Influence Of Aerosol Water In The Organic Phase On The Mass, Properties And Source Apportionment Of Secondary Organic Aerosol In A Source-Oriented Model

Secondary organic aerosol (SOA) is hygroscopic meaning that water partitions into the organic phase to a degree determined by the organic chemical composition and the ambient relative humidity. In addition to contributing to the SOA mass, the absorbed water also alters the mean molecular weight of the organic phase and the activity coefficients of SOA's constituent compounds, which in turn influences SOA's gas-particle partitioning. Classical SOA treatment in aerosol transport models does not account for water uptake and its subsequent impacts. In this work, we use the UCD/CIT source-oriented air quality model to simulate water uptake by OA on source-oriented particle types and assess its influence on the mass, properties and source-apportionment of SOA. The model is run for a summer month over two distinct regions: California and the Southeast United States. Internally mixed aerosol calculations predict partitioning of water into the condensed organic phase when the relative humidity is higher than 60%. Daily-averaged organic water concentrations are approximately ~0.5 μg m^-3 under typical conditions but depend strongly on relative humidity. On average, the model predicts a 100% increase in the combined SOA-water mass for every 10% increase in relative humidity above 60%. The water in the organic phase is sufficient to substantially change the mean molecular weight of the SOA-water mixture and the activity coefficients of the SOA constituents to push gas-particle equilibrium towards the particle phase. For simplicity, POA (being hydrophobic) and SOA (being hydrophilic) are treated as separate phases. Externally mixed aerosol calculations predict similar levels of particle water and SOA enhancement because the functional characteristics of the POA emitted from different sources does not yet influence SOA formation on those particles. Each externally mixed particle type therefore develops similar SOA composition. Tests of water uptake into the organic phase on the solubility of POA and SOA and composition of POA as a result of atmospheric aging are ongoing.