Global Simulations of Phase State of Secondary Organic Aerosols with GEOS-Chem

REGINA LUU, Meredith Schervish, Nicole June, O'Donnell Samuel, Shantanu Jathar, Jeffrey R. Pierce, Manabu Shiraiwa, University of California, Irvine

     Abstract Number: 699
     Working Group: Aerosols Spanning Spatial Scales: Measurement Networks to Models and Satellites

Abstract
Secondary organic aerosols (SOA) phase states have strong implications for aerosol effects on climate and air quality. The phase state of SOA particles informs SOA formation and partitioning, multiphase chemistry, and ice nucleation. Glass transition temperature (T_g) denotes a nonequilibrium phase state change from a solid to a more malleable semi-solid state as temperature increases. Regional modeling studies over the contiguous US have shown that SOA in the western US has higher surface T_g and SOA viscosity compared to the eastern US, resembling geospatial relative humidity patterns. On the global scale, SOA is predicted as mostly liquid in tropical and polar air with high relative humidity, semi-solid at the mid-latitudes, and solid over arid lands . However, most large-scale climate models do not intrinsically account for SOA phase state and its consequences on gas-particle partitioning. Here, we aim to implement SOA phase state prediction methodology into the global transport model GEOS-Chem to evaluate spatial-temporal variations in the atmosphere. We use the volatility basis set (VBS) and molecular corridors to assign molar mass and O:C ratios of organic compounds to estimate T_g. The prediction of SOA viscosity was derived from the Angell plot of fragility while considering the Gordon-Taylor mixing rule and hygroscopic growth of SOA particles. Our results suggest agreement with previous findings on the global distribution of SOA phase states. Then sensitivity studies varying assumed O:C and the hygroscopicity parameter by +/- 50%, indicate a significant dependence of T_g on the hygroscopicity parameter but minimally on O:C. Although initial results presented here are from offline calculations, with an online integration of SOA phase state predictions, large-scale models like GEOS-Chem can have more comprehensive evaluations of global atmospheric aerosol processes and the impacts on air quality, public health, and the climate.