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Advances in understanding of tropospheric halogen chemistry

Session Chairs: Becky Alexander, University of Washington and Alfonso Saiz Lopez, Spanish National Research Council (CSIC)

• Modelling the multiscale impacts of tropospheric halogen chemistry
• Presented by: Dr. Qinyi Li, Professor, Shandong University
  
  Tropospheric halogen (chlorine, bromine, and iodine) chemistry has significant and complicated effects on atmospheric oxidants, air pollutants, and greenhouse gases. Field measurements have suggested the ubiquitous existence of halogen species across various environments in the troposphere; laboratory experiments have revealed detailed interactions between reactive halogen species and carbon-, nitrogen-, and sulfur-containing species. However, halogen species and their chemical mechanisms are not included in most atmospheric models, limiting the understanding of atmospheric chemistry as well as the causes of air pollution and climate change. In this series of studies, we incorporated newly proposed chemical mechanisms of halogens into multiscale atmospheric models and comprehensively quantified the impacts of halogen chemistry on atmospheric oxidants, air pollutants, and greenhouse gases in polar boundary layer, marine boundary layer, urban air, free troposphere, etc.
   
• Aircraft-based Observations of Arctic Ozone and Reactive Bromine Chemistry and the Influence of Oil Field Combustion Emissions
• Presented by: Kerri Pratt, Professor, University of Michigan
  
  During springtime, the near-surface atmosphere in the Arctic is often depleted in ozone, due to a multiphase catalytic reaction cycle known as the bromine explosion cycle. Molecular bromine (Br2) is produced from photochemical condensed-phase and multiphase reactions in the surface snowpack and aerosols. Photolysis of Br2 produces bromine atoms that react with ozone to produce bromine monoxide (BrO), which go on to react with HO2 or NO2 to produce products that react with bromide-containing surfaces to produce Br2. The CHACHA (CHemistry in the Arctic: Clouds, Halogens, and Aerosols) field campaign was conducted during Feb. – Apr. 2022 with two research aircraft based out of Utqiaġvik, Alaska. Flights were conducted over the snow-covered sea ice, tundra, open leads (sea ice fractures), and the North Slope of Alaska oil fields. Gaseous reactive bromine compounds, including Br2 and BrO, were measured using chemical ionization mass spectrometry (CIMS) and multi-axis differential optical absorption spectroscopy (MAX-DOAS), with coincident in-situ ozone measurements. In addition, the role of nitrogen oxides (NOx) on bromine chemistry was investigated through measurements of nitrogen dioxide (NO2) via MAX-DOAS and nitric acid (HNO3) and peroxynitric acid (HO2NO2) via CIMS. This study is improving our understanding of atmospheric chemistry in the rapidly changing Arctic, with sea ice loss and increasing development and resource extraction.
   
• Global impact of halogen chemistry on secondary aerosols
• Presented by: Xuan Wang, Assistant Professor, City University of Hong Kong
  
  Abstract coming soon!

Atmospheric Chemistry in Public Health and Regulatory Applications

Session Chairs: Jim Kelly, US EPA and Emma D'Ambro, US EPA

• Utilizing Spatial Measurements of Volatile Organic Compounds to Quantify Community Exposures and Health Risk: Results from the Hazardous Air Pollution and Monitoring Assessment Project (HAP-MAP)
• Presented by: Dr. Pete De Carlo, Associate Professor, Johns Hopkins University
  
  Fenceline communities frequently bear the brunt of environmental pollution, and current regulatory monitoring approaches fail to generate the data needed to document these disproportionate exposures. We addressed this critical data gap by combining state-of-the-art analytical instrumentation deployed on a mobile van to characterize spatial concentration gradients for hazardous air pollutants (I.e. particulate matter, metals, and VOCs) in a highly industrialized region along the Mississippi River corridor in Louisiana. Gradients in hazardous air pollutants were observed with higher levels, and consequently higher exposure risk, found near facility fencelines. Near to chemical production facilities we observed areas with high ethylene oxide using multiple spectroscopic methods. These observed differences in concentration and therefore exposure for a suite of hazardous air pollutants were then placed in a cumulative risk framework to show the spatial variability of cancer and non-cancer risks at the neighborhood scale. This presentation will give an overview of the HAP-MAP study design and results.
   
• Predicting wintertime particulate sulfur in Fairbanks, Alaska during the ALPACA campaign
• Presented by: Kathleen Fahey, Physical Scientist, U.S. Environmental Protection Agency
  
  Fairbanks, Alaska, and surrounding communities are often subject to high PM concentrations in the winter. The cold and dark conditions are often associated with strong surface-based temperature inversions and air stagnation, trapping high emissions (e.g., space heating emissions from residential wood and oil combustion) close to the surface. During wintertime PM pollution episodes, PM2.5 is made up mostly of OC, EC, and SO4. While much of the carbon comes from residential wood combustion, there are several sulfur sources in the Fairbanks area, so high SO4 concentrations are likely a mix of primary and secondary contributions. Previous CMAQ modeling for the area showed large underpredictions of SO4 during the winter, much like other locations that experience a wintertime build-up of SO4 not reflected in models. It suggests that CTMs may be missing important sulfur chemistry active under cold, low-light conditions when contributions from the major gas-phase (OH) and in-cloud (H2O2, O3) oxidation pathways for SO2 are limited. In this study, we develop a modeling platform (i.e., emissions, meteorology, and air quality modeling) to investigate the evolution of PM2.5 over Fairbanks and North Pole, Alaska, during the recent Alaskan Layered Pollution and Chemical Analysis (ALPACA) winter air quality study. A goal of this work is to provide an improved modeling tool to inform development of effective plans to reduce high wintertime PM2.5 in Fairbanks and similar locations.
   
