Global Simulations of Phase State and Equilibration Time Scales of Secondary Organic Aerosols with GEOS-Chem
REGINA LUU, Meredith Schervish, Nicole June, Samuel O'Donnell, Shantanu Jathar, Jeffrey R. Pierce, Manabu Shiraiwa, University of California, Irvine
Abstract Number: 33
Working Group: Aerosol Processes and Properties in Changing Environments in the Anthropocene
Abstract
The phase state of secondary organic aerosols (SOA) can range from liquid through amorphous semisolid to glassy solid, which is important to consider as it influences various multiphase processes including SOA formation and partitioning, multiphase chemistry, and cloud activation. In this study, we simulate the glass transition temperature and viscosity of SOA over the globe using the global chemical transport model, GEOS-Chem. The simulated spatial distributions show that SOA at the surface exist as liquid over equatorial regions and oceans, semisolid in the midlatitude continental regions, and glassy solid over lands with low relative humidity. The predicted SOA viscosities are mostly consistent with the available measurements. In the free troposphere, SOA particles are mostly predicted to be semisolid at 850 hPa and glassy solid at 500 hPa, except over tropical regions including Amazonia, where SOA are predicted to be low viscous. Phase state also exhibits seasonal variation with a higher frequency of semisolid and solid particles in winter compared to warmer seasons. We calculate equilibration time scales of SOA partitioning (τeq) and effective mass accommodation coefficient (αeff), indicating that τeq is shorter than the chemical time step of GEOS-Chem of 20 min and αeff is close to unity for most locations at the surface level, supporting the application of equilibrium SOA partitioning. However, τeq is prolonged and αeff is lowered over drylands and most regions in the upper troposphere, suggesting that kