Biogenic Secondary Organic Aerosols over the Bay of Bengal: Interplay with Aerosol Liquid Water Content and Biomass Burning

Organic aerosols contribute significant uncertainty to climate-forcing feedbacks due to poorly constrained secondary organic aerosol (SOA) formation pathways. This study investigates biogenic SOA (BSOA) in the wintertime marine atmospheric boundary layer over the Bay of Bengal─a critical receptor region at the intersection of distinct outflows from the Indo-Gangetic Plain (IGP) and Southeast Asia (SEA). Through measurements of isoprene, α-/β-pinene, and β-caryophyllene oxidation products in PM2.5, we find β-caryophyllene SOA to be the most abundant BSOA class. BSOA concentrations correlate with levoglucosan in both outflows, suggesting a potential link to the oxidation of precursors coemitted with biomass-burning products. While aerosol liquid water content (ALWC) is comparable across outflows, BSOA exhibit significant linear relationships with anthropogenic non-sea-salt-SO42– and ALWC specifically within the SEA outflow. These associations, coupled with observed NH4+/SO42– equivalent ratios (0.77 ± 0.12), suggest that acid-catalyzed aqueous-phase processing may contribute to BSOA formation in the SEA outflow over the open ocean. This covariability highlight the importance of distinct continental outflow signatures to understand BSOA evolution in marine environments, while acknowledging the limitations of indirect acidity proxies and idealized ALWC assumptions.