• Air Quality Modeling and Emissions Inventory to Assist SIP Developments
• Presented by: Sang-Mi Lee, Planning and Rules Manager, South Coast Air Quality Management District
  
  The South Coast AQMD has developed a comprehensive chemical transport modeling framework that integrates WRF, CMAQ, SMOKE, and MEGAN. Emissions from key mobile sources for the CMAQ modeling are based on real-world movement data gathered from various sensors and respondents. For instance, the precise locations of aircraft, ocean going vessels and on-road vehicles at specific dates and times are captured and incorporated into the modeling. This platform is used to develop strategies aimed at reducing emissions to meet the National Ambient Air Quality Standards (NAAQS) in the greater Los Angeles area (South Coast Air Basin). 
  
  For example, to achieve the 2015 8-hour ozone standard of 70 ppb, the modeling indicated that an approximately 80% reduction in NOx emissions would be required by 2037 within the South Coast Air Basin. The 2022 Air Quality Management Plan outlined a series of control strategies designed to achieve these NOx reductions.
  
  In addition to grid-scale chemical transport modeling, a hybrid modeling technique has been developed to address the high variability in pollutant concentrations near emission sources. This approach combines CMAQ with AERMOD and was applied to assess PM2.5 concentration distribution near major freeways. The hybrid technique was also used in the state implementation plan to meet the 2012 annual PM2.5 standard. The plan was submitted to the U.S EPA for its review and approval.
   
• Role of chemical production and depositional losses on formaldehyde in the Community Regional Atmospheric Chemistry Multiphase Mechanism (CRACMM)
• Presented by: Nash Skipper, Physical Scientist, U.S. Environmental Protection Agency
  
  Formaldehyde (HCHO) is an important air pollutant with direct cancer risk and ozone forming potential. HCHO sources are complex because it is both directly emitted and produced from oxidation of most gas phase reactive organic carbon (ROC). We update the production of HCHO from ROC in the Community Regional Atmospheric Chemistry Multiphase Mechanism (CRACMM) in the Community Multiscale Air Quality (CMAQ) model. Production of HCHO from isoprene and monoterpenes is increased, correcting an underestimate in the current version. Simulated 2019 June–August surface HCHO during peak photochemical production (11am–3pm) increased by 32% over the southeastern US and by 13% over the entire contiguous US. The increased HCHO compares more favorably with satellite- and aircraft-based observations. Evaluation against hourly surface observations indicates a missing nighttime sink for HCHO which can be improved by adding bidirectional exchange of HCHO and a leaf wetness dependent deposition process which increases nighttime deposition. The ability of CRACMM to capture peak levels of HCHO at midday is improved though still underestimated at southeastern US sites. Using established risk assessment methods, lifetime exposure of the contiguous U.S. population (~320 million) to ambient HCHO levels predicted here may result in 6200 lifetime cancer cases, 40% of which are from controllable anthropogenic emissions of nitrogen oxides and reactive organic compounds.

BVOC multiphase chemistry - what do we know and what are we missing?

Session Chairs: Celia Faiola, UC Irvine and Domenico Taraborrelli, Forschungszentrum Jülich

• Formation of highly oxygenated molecules across different chemical regimes -Impact of NO and HO2 on secondary organic aerosol formation
• Presented by: Yarê Baker, Leibniz Institute for Tropospheric Research
  
  One major source of secondary organic aerosol (SOA) are highly oxygenated molecules (HOMs) from the atmospheric oxidation of biogenic volatile organic compounds. The key species in HOM formation are peroxy radicals (RO2), and many studies focus on their possible reaction pathways. The fate of HOM-RO2 is defined by the importance of reactions with other RO2, HO2 and NO, as well as by the overall reactivity of their surroundings. However, laboratory studies often cannot capture the complexity of the real atmosphere resulting in an overemphasis of RO2 cross-reactions. We present a set of α-pinene photooxidation experiments that systematically varied these parameters to approach atmospheric relevance. Experiments were performed under steady-state conditions in the continuously stirred tank reactor SAPHIR-STAR. The chemical reaction regime was shifted by increasing the concentrations of HO2 and NO, separately and in mixture, while keeping the primary α-pinene oxidation conditions constant. Ammonium sulfate aerosol was added in each regime to observe the condensation behavior of the HOM gas-phase products and to gain insight into the volatility of the produced HOMs. Our results indicate that laboratory experiments with dominance of RO2+RO2 cross reactions can lead to an overestimation of the SOA formation, and thus the according SOA yield cannot be directly transferred to the atmosphere. Our mechanistic considerations illustrate how the HOM product distribution depends on HO2 and NO concentrations and explain the reduced SOA formation by changes in the HOM products and their volatility. 
   
• Low-volatility accretion product formation from RO2 + RO2: Exploring uncharted chemistry with GECKO-A
• Presented by: Lauri Franzon, PhD Student, University of Helsinki
  
  The lowest volatility gas-phase molecules formed from atmospheric organic chemistry are likely accretion products from peroxy radical recombination (RO2 + RO2). Until recently, it was thought that organic peroxides were the only type of accretion product formed from this reaction, but thanks to Peräkylä et.al. (JACS, 2023) we now know that ether and ester products are also possible in the event of a rapid alkoxy radical (RO) decomposition occurring in the triplet state RO dimer intermediate 3(RO...OR). We do not know the full extent of this mechanism’s atmopsheric importance, but we can make an educated guess using the automated mechanism generation software GECKO-A (Aumont et.al, ACP, 2005). 
  
