM sub‐systems, revealing that for species ranging in lifetime from days to months, transport from convection along the East Asia Subtropical Front was generally associated with more UTLS pollutants than transport from convection over South Asia. The convective transport diagnostic is used to isolate three convective transport events over eastern Asia which had distinct chemical tracer relationship behaviors, indicating the different economical behaviors of the contributing source regions. One of these transport events is explored in greater detail, where a polluted air mass was sampled from convection over the Northeast China Plain which may have been high enough in altitude to impact the composition of the stratosphere. Overall, the presented diagnosis of convective transport contribution to ACCLIP airborne sampling indicates a key scientific success of the campaign and enables process studies of the climate interactions from the two ASM sub‐systems.