We present results from a Very Large Methane Release (VLMR) experiment evaluating methane retrievals from the Geostationary Operational Environmental Satellites (GOES) Advanced Baseline Imagers (ABIs) and multiple low-Earth-orbit imagers with high point-source detection limits. The experiment coordinated observations of a U.S. gas pipeline blowdown with nine satellites, two aircraft, and a truck-based mobile laboratory. We used the GOES-16, -18, and -19 ABIs with revisits every 10 min to 7 s to quantify release magnitude and uncertainty. Best methane retrieval precision (7–8%) was achieved in the 7-s and 30-s mesoscale scan modes averaged to 5 min. Source-rate and mass estimates are broadly consistent across measurement platforms. Detectable emissions totaled 370±30 t over 44–65 min from two release points, ~25% lower than bottom-up expectations based on pipeline volume and nominal pressure, likely due to late-stage emissions below satellite detection limits. Our work provides a framework for evaluating high-detection-limit methane point-source imagers.

Abstract Biomass burning (BB) is a major global sources of ozone (O 3 ) precursors, including nitrogen oxides (NO x ), carbon monoxide (CO), and volatile organic compounds (VOCs). Here, we combine Community Earth System Model 1 with airborne measurements from the NASA Atmospheric Tomography mission conducted during 2016–2018 to assess the global impact of aged BB emissions on O 3 from the surface to the lower stratosphere. The model captures the broad spatial distribution of BB‐related tracers and O 3 in the Atlantic, but it significantly underestimates BB‐related aerosol concentrations by an order of magnitude above 3 km in the tropical and Northern Hemispheric Pacific, leading to an underestimation of BB‐induced O 3 concentrations. In the upper troposphere and lower stratosphere over the tropical Atlantic, BB‐induced O 3 enhancements are most pronounced during August to November, with peak values reaching up to 30 ppbv in October. These enhancements are primarily driven by the oxidation of BB‐emitted VOCs and CO, followed by the photolysis of the NO 2 . The Asian summer monsoon circulation plays a crucial role in lofting these precursors into higher altitudes, where NO x is converted into its reservoir form, peroxyacetyl nitrate. These findings demonstrate BB's extensive vertical and horizontal influence on atmospheric O 3 , underscoring the importance of accurately representing BB processes in climate and air quality models to better assess their impacts on global air pollution and climate change.

Abstract We thank Millet et al. for their comment in discussing the methodology and conclusions reached in Francoeur et al. (2025, https://doi.org/10.1029/2025gl115286). In general, we agree with the key points of the comment, although with significant caveats. Mainly, we agree that the thermal infrared is capable of constraining ethane emissions from space. While we also agree that spectral signals do not change in a 1:1 manner with ethane emissions, our chemical transport and radiative transfer modeling suggest that the relationship is 0.72:1. Lastly, while spectral signals are affected by vertical sensitivity and regional background, these factors are not large enough to significantly alter how trends in spectral signals reflect trends in ethane emissions. We hope that the differences observed between Francoeur et al. (2025, https://doi.org/10.1029/2025gl115286) and Brewer et al. (2024, https://doi.org/10.1038/s41467‐024‐52247‐z) lead to further study that advances the ability to detect atmospheric composition in the thermal infrared.

Biomass burning (BB) is the second largest global source of organic aerosol (OA), with its impact on climate and air quality expected to rise with global warming. Aircraft measurements of BB plumes show a rapid increase in the OA oxidation state, which could not be reproduced in simulation chambers based on gas-phase oxidation. Here, we investigate isolation key aging processes of BB emissions in controlled chamber experiments, including the effects of ultraviolet-A irradiation, OH-driven secondary OA formation, heterogeneous OH oxidation or ozonolysis, and evaporation. We find that ultraviolet-A irradiation can drive rapid intraparticle OA aging within hours, proceeding an order of magnitude faster than gas-phase oxidation. Other mechanisms were even slower in producing oxygenated OA (OOA). Based on our experiments, we estimate that this process proceeds with a quantum yield of ∼0.4%, meaning that four in every thousand photons absorbed by organic chromophores in BBOA generate radicals in the particle phase that rapidly oxidize the particulate organics. We further analyzed aircraft measurements of wildfire and prescribed burn plumes, showing that photoinduced processes can account for much of the rapid OOA formation. We estimate that, in BB-influenced regions, over half of OOA may originate from photo-oxidized primary OA within the first day. Our findings challenge the prevailing paradigm that gas-phase oxidation dominates BBOA aging and highlight a previously underappreciated, but dominant role of intraparticle photochemistry.

