Particle partitioning potential of organic compounds is highest in the Eastern US and driven by anthropogenic water

Abstract. Gas-phase water-soluble organic matter (WSOM_g) is ubiquitous in the troposphere. In the summertime, the potential for these gases to partition to particle-phase liquid water (H₂O_ptcl) where they can form secondary organic aerosol (SOA_AQ) is high in the Eastern US and low elsewhere, with the exception of an area near Los Angeles, CA. This spatial pattern is driven by mass concentrations of H₂O_ptcl, not WSOM_g. H₂O_ptcl mass concentrations are predicted to be high in the Eastern US, largely due to sulfate. The ability of sulfate to increase H₂O_ptcl is well established and routinely included in atmospheric models; however WSOM_g partitioning to this water and subsequent SOA formation is not. The high mass concentrations of H₂O_ptcl in the southeast (SE) US but not the Amazon may help explain why biogenic SOA mass concentrations are high in the SE US but low in the Amazon. Furthermore, during the summertime in the Eastern US, the potential for organic gases to partition into liquid water is greater than their potential to partition into organic matter (OM) because concentrations of WSOM_g and H₂O_ptcl are higher than semi-volatile gases and OM. Thus, unless condensed phase yields are substantially different (> ~ order of magnitude), we expect that SOA formed through aqueous-phase pathways (SOA_AQ) will dominate in the Eastern US. These findings also suggest that H₂O_ptcl is largely anthropogenic and provide a previously unrecognized mechanism by which anthropogenic pollutants impact the amount of SOA mass formed from biogenic organic emissions. The previously reported estimate of the controllable fraction of biogenic SOA in the Eastern US (50%) is likely too low.