Seasonal Assessment of Secondary Organic Aerosol Formed through Aqueous Pathways in the Eastern United States

Despite the importance of aqueous secondary organic aerosol (aqSOA), several uncertainties remain regarding its formation. These include the seasonal contributions of aqSOA to total SOA mass and the relative proportions formed via reversible versus irreversible pathways. In this study, we use measurements of seasonal water-soluble organic compounds in the particle and gaseous phases (i.e., WSOC_p and WSOC_g, respectively) at a site in the eastern U.S. to quantify aqSOA concentrations as well as its reversible and irreversible fractions within each season. We show evidence that aqSOA concentrations present a significant contribution to total SOA mass during the nighttime in the eastern U.S. with ~30% and ~50% in the cold and warm seasons, respectively. Further, aqSOA mass is mostly formed through irreversible pathways except during the warm seasons, where reversible partitioning of WSOC_g accounts for ~10% of the total SOA mass (~20% of aqSOA mass). Comparisons between the seasonally measured aqSOA concentrations and their counterpart Community Multiscale Air Quality (CMAQ) modeled mass provide evidence that accounting for additional reversible and irreversible aqSOA pathways could improve model predictions of SOA concentrations and close the current gap between those predictions and field observations. This work holds valuable implications for the atmospheric chemistry of multiphase aerosol formation and its accurate representation in chemical transport models.