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 As large wildfires become more frequent and severe in North America due to heightened fuel aridity and fire weather conditions driven by anthropogenic climate change, wildfire smoke has an increasingly important influence on air quality. Transported smoke can impact urban boundary layer ozone (O 3 ) directly, via the entrainment of O 3 formed in smoke, or indirectly, through the influence of pyrogenic volatile organic compounds (VOCs) and aerosols on local O 3 production. Widespread smoke impacts from the record‐breaking 2023 Canadian wildfire season coincided with the July–August 2023 Airborne Emissions and Reactions Observed from Megacities to Marine Areas (AEROMMA) aircraft‐based field campaign, which provided extensive in situ observations of urban chemistry in North American megacities. Vertically resolved trace gas and aerosol measurements from the August 2 AEROMMA research flight in Chicago indicated the entrainment of diffuse smoke from an aged, lofted wildfire plume into the urban boundary layer and were used to constrain a zero‐dimensional photochemical model and a radiative transfer model. We perturbed the models across a range of smoke densities and urban nitrogen oxide emissions and found net pyrogenic O 3 enhancements of 3–17 ppbv, with the largest contribution from transported smoke O 3 (65%–84%), followed by locally produced photochemical O 3 from smoke VOCs (18%–46%), and finally smoke aerosol shading (−12%–0%). Although these results are specific to the observed August 2 smoke plume and Chicago urban chemistry, the modeling framework presented could be applied to other smoke‐impacted locations to disentangle the various effects of wildfire smoke on urban O 3 .

Wildfires are becoming more prevalent and emissions from wildfires can have an impact on public health and atmospheric chemistry, with emissions often travelling long distances from the source. Biomass burning produces carbon monoxide (CO), formaldehyde (HCHO), and nitrogen dioxide (NO 2 ), among other species. Understanding how these emissions evolve downwind is important to understanding these impacts. Enhancement rates of these molecules were calculated using single-overpass TROPOspheric Monitoring Instrument (TROPOMI) measurements, MODerate resolution Imaging Spectroradiometer (MODIS) fire radiative power (FRP), a 1-D advection model, and the Gaussian-flux method for sixteen plumes as they aged downwind from nine boreal forest wildfires in Western Canada during summer 2023. Rates were also analysed and compared to data provided for three fires measured during the summer 2019 Fire Influence on Regional to Global Environments and Air Quality (FIREX-AQ) campaign. Calculated rates were generally within but slightly lower than provided FIREX-AQ ranges due to diurnal differences between satellite and in-situ measurements. Spatiotemporal variation across all summer 2023 plumes confirm CO emission primarily from the fire centre, with HCHO and NO 2 showing primarily secondary formation potentially due to mixing with atmospheric oxidants. Enhancement ratios (EnhR) of calculated vertical column density enhancements for HCHO/CO, NO 2 /CO, and HCHO/NO 2 showed moderate to low average correlation (R 2 = 0.40, 0.23, 0.49, respectively), with average slopes consistent with in-situ and satellite literature values, within uncertainty, for HCHO/CO and NO 2 /CO. Our results demonstrate the effectiveness of TROPOMI and the Gaussian-flux method for analysing downwind evolution of wildfire emissions.

