Aqueous processing of water-soluble organic compounds in the eastern United States during winter

Aqueous multi-phase processes are significant contributors to organic aerosol (OA) mass in the atmosphere. This study characterizes the formation of water-soluble organic matter during the winter in the eastern United States through simultaneous measurements of water-soluble organic carbon in the gas and particle phases (WSOCg and WSOCp, respectively). The formation of secondary WSOCp occurred primarily through two pathways: (1) absorptive partitioning of oxygenated organics to the bulk OA and (2) aqueous phase processes. WSOCp formation through the former pathway was evident through the relationship between the fraction of total WSOC in the particle phase (F-p) and the total OA concentration. Conversely, evidence for nighttime aqueous WSOCp formation was based upon the strong enhancement in F-p with increasing relative humidity, indicating the uptake of WSOCg to aerosol liquid water (ALW). The F-p-RH relationship was only observed for temperatures between 0-10 degrees C, suggesting conditions for aqueous multi-phase processes were enhanced during these times. Temperature exhibited an inverse relationship with ALW and a proportional relationship with aerosol potassium. ALW and biomass burning precursors were both abundant in the 0-10 degrees C temperature range, facilitating aqueous WSOCp formation. To assess the impact of particle drying on the WSOCp concentrations, the particle measurements alternated between ambient and dried channels. No change was observed in the concentration of particles before and after drying, indicating that the WSOCp formed through the uptake of WSOCg into OA and ALW remained in the condensed phase upon particle drying at all temperature ranges. This work contributes to our understanding of sources, pathways, and factors affecting aqueous aerosol formation in the winter.