Chemical Characterization of Water-Soluble Organic Aerosol in Contrasting Rural and Urban Environments in the Southeastern United States.

We developed a novel system for direct and online characterization of water-solubility of organic aerosol (OA) by coupling a Particle Into Liquid Sampler (PILS) to a High-Resolution Time-of-Flight Aerosol Mass Spectrometer (HR-ToF-AMS). We showed that approximately 88% and 77% of OA are water-soluble in rural Centreville, Alabama and urban Atlanta, Georgia, respectively. The water-solubility of OA factors, resolved with Positive Matrix Factorization analysis of AMS data, is directly investigated for the first time. Above 80% of isoprene-derived OA is water-soluble and its water-soluble fraction has the least variability among all OA factors. This is consistent with that the majority of this factor represents OA formed through the aqueous-phase reaction of isoprene epoxydiols. More-oxidized oxygenated OA is dominantly water-soluble, consistent with this factor representing highly oxidized compounds. Less-oxidized oxygenated OA has the lowest water-solubility among all secondary OA factors, which agrees with the hypothesis that this factor in the southeastern U.S. includes contributions from organic nitrates. While hydrocarbon-like OA is largely water-insoluble, biomass burning OA and cooking OA have the largest range of water-soluble fraction. This study on the water-solubility of OA factors provides insights for interpretation of OA factors and improves understanding of the complex OA sources in the atmosphere.