The first 5 hours of aging in biomass burning plumes can strongly affect ozone photochemistry. We examine how volatile organic compounds (VOCs), nitrogen oxides, and nitrous acid influence hydroxyl radical, ozone, and peroxyacetyl nitrate (PAN) based on three aircraft campaigns over the United States. Our analyses reveal variable, highly elevated hydroxyl radical concentrations in the first 2 hours, resulting in evident fire-to-fire variability in VOCs oxidation and in ozone and PAN production. About 40 to 70% of the variability is explained by chemical aging. Ozone production in the plumes is usually VOC-limited for the first 2 hours and then nitrogen oxide limited downwind. Box model results for hydroxyl radical, ozone, and most VOCs, using the full, explicit Master Chemical Mechanism (MCM) mechanism, suggest no major gaps in the current best knowledge of gas-phase chemistry. However, the MCM sometimes overestimates PAN due to underestimated nitrogen oxide sinks. GEOS-Chem, a widely used chemical transport model with a reduced mechanism, generally underperforms because of incomplete VOC representation. We identify these critical pathways to guide future model development.

The operation of geostationary (GEO) instruments such as the Tropospheric Emissions: Monitoring of Pollution (TEMPO) provides unprecedented hourly nitrogen dioxide (NO2) observations compared to the once-daily data from a low-Earth orbit (LEO) platform like the TROPOspheric Monitoring Instrument (TROPOMI). This study investigates the performance and challenges of using TEMPO versus TROPOMI measurements to constrain anthropogenic nitrogen oxides (NOx) emissions. The accuracy of TEMPO and TROPOMI NO2 tropospheric columns are assessed using Pandora observations, finding a low bias of 9%-12.3% in TEMPO, and TROPOMI data during August 2023, while TEMPO midday and late afternoon observations are less of low bias. Top-down NOx emissions derived by midday TEMPO and TROPOMI data are generally consistent over urban areas, being 5%-20% lower than bottom-up emissions provided by the 2021 GReenhouse gas And Air Pollutants Emissions System (GRA2PES), and align with 2023 GRA2PES emissions, demonstrating the reliability of using satellite data for timely updates of bottom-up inventories. However, assimilating additional morning/late afternoon TEMPO data leads to the poorest top-down NOx emissions, likely resulting from larger negative measurement biases. NOx emission inversions effectively mitigate NOx overprediction, though the top-down NOx emissions might be over-corrected in urban cores. NOx emissions optimization also improves ozone forecasts by reducing the model's positive biases, especially when assimilating midday TEMPO data. Our study suggests that TEMPO midday observations provide better constraints on the magnitude and spatiotemporal variation of anthropogenic NOx emissions than TROPOMI, while morning TEMPO v3 data should be used cautiously due to potential negative impact on NOx emissions inversion.

Tropospheric ozone over the South and Tropical Atlantic plays an important role in the photochemistry and energy budget of the atmosphere. In this remote region, tropospheric ozone estimates from reanalysis datasets show the largest discrepancies. The present study characterises the vertical and horizontal distribution of tropospheric ozone over the South and Tropical Atlantic during February and October 2017 using amultispectral synergism called IASI + GOME2, two global chemistry reanalysis products-the Copernicus Atmosphere Monitoring Service reanalysis (CAMS reanalysis) and the Tropospheric Chemistry Reanalysis version 2 (TCR-2)-and in situ airborne measurements from the Atmospheric Tomography Mission (ATom). In February, biomass burning in West and Central Africa and deep convection over the Gulf of Guinea strongly influence the region. IASI + GOME2 captures enhanced ozone abundances in the Southern Hemisphere and low ozone concentrations in the Northern Hemisphere, exhibiting excellent agreement with ATom-2 profiles. In contrast, both reanalyses underestimate lower-tropospheric ozone influenced by biomass-burning outflow and overestimate ozone in the Northern Hemisphere due to excessive contributions from stratospheric intrusion and North American anthropogenic emissions. In October, tropospheric ozone enhancement associated with biomass-burning outflow from Austral Africa is consistently depicted by observations and reanalyses. These results emphasize the need to evaluate the seasonal variability of each of the multiple sources of ozone precursors within atmospheric chemistry reanalyses.

