Aqueous‐Phase Processing of Organic Aerosols at the Southern Great Plains: Insights From Integrated Chemical and Microphysical Analyses

Abstract Aqueous‐phase reactions are hypothesized to be a major pathway for secondary organic aerosol (SOA) formation in humid, cloud‐influenced environments, yet direct observational constraints remain limited. Here we investigated aqueous SOA (aqSOA) formation and evolution during the spring 2016 Holistic Interactions of Shallow Clouds, Aerosols, and Land Ecosystems (HI‐SCALE) campaign at the U.S. Department of Energy Southern Great Plains (SGP) site using high‐resolution aerosol mass spectrometer (HR‐AMS) measurements integrated with particle size distribution and meteorological observations. Positive matrix factorization (PMF) of HR‐AMS spectra resolved eight organic aerosol (OA) factors, including biomass burning OA (BBOA) factors with varying levels of oxidation, biogenic SOA (BSOArr), sulfate‐associated oxygenated OA (SOOA), nitrate‐associated oxygenated OA (NOOA), aged primary, anthropogenic OA, and nitrogen‐enriched OA. Four high RH periods associated with cloud presence and stable air masses were identified as favorable for aqueous processing, three of which were cloud‐ or humid‐boundary‐layer events with similar chemical evolution. During these events, SOOA, BSOA, and the most aged BBOA factor (BBOA3) collectively contributed 79% of OA mass (vs. 64% outside these events), with BSOA exhibiting the largest fractional increase. Growth‐rate analysis revealed size‐dependent particle growth consistent with volume‐controlled aqueous reactions. Correlations of aqSOA tracer ions (CHO + , CHO 2 + , CH 2 O 2 + , and C 2 H 2 O 2 + ) with BBOA3 and BSOA, and CH 3 SO 2 + with SOOA, further support aqSOA formation. These findings provide direct field evidence that aqueous‐phase chemistry significantly influences aerosol composition and evolution at SGP and demonstrate an integrated chemical‐microphysical framework for diagnosing aqSOA processing in ambient AMS data sets.