  We have analysed a large set of predicted accretion products from pairs of RO2 formed from atmospheric oxidation of n-decane, toluene, seven monoterpenes, and β-caryophyllene, in which product branching ratios were estimated with a parametrization of computational results from our group (Hasan et.al, JPCA, 2020 & 2023, Franzon JPCA 2023) compared to Vereecken’s SAR-predicted RO reaction rates (PCCP, 2009 & 2010). As a result, we have discovered several new reactivity trends that determine which RO2 + RO2 reactions are the most important for the formation of low-volatility organic compounds. The results are qualitative rather than quantitative due to resting on multiple simplifications, but we believe they still provide invaluable data for future experimental and computational work on RO2 + RO2.
   
• Peroxyhemiacetals Form Carboxylic Acids and Dimer Esters in Pinene Secondary Organic Aerosol
• Presented by: Christopher Kenseth, Postdoctoral Fellow, NSF Atmospheric and Geospace Sciences, University of Washington
  
  Advanced mass spectrometric techniques have been employed for decades to infer structures and develop formation mechanisms of molecular products in secondary organic aerosol (SOA). These structures and mechanisms have been widely adopted (e.g., in explicit chemical mechanisms) yet in many cases remain unconstrained, leading to potential misinterpretation and significant uncertainty with respect to the composition, properties, and associated impacts of SOA. In this work, we unambiguously determine the structures of the most abundant monomeric and dimeric products identified using liquid chromatography/electrospray ionization mass spectrometry (LC/ESI-MS) in SOA from ozonolysis of α-pinene and β-pinene—substantial global SOA sources—through independent synthesis of authentic standards. Based on targeted laboratory experiments featuring isotopically labeled compounds and tailored precursors together with quantum chemical calculations of multigenerational peroxy and alkoxy radical chemistry, we show that these products are formed in the particle phase and demonstrate that the particle-phase production of (acyl)peroxyhemiacetals, and their subsequent conversion to carboxylic acids and dimer esters via Baeyer-Villiger decomposition and nucleophilic addition of alcohols, represents a key process in SOA formation.

Chemical Mechanism Development under Changing Regimes

Session Chairs: Luc Vereecken, Forschungszentrum Jülich and Rebecca Schwantes, NOAA Chemical Sciences Laboratory

• The Formation of Accretion Products in the Self- and Cross-Reactions of Small Alkene-Derived Peroxy Radicals
• Presented by: Sara Murphy, Postdoctoral Scholar, University of California Los Angeles
  
  In this work, we examine the formation of a peroxide accretion product (ROOR) in the self- and cross-reactions of organic peroxy radicals (RO2) derived in the OH-initiated oxidation of 2-methylpropene, propene, and cis-2-butene in the presence of ethene. We observe the formation of ROOR products via the self- and cross-reactions of the various RO2 isomers formed in these systems and measure the formation rate of these ROOR relative to the formation of the ethene-derived ROOR, which has been measured previously. With these results, we examine the relationship between ROOR formation and peroxy radical structure and the relationship between the formation of ROOR in cross-reactions compared to ROOR formation in the corresponding self-reactions.
   
• Observations and Constraints on the Sulfur Budget over the Marine Environment
• Presented by: Christopher Jernigan, Post-doctorial Associate, NOAA/CIRES
  
  The photochemical oxidation of dimethyl sulfide (DMS) by the hydroxyl radical (OH) contributes to the production and growth of particles and cloud condensation nuclei (CCN) in the marine boundary layer (MBL). DMS oxidized by OH can go on to form a stable intermediate, hydroperoxymethyl thioformate (HPMTF), which is both globally ubiquitous and efficiently lost to multiphase processes in the marine atmosphere. Recent work has shown that clouds act as a significant and irreversible loss path for HPMTF, sequestering sulfur and greatly reducing the production of long-lived climate relevant species (e.g. SO2 and OCS). Until recently, there have been few observational constraints on the role of cloud type and abundance on the spatiotemporal distribution of HPMTF and other soluble molecules in the MBL. Here we present aircraft observations from the NASA DC-8 of key sulfur species throughout the sulfur oxidative pathway (e.g. DMS, HPMTF, DMSO, SO2) utilizing various mass spectrometers and optical instruments. We implement a constrained steady state analysis in tandem with constrained box modeling to evaluate the current understanding of sulfur chemistry within the marine environment.
   
• Impacts of halogen chemistry and photolysis of particulate nitrate on ozone
• Presented by: Golam Sarwar, Research Physical Scientist, U.S. Environmental Protection Agency
  
  We examine the impacts of halogen chemistry and aerosol nitrate (pNO3) photolysis on ozone using the Community Regional Atmospheric Chemistry Multiphase Mechanism v2 with additional updates (CRACMM2). We implement halogen (Br, Cl, and I compounds) chemistry and pNO3 photolysis into CRACMM2 and perform three annual simulations using the Community Multiscale Air Quality model over the Northern Hemisphere with 108-km horizontal grid spacings. The first simulation uses CRACMM2, the second simulation uses CRACMM2 with halogen chemistry, and the third simulation uses CRACMM2 with both the halogen chemistry and pNO3 photolysis. Model results suggest that halogen chemistry reduces ozone while pNO3 photolysis enhances ozone and compensates the halogen mediated loss. We compare model ozone mixing ratios with available surface and ozonesonde measurements and find that CRACMM2 exhibits a large negative bias in spring over the entire U.S., the halogen chemistry further deteriorates it, but the pNO3 photolysis improves and largely eliminates the springtime negative bias. For the western U.S., CRACMM2 underpredicts the daily maximum 8-hour average ozone in all months, the halogen chemistry further deteriorates it, and the pNO3 photolysis improves and alleviates the negative bias. Detailed comparisons between model predictions and observations will be presented to assess the mixed impacts of halogen chemistry and pNO3 photolysis across different regions of the continental U.S.
  Disclaimer: The views expressed in this abstract are those of the authors and do not necessarily represent the views or policies of the U.S. Environmental Protection Agency (EPA).

Chemical Mechanisms Important at Regional Scales, Across Diverse Regions

Session Chairs: Ilona Riipenen, Stockholm University and Anoop Mahajan, Indian Institute of Tropical Meteorology

• Multiphase Formation of Secondary Aerosol Mediated by Particulate Nitrate Photolysis
• Presented by: Dr. Ruifeng Zhang, King Abdullah University of Science and Technology
  
  Particulate nitrate is one of the most abundant inorganic components in ambient fine particles. Its photolysis has been considered an important source of reactive species (e.g., OH, NO2, and NO2-/HNO2) in atmospheric particles and cloud droplets. These oxidants facilitate the transformation of atmospheric inorganic and organic precursors into secondary aerosols. In our study, we investigated the SO2 oxidation mediated by the particulate nitrate photolysis, revealing a substantial sulfate production rate. Kinetic modeling simulations indicated that the oxidation processes involving NO2-/HNO2 play a pivotal role in sulfate formation. Furthermore, we assessed the impact of halide ions on sulfate production during nitrate photolysis. Halide ions exhibit high surface activity and tend to accumulate at interfaces, which increases the concentration of counterions (i.e., cations). This accumulation facilitates the closer proximity of nitrate ions to the interface due to the formation of a double layer of interfacial halide ions and subsurface counterions, thereby enhancing nitrate photolysis due to the incomplete solvent cages at the interface. Laboratory experiments demonstrated that the presence of halide ions (e.g., Cl-, Br-, and I-) significantly increases sulfate production via nitrate photolysis. Subsequently, we further explored the oxidation of glyoxal by nitrate photolysis in the presence and absence of brown carbon (BrC). Our findings revealed that formate is the predominant product during glyoxal oxidation facilitated by sodium nitrate photolysis (i.e., absence of BrC). In the presence of BrC (i.e., in the ammonium nitrate system), we observed phase separation and the formation of oxamide. Moreover, glyoxal decay was significantly accelerated in the presence of BrC, attributed to photosensitization effects.
   
• Amazing Amazonian Atmospheric Chemistry
• Presented by: Prof. Jonathan Williams, Max Planck Institute for Chemistry
  
  The Amazon rainforest is the largest source of volatile organic compounds (VOC) to the global atmosphere. These VOC emissions are important as they can influence the atmospheric oxidation capacity, the Earth’s radiative budget, and regional precipitation. They also facilitate communication between animals, plants, and insects. Since 2012 we have been using sensitive mass spectrometers to characterize the VOC from a measurement tower sited in the pristine rainforest north east of Manaus, Brazil. Data has been collected from multiple heights (80, 150 and 325m), dry and wet seasons, and multiple years.
  In 2022-2023 the CAFÉ-BRAZIL airborne campaign allowed VOC measurements to be extended to 14.5 km and across the entire Amazon basin. We found that convection of VOC by day is rendered ineffective by photochemical loss, but nocturnal deep convection exports considerable BVOC to high-altitude, extending the chemical influence of the rainforest to 14.5 km, and priming it for complex organic photochemistry at dawn. Close to the forest (325m) VOC mixing ratios peaked between 12:00 and 14:00, closely correlated with light and temperature. However, at 10km the VOC diel cycle reached a maximum at 04:00. The impact of these biogenic emissions on the Amazonian atmosphere was assessed with a global chemistry model by reducing emissions by 75%, which was equivalent to measurements over deforested areas.
   
• Upper Atmospheric Aerosol Formation: A Sky Full of Heterogeneity
• Presented by: Mingyi Wang, Assistant Professor, University of Chicago
  
  Aerosol formation in the upper atmosphere is particularly important because it may account for at least a third of global cloud condensation nuclei (CCN). Additionally, this process serves as a natural analog for climate interventions that involve introducing particles into the stratosphere. However, the precursor vapors and the mechanisms that drive aerosol formation at high altitudes remain poorly understood.
  
  This talk will present results that illustrate how laboratory experiments can help to fill this knowledge gap. Using experiments performed in the CERN CLOUD chamber, we show that nitric acid, sulfuric acid and ammonia form particles synergistically, at rates orders of magnitude faster than those from conventional ternary H2SO4 nucleation. The importance of this mechanism depends on the availability of ammonia, which has recently been observed at unexpectedly high levels in the upper troposphere over the Asian monsoon region. Once particles are formed, the co-condensation of ammonia and abundant nitric acid alone is sufficient to drive rapid particle growth to CCN sizes with only trace sulfate. Our model simulations further confirm that ammonia is efficiently convected aloft during the Asian monsoon, driving rapid multi-acid HNO3–H2SO4–NH3 nucleation in the upper atmosphere and producing INP that spread across the mid-latitude Northern Hemisphere.

Chemistry Heterogeneity Within the Wildfire Smoke Plume

Session Chairs: Georgios Gkatzelis, Forschungszentrum Jülich and Rebecca Buchholz, National Center for Atmospheric Research

• Simplifying Smoke: Methods of Untangling Smoke Complexity and Heterogeneity, What They Reveal, and Why It Matters for Air Quality
• Presented by: Zachary Decker, Research Scientist, CIRES & NOAA
  
  When smoke is formed it may be a mostly consistent, or homogeneous, mixture of gas and particles. As the smoke moves over time, photolysis and dilution will perturb this homogeneous mixture into a system of varied chemistry depending on the location within the smoke plume (e.g. center to edge). This induced heterogeneity is then further jumbled by internal mixing of the plume. As a result, the plume structure is highly complex and analyses are often limited to specific regions or an average. In this talk, I will describe current qualitative and quantitative methods which un-jumble the complexity of smoke heterogeneity. Once the plume structure is untangled, the underlying chemical mechanisms can be more easily interpreted. For example, the relative importance of various mechanisms for the formation of secondary organic aerosol (SOA) precursors can be determined. Even more, one can differentiate the effects of dilution and oxidation of organic aerosol (OA). With these tools we can better understand the importance of key chemical mechanisms on air quality impacts.
   
• Wildfire Growth Database Fusing Polar-Orbiting and Geostationary Satellite Observations and Agency Fire Records
• Presented by: Crystal McClure, Atmospheric Science, Sonoma Technology
  
  Better understanding of wildfire progression and fire behavior is of significant concern for research and fire management communities. Advances in these topics require a long-term, consistent, high-resolution dataset. To date, datasets with high spatiotemporal resolution cover limited timeframes, focus on large fires, and/or use a single observation source. We have developed a methodology named Firelytics for computing daily-to-hourly wildfire growth that includes all fire sizes and fuses polar-orbiting and geostationary satellite observations and multi-agency fire records. First, we use sub-daily active thermal anomaly retrievals from the Visible Infrared Imaging Radiometer Suite (VIIRS) aboard the polar-orbiting satellites SNPP and NOAA-20 to map detailed fire perimeter and compute the size and growth areas of active fires. We then incorporate continuous active fire observations from the Geostationary Operational Environmental Satellites (GOES) Advanced Baseline Imager (ABI) to estimate hourly fire growths. These data are then fused with multiple agency records to provide the names, causes, and context for the fires. The resulting data provide information on hourly fire growth variability and are highly correlated with aircraft infrared measurements. The high spatiotemporal resolution and consistency of fire Firelytics data presents a significant upgrade over current wildfire databases for multiple application areas, including fire weather analysis, fire behavior modeling, wildfire potential predictions, smoke modeling, and near-real-time fire monitoring. We will present the latest updates on Firelytics data and their usage, and a new data dashboard application.
   
• Tracking wildfire chemistry across diverse spatial scales
• Presented by: Lu Xu, Assistant Professor, Washington University in St. Louis
  
  Wildfires pose imminent threats to air quality and significantly impact global atmospheric composition. Understanding the effects of wildfire emissions on the atmospheric composition requires addressing two questions. First, what drives the variability in plume composition (e.g., organic aerosol) during near-field evolution? Second, how does the plume composition evolve during long-range transport? To answer these questions, we analyze data from recent laboratory experiments and field campaigns focused on characterizing wildfire emissions and their evolution. Our analysis sheds light on the near-field variability of organic aerosol in wildfire plumes and reveals the chemical evolution of key species (e.g., organic aerosol, ozone, brown carbon, volatile organic compounds, NOy species) across vast distances downwind. The outcomes of this research will enhance our ability to assess air quality in regions near wildfire sources, quantify wildfire contributions to regional ozone levels, and inform air quality policies to mitigate pollution during wildfire events in populated areas.
   
• The Measured Impact of Wildfires on Ozone in Western Canada
• Presented by: Stephanie R. Schneider, McMaster University
  
  The impacts of wildfires on atmospheric ozone (O3) are difficult to characterize due to the many factors that affect the formation rate of O3 and the episodic nature of fire events. To understand the factors which impact O3 mixing ratios during wildfires, we address the variability in wildfire events in time using a data-driven analysis with ambient air pollution measurements in Western Canada from 2001-2019. Wildfire events are identified using the automated Trajectory-Fire Interception Method (TFIM), which quantifies interceptions between HYSPLIT back-trajectories and wildfire hotspots. We show that the time of wildfire influence, fire distance and PM2.5 mixing ratio influence the local O3 measured during wildfire periods in an additive way. To elucidate the role of urban pollution mixing with wildfire plumes on O3 measurements, a collection of air quality stations forming a path through the city of Edmonton, Alberta, Canada is used to track changes to O3 mixing ratio as wildfire plumes interact with the urban air. From this, we show that O3 measurements during wildfire periods are increased in rural areas east of the city, relative to the western rural and urban stations. This trend was not observed for wildfire events which did not travel through the city, further emphasizing the role of the urban plume. Overall, our results highlight the many factors that need to be considered to understand the heterogeneity of O3 formation during wildfire events. 

Data-Driven Innovations in Atmospheric Chemistry: From Data to Discovery

Session Chairs: Makoto Kelp, Stanford University and Sam Silva, University of Southern California

• Automated global detection of methane super-emitters using machine learning and satellite data
• Presented by: Berend J. Schuit, PhD Candidate, SRON Netherlands Institute for Space Research; GHGSat Inc.
  
  A reduction in anthropogenic methane emissions is vital to limit near-term global warming. Mitigating the largest methane super-emitters provides an important opportunity for significant short-term reductions in global emissions. Since late 2017, the TROPOspheric Monitoring Instrument (TROPOMI) has been in orbit providing daily global coverage of atmospheric methane mixing ratios at a resolution of up to 7x5.5 km2, enabling the detection of these super-emitters at global scale for the first time. However, TROPOMI produces millions of observations each day, which makes manual inspection of all the data infeasible. Therefore, we have developed a Machine Learning pipeline to efficiently scan the TROPOMI data to detect methane emission plumes. This two-stage pipeline consists of a Convolutional Neural Network followed by a Support Vector Classifier. We trained the models on manually labelled pre-2021 methane plumes in TROPOMI data, and subsequently analysed all TROPOMI data of 2021 using the trained models to find ~3000 methane plumes originating from the oil/gas industry, coal mines, and urban areas.
  This presentation covers the developed machine learning pipeline, discusses insights obtained on the global distribution of methane super-emitters, shows how the ML pipeline is currently applied operationally on new TROPOMI data by our group at SRON (earth.sron.nl/methane-emissions), and demonstrates how it is sometimes possible to follow-up on detections in TROPOMI data using high spatial-resolution (~20x20 m2) satellite instruments to identify the exact sources/facilities responsible for the emission plumes observed with both satellite instruments.
   
• Deep Learning for Surface Ozone Emulators with Uncertainty Quantification
• Presented by: Kelsey Doerksen, University of Oxford
  
  Air pollution is a global hazard, and as of 2023, 94% of the world's population is exposed to unsafe pollution levels. Surface Ozone (O3), an important pollutant, and the drivers of its trends are difficult to model, and traditional physics-based models fall short in their practical use for scales relevant to human-health impacts. Deep Learning-based emulators have shown promise in capturing complex climate patterns, but overall lack the interpretability necessary to support critical decision making for policy changes and public health measures. We implement an uncertainty-aware Convolutional Neural Network architecture to predict the Multi-mOdel Multi-cOnstituent Chemical data assimilation (MOMO-Chem) model's surface ozone residuals (bias) using Bayesian and quantile regression methods. We demonstrate the capability of our techniques in regional estimation of bias in North America and Europe, and highlight the uncertainty quantification scores between our methodologies and discern which ground stations are optimal and sub-optimal candidates for MOMO-Chem bias correction. We evaluate the impact of land-use information in surface ozone residual modelling and discuss how to extract meaningful scientific understanding of air pollution from our results.

Fundamental Studies - Basic Processes Shaping the Atmosphere

Session Chairs: Matti Rissanen, University of Helsinki & Tampere University and Rebecca Caravan, Argonne National Laboratory

• Atmospheric Gas Phase Oxidation of Sulfur and Other Nonhydrocarbons
• Presented by: Dr. Jing Chen, University of Copenhagen
  
  Autoxidation is a key process in the atmospheric oxidation of volatile organic compounds, where oxygen-containing functional groups are introduced through O₂ addition to alkyl radicals followed by unimolecular isomerization, leading to the formation of highly oxygenated molecules. In such a process, the carbon atoms remain at 4 valence, allowing for straightforward theoretical modeling using single-reference methods. However, the oxidation of nonhydrocarbons, such as sulfur (S), nitrogen (N), and phosphorus (P) containing compounds, often involves valence change of these atoms that typically carry multireference characters, challenging traditional theoretical approaches.
  
  We theoretically investigated the atmospheric fate of dimethyl sulfide (DMS) and explained the gas-phase formation of methane sulfonic acid (MSA), a known product from DMS oxidation whose formation mechanism remained unclear for decades. Our theoretical investigations reveal that temperature critically influences the atmospheric fate of DMS. In warmer regions, sulfuric acid (SA) is the predominant product, whereas in colder climates, MSA formation is favored in the gas phase. This work highlights the necessity of multireference treatment for accurate theoretical modeling of nonhydrocarbon oxidation processes in the atmosphere, providing new insights into the theoretical treatment of the oxidation of sulfur and other nonhydrocarbon species.
   
• Photophysical Oxidation: The role of Far-from-Equilibrium Chemistry in Atmospheric Mechanisms
• Presented by: Scott Kable, Professor, University of New South Wales
  
  In the atmosphere, a gas phase molecule has four principal fates. It can i) absorb light and photolyse, ii) react with a highly reactive species such as OH, NO3 or O3, iii) undergo wet or dry deposition, or iv) be long-lived, and hence globally distributed throughout the atmosphere. In this seminar, I will explain our discovery of a fifth process where i) and ii) combine and present early results on possible impact in atmospheric chemistry. 
  After a molecule absorbs UV light and excited to a higher electronic state it may undergo a photochemical reaction, producing radicals or molecules which then oxide further in the atmosphere (photochemical oxidation, PCO). The excited molecule may also collisionally-cool, returning to equilibrium with its environment. In the time between excitation and re-equilibration, these molecules are in a far-from-equilibrium state with very significant vibrational energy. These photo-activated molecules may react with ambient O2 in a process called photophysical oxidation (PPO).
  We published recently (Welsh et al., Nat. Chem. 2023, 15, 1350) the first example of vibrationally excited HCHO, formed after excitation to the S1 state and subsequent internal conversion to S0, reacting with O2 to form HO2 + HCO. The quantum yield of this process in HCHO was small – about 0.01. However it occurs at lower energy (longer wavelength) than radicals can be formed by direct photolysis, thereby opening up new pathways for radical production.
  In this seminar, I will describe briefly the PPO process in HCHO and then describe PPO in methacrolein (MACR), where radical products are almost entirely formed by PPO, and not PCO. I will then show some very early GEOS-Chem modelling of the impact of PPO on HO2 yield in HCHO atmospheric chemistry.
  