Ozone profile measurements at high temporal and vertical resolution are needed to better understand physical processes driving tropospheric ozone variability and to validate the tropospheric ozone measurements from spaceborne missions such as TEMPO (Tropospheric Emissions: Monitoring Pollution). As part of the Tropospheric Ozone Lidar Network (TOLNet) efforts allocated to provide such measurements, and leveraging on the experience of more than 20 years of ozone lidar measurements at Table Mountain Facility, the JPL lidar group developed the SMOL (Small Mobile Ozone Lidar), an affordable differential absorption lidar (DIAL) system covering all altitudes from 150 m to 10 km a.g.l. In this abstract, we will review the main characteristics of SMOL, the preliminary results of its first field deployment during the Synergistic TEMPO Air Quality Science (STAQS) and Atmospheric Emissions and Reactions Observed from Megacities to Marine Areas (AEROMMA) campaigns in summer 2023, as well as discuss upcoming deployments and collaborations.

Declining nitrogen oxide (NOx = NO + NO2) emissions have transformed oxidation pathways in urban atmospheres, with implications for air quality. Organic peroxy radicals (RO2), key intermediates in volatile organic compound oxidation, typically react with NO to form ozone (O3). Under lower-NO conditions, alternative RO2 fates, including isomerization forming highly oxidized organic molecules (HOMs), can enhance secondary organic aerosol (SOA) production. We combine aircraft observations over four major North American cities with geostationary satellite data to characterize isoprene-derived RO2 fate across urban environments. We infer RO2 bimolecular lifetimes (τbi) as a proxy for isomerization potential, finding longer τbi (17 ± 11 seconds) in New York, Chicago, and Toronto compared to Los Angeles (7 ± 6 seconds). Satellite measurements reveal that long τbi is widespread across urban North America, suggesting that declining NOx is likely to lead to greater HOM formation in urban regions. These findings indicate that atmospheric models omitting RO2 isomerization chemistry may incorrectly simulate organic oxidation and the subsequent oxidation state of volatile organic compounds and SOA.

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.

Wildfires are the largest terrestrial source of atmospheric ammonia (NH3), yet their impacts on NH3 concentrations and ammonium (NH4 +) deposition remain poorly quantified. In this study, we evaluate the effects of the record-breaking 2023 Canadian wildfire season on NH3 concentration and NH4 + deposition across the Upper Midwest. This study integrates satellite observations, ground-based data, and in situ aircraft measurements. In May-June 2023, NH3 concentrations increased at 83% of ground sites, and NH4 + deposition flux rose at 100% of ground sites in the Upper Midwest. Satellite data showed significantly higher column-averaged NH3 in 47% of grid cells in the Upper Midwest. On 1 August, a smoke plume over the Midwest corresponded with an AEROMMA flight observing enhanced NH3, NH4 +, carbon monoxide, and acetonitrile. These findings highlight the substantial impact of wildfire smoke on NH3 and NH4 + at regional scales, with implications for nitrogen cycling, air quality, and atmospheric modeling.

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.

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.

Despite ongoing reductions in emissions of ozone (O3) precursors, nitrogen oxides (NOx = NO + NO2) and volatile organic compounds (VOCs), the three largest urban areas in the United States ─ New York City (NYC), Chicago, and Los Angeles (LA) ─ continue to exceed national air quality standards for O3. Airborne measurements during the 2023 Atmospheric Emissions and Reactions Observed from Megacities to Marine Areas (AEROMMA) campaign investigated nonlinear O3 photochemistry in these cities. We report mean ozone production efficiency (OPE), the enhancement ratio of Ox (= O3 + NO2) to NOx oxidation products, of 9 ± 4 (1σ), 6 ± 3, and 6 ± 3 ppbv ppbv-1 in NYC, Chicago, and LA, respectively. Compared to historical values, OPE has increased in NYC but remains constant in LA. We find that OPE during AEROMMA has a nonlinear, inverse relationship with total reactive nitrogen (NOy, a proxy for initial NOx) and a positive correlation with the nonmethane VOC to NOy enhancement ratio. A zero-dimensional photochemical model supports these observed OPE dependences on NOx and VOCs and shows that OPE is a distinct metric from total O3 production that is informative to the development of O3 pollution control strategies. We find that OPE values have higher variability, and a larger increase with NOx emissions reductions, in areas that experience NOx-sensitive rather than NOx-saturated O3 photochemistry; nonetheless, NOx reductions under NOx-sensitive conditions still reduce total O3 production despite the corresponding increase in OPE.