Jinsoo Park

Wildfire smoke contains a large amount of phenolic hydrocarbons (HCs) that react with atmospheric oxidants, producing secondary organic aerosol (SOA). In this study, the UNIfied Partitioning Aerosol‐phase Reaction (UNIPAR) model was integrated into the Community Multiscale Air Quality Modeling System (CMAQ) to predict SOA over the West Coast of the United States during the Fire Influence on Regional to Global Environments Experiment‐Air Quality (FIREX‐AQ) campaign. UNIPAR predicts SOA formation via multiphase reactions of explicitly predicted products from the oxidation of precursor HCs. In CMAQ‐UNIPAR, various phenolic HCs were surrogated with phenol, cresol, and catechol, and their parameters were established for three oxidation pathways (OH radicals, O ₃ , and NO ₃ radicals). Phenolic SOA reached nearly 80% of the total SOA in McCall, ID (remote area), followed by terpene SOA. Alkane SOA, driven by anthropogenic emissions, was minimal in remote areas, but still substantial in Sacramento, CA. Simulations showed that both partitioning and heterogeneous reactions of reactive organics were equally important for SOA formation. Phenolic SOA actively formed via reactions with OH radical in daytime. In a very low NO _x environment, phenolic SOA is also produced through the ozonolysis of catechol at nighttime, though not as abundantly as its daytime production. Under this circumstance, NO _x increments positively impacted phenolic SOA concentrations more than terpene SOA in remote areas. Ozone production increases with increasing NO _x in NO _x ‐limited regimes owing to high fluxes of smoke‐origin HCs; in turn, this ozone elevates OH radicals that rapidly react with phenolic HCs (catechol), effectively increasing SOA.

Aerosol mass and number concentrations have decreased substantially over East Asia in response to recent emission controls, accompanied by a shift in dominant inorganic composition from sulfate to nitrate. Tracking how these changes affect aerosol and cloud condensation nuclei (CCN) properties is essential for constraining aerosol–cloud interactions, which remain one of the largest uncertainties in climate projections. However, such changes have rarely been assessed through long-term in situ observations, particularly over downwind oceanic regions. This study presents ship-based observations of aerosol and CCN properties over the Yellow Sea, a key receptor region for East Asian continental outflow, during late winter to spring from 2020 to 2025. When combined with earlier datasets, a recent decadal record showed that aerosol number concentration (NCN) and CCN number concentration at 0.6% supersaturation (NCCN0.6%) decreased by 36% and 44%, respectively, over 2017–2025. Despite the sulfate-to-nitrate shift, the CCN-derived hygroscopicity values (κCCN: 0.08–0.19) did not increase and remained systematically lower than those derived from bulk chemistry (κchem: 0.39 ± 0.07), indicating that bulk composition poorly represents CCN-relevant particles. Separating the contributions of size distribution and hygroscopicity showed that the decline in NCCN at higher supersaturations (≥0.4%) was driven primarily by changes in particle size distribution, while at lower supersaturations (≤0.2%) both size distribution and hygroscopicity contributed to the observed variability. Back trajectory analysis further indicated that declines in NCN and NCCN0.6% were driven by changes within transport pathways rather than by shifts between them, consistent with the effects of emission reductions across East Asia. These findings highlight the effectiveness of emission controls in reducing aerosol and CCN over the Yellow Sea, while revealing that the sulfate-to-nitrate shift has not enhanced CCN activity at cloud-relevant particle sizes.

This study investigates an ozone intrusion event observed during the Pre‐Asian Summer Monsoon Chemical and Climate Impact Project in August 2021, using 26 consecutive daily ozonesonde measurements over South Korea. A pronounced enhancement in total column ozone was observed between 17 and 19 August, which can be largely attributed to an ozone intrusion in the upper troposphere–lower stratosphere (UTLS), accounting for approximately 60% of the increase. The upper tropospheric circulation patterns demonstrate a clear signature of anticyclonic Rossby wave breaking (AWB) on the northeastern edge of the Asian summer monsoon anticyclone, aligned with the summertime jet stream. This AWB, accompanied by a cut‐off low and tropopause folding, facilitated downward transport of stratospheric ozone into the upper troposphere. In addition, the ozone variability is investigated in two chemical reanalysis data sets: Modern‐Era Retrospective Analysis for Research and Applications, Version 2 (MERRA‐2) and European Centre for Medium‐Range Weather Forecasts (ECMWF) Atmospheric Composition Reanalysis 4 (EAC4). MERRA‐2 and EAC4 capture the ozone intrusion event with relevant synoptic‐scale circulation patterns and ozone variability. However, discrepancies of ozone data in the chemical reanalyses were found in vertical ozone structures and persistence in the troposphere. MERRA‐2 better represented the secondary ozone peak in the UTLS but underestimated lower‐tropospheric ozone. In contrast, EAC4 showed a systematic positive bias particularly in the stratosphere and near the surface. Continued integration of temporally high‐resolution ozone measurements is beneficial for understanding synoptic‐scale ozone variability and evaluating emerging chemical reanalyses.

This study investigates an ozone intrusion event observed during the Pre-Asian Summer Monsoon Chemical & Climate Impact Project (Pre-ACCLIP) in August 2021, using 26 consecutive daily ozonesonde measurements over South Korea. A pronounced enhancement in total column ozone was observed between 17 and 19 August, which can be largely attributed to an ozone intrusion in the upper troposphere–lower stratosphere (UTLS), accounting for approximately 60% of the increase. The upper tropospheric circulation patterns demonstrate a clear signature of anticyclonic Rossby wave breaking (AWB) on the northeastern edge of the Asian summer monsoon anticyclone, aligned with the summertime jet stream. This AWB, accompanied by a cut-off low and tropopause folding, facilitated downward transport of stratospheric ozone into the upper troposphere. In addition, the ozone variability is investigated in two chemical reanalysis datasets: Modern-Era Retrospective Analysis for Research and Applications, Version 2 (MERRA-2) and ECMWF Atmospheric Composition Reanalysis 4 (EAC4). MERRA-2 and EAC4 capture the ozone intrusion event with relevant synoptic-scale circulation patterns and ozone variability. However, discrepancies of ozone data in the chemical reanalyses were found in vertical ozone structures and persistence in the troposphere. MERRA-2 better represented the secondary ozone peak in the UTLS but underestimated lower-tropospheric ozone. In contrast, EAC4 showed a systematic positive bias particularly in the stratosphere and near the surface. Continued integration of temporally high-resolution ozone measurements is beneficial for understanding synoptic-scale ozone variability and evaluating emerging chemical reanalyses.