  Acknowledgments: This work was supported by the Australian Research Council, the Australian National Computational Infrastructure, as well as our institutions.
   
• Autoxidation in ethers
• Presented by: Paul Wennberg, Caltech
  
  Ethers are used ubiquitously as solvents and many are quite volatile. The subsequent photochemistry of ethers is, however, poorly understood. Together with Caltech PhD student Hongmin Yu and U. Copenhagen Postdoctoral Research Fellow Jing Chen, I explore the rate coefficients for hydrogen shifts to the nacent peroxyradicals (autoxidation). We show that this chemistry is quite fast and, for many ethers, will dominate their photodegradation in the atmosphere - even in environments with significant concentrations of nitric oxide.
   
• Tracking the Criegee Intermediates Reaction Networks in Ozonolysis of a Series of Acyclic and Endocyclic Alkenes
• Presented by: Prof. Denisia Popolan-Vaida, Department of Chemistry, University of Central Florida
  
  The reaction networks that describe the complex chemical processes involved in atmospheric and combustion systems are typically dominated by reactive intermediates that usually occur in small concentrations. Identification and quantification of these reactive species within complex reactive mixtures are key for the development of a fundamental, chemically accurate description of complex atmospheric and combustion systems. Criegee intermediates (CIs), are carbonyl oxide reactive intermediates formed during the ozonolysis of unsaturated organic compounds, known to play an important role in the chemistry of the atmosphere. CIs have the unique capability to add both carbon and oxygen mass to the co-reactant through either 1,3-dipolar cycloaddition or insertion mechanisms that can lead to the production of high-molecular-weight, low-volatility products, which have been linked to the formation of secondary organic aerosols (SOA), in just a few reaction steps. The unimolecular decomposition of CIs is recognized as an important source of OH in the atmosphere.
  The reaction networks of CIs formed in the ozone-assisted oxidation reaction of a series of C3 to C7 acyclic and C5 to C7 endocyclic alkenes were investigated in an atmospheric pressure jet stirred reactor to understand the effect of CIs carbon number and reactive environment on the products distribution and the formation of high-molecular-weight, low-volatility compounds. Molecular beam high-resolution mass spectrometry and tunable synchrotron single-photon ionization in conjunction with ab initio threshold energy calculation were employed for the detection and identification of gas-phase reactive intermediates and final products. Temperature-dependent analysis employing DART ionization-Orbitrap mass spectrometry was used to facilitate the detection of high-molecular-weight, low-volatility reaction products. 
  The ozone-assisted oxidation of acyclic C3 to C7 alkenes were observed to lead to complex CIs reaction networks that involve both CIs unimolecular and bimolecular reactions. For instance, formaldehyde oxide CI formed in the ozone-assisted oxidation of 1-propene and 1-butene is observed to dominate the reaction networks. Products corresponding to the sequential CIs additions to ozonolysis reaction co-products like water, alkenes, aldehydes, alcohols, and carboxylic acids are also detected. However, oligomerization does not appear to be favored by increasing the CIs carbon number. No evidence for the formation of bimolecular products is observed in the case of long chain CIs formed in the ozone-assisted oxidation of endocyclic alkenes, i.e., C5 to C7 alkenes. Instead, long chain CIs are observed to undergo unimolecular decomposition followed by autooxidation. 
  The results of our studies represent an important step towards understanding the CIs reactive behavior in complex reactive environments where several co-reactants are present. It provides valuable information for future kinetic modeling work and can help connect our current understanding of the role and reactivity of CIs in the gas phase to heterogenous environments.
• Acyl Peroxy Radicals as Atmospheric Oxidants
• Presented by: Dr. Nanna Myllys, University of Helsinki
  
  In this presentation, I will discuss the role of acyl peroxy radicals in oxidation of unsaturated hydrocarbons under atmospheric and laboratory conditions. Our quantum chemical calculations show that the accretion reaction between acyl peroxy radical and unsaturated hydrocarbon occurs through very low barrier. Formed perester alkyl radical rapidly oxidize. Subsequent reactions such as hydrogen shift, ring opening, and dimerization are studied. Our chamber experiments show that the reaction indeed takes place and the product compounds are large and highly functionalized.

Instruments and Observations

Session Chairs: Frank Winiberg, NASA Jet Propulsion Laboratory

• New science enabled through recent developments in LIF-based NOx instrument capabilities
• Presented by: Dr. Andrew Rollins, NOAA
  
  Measurements of nitrogen oxides (NO, NO2) in low NOx regimes of the atmosphere and laboratory are challenging, yet key for diagnosing radical chemistry and reactive nitrogen cycling and budgets. Recent interest in low NOx chemistry both in remote parts of the atmosphere, and in increasingly clean urban areas, have enhanced interest in reliable techniques for measuring NOx even at 10 pptv or below. In the past few years, we have developed a single-photon laser induced fluorescence (LIF) technique for measuring NO at single pptv levels and demonstrated the utility and robustness of this new technology during multiple airborne and ground-based campaigns. In addition, we have used a new variant of a photolytic NO2 converter to measure NO2 in the upper troposphere where measurements and models of NOx abundance have frequently diverged.
  
  In this presentation I will describe details of the new NO-LIF instrument and how this technique can overcome some of the disadvantages of other NOx measurement techniques. Examples of recent field campaigns that benefited from these measurements include measuring NO and NO2 in the upper troposphere (ACCLIP 2022 and AEROMMA 2023), and in the pristine marine boundary layer (BLEACH campaign, 2022-2023). I will give an overview of key results from those campaigns and their relevance to informing atmospheric chemical mechanisms.
   
• Determination of isomerisation reactions of organic peroxides of selected unsaturated aldehydes from the measurement of the temperature dependent rate coefficients of their reaction with OH radicals.
• Presented by: Dr. Anna Novelli, Forschungszsentrum Jülich
  
  Isomerisation reactions of organic peroxy radicals (RO2) can contribute substantially to the loss rate of RO2 radicals in the troposphere recycling radical species and contributing to the formation of highly oxidized molecules. 
  In this study, the potential isomerisation reaction of RO2 radicals formed from the reaction of OH with five different unsaturated aldehydes (acrolein, methacrolein, trans-crotonaldehyde, tiglic aldehyde, and 3-methyl crotonaldehyde) was determined using the measurement of OH radical lifetime. An instrument using laser flash photolysis combined with the OH detection by laser induced fluorescence, typically used for field or chamber measurement, was modified allowing heating and/or cooling of the sampled air in the flow tube where the OH radical decay is observed. This allowed the measurement at atmospheric pressure and in air of the temperature dependent rate coefficient between 280 and 340 K of the OH reaction with the unsaturated aldehydes. If isomerisation reaction of RO2 radicals are relevant within the residence time in the flow tube, OH can be regenerated. Therefore, the observed OH decay deviates from a single-exponential decay expected from a pseudo-first order loss reaction. This allows to determine the rate coefficient of the peroxy radical isomerisation reaction using model calculations, in which reaction rates are optimized to best describe the observed OH radical decay. This method is one of few direct measurements of atmospherically relevant peroxy radical isomerisation reaction coefficients that have been reported in literature so far. Experimentally derived values from the different unsaturated aldehydes are compared to quantum-chemical calculations and to rate coefficients reported in literature.

Nighttime Chemistry

Session Chairs: Claudia Mohr, Stockholm University and Carl Percival, NASA Jet Propulsion Laboratory

• The suppression of nighttime chemistry during polluted episodes in megacity Delhi
• Presented by: Dr. Sophie Haslett, Stockholm University
  
  Delhi is one of the world’s most polluted cities, experiencing its most severe episodes during winter due to stagnant meteorology and a compressed boundary layer. However, results from our measurement campaign in 2019 indicate that the continued emission of NOx throughout the night – likely the result of heavy-duty traffic and biomass burning – has the unusual effect of suppressing common nighttime chemical processes that rely on the formation of the nitrate radical (NO3) and dinitrogen pentoxide (N2O5).
  
  This has ramifications for oxidation processes in Delhi. Nighttime concentrations of NO3, which is generally associated with nocturnal oxidation, are almost negligible. As a result of this, the atmospheric processing of primary aerosol is unusually low. Secondly, although the concentration of particulate chloride in Delhi’s atmosphere is remarkably high, the suppression of nighttime N2O5, an important precursor for the production of reactive chloride species, means that chlorine radicals are not released at sunrise at the high concentrations that might have been expected. Our results indicate that a reduction in nighttime NOx emissions may have the potential to increase oxidation via NO3 during the night and via the chlorine radical during the day. This could, counter-intuitively, result in an overall increase in secondary aerosol and haze in the city.
   
• Rapid night-time nanoparticle growth observed in Delhi driven by biomass-burning emissions
• Presented by: Dr. Suneeti Mishra, PSI
  
  Biomass burning, both natural and human-induced, is a significant source of particulate pollution, impacting air quality, climate, and human health worldwide. Delhi, one of the most densely populated cities globally, faces severe winter haze events driven by this particulate pollution. However, the mechanisms behind these events are not fully understood. Our study reveals that intense nocturnal particle growth, driven by biomass-burning emissions, plays a crucial role in the development of haze in Delhi. Despite unfavorable conditions for new particle formation—such as the lack of photochemical production of low-volatility vapors and high vapor loss due to extreme pollution—these growth events occur systematically and account for 70% of the particle concentrations during haze periods. This particle growth is primarily caused by the condensation of organic vapors from biomass burning, triggered by sharp night-time temperature drops and increased emissions. This process is particularly significant in Delhi, where it contrasts with other urban areas globally, where daytime oxidation of vapors typically drives haze formation. Given the widespread practice of uncontrolled biomass burning for heating and cooking across the Indo-Gangetic plain and around the world, this growth mechanism likely contributes to the formation of ultrafine particles, posing health risks to millions and influencing regional climate. Our findings suggest that regulating biomass-combustion emissions could be key to reducing nocturnal haze and improving public health in such polluted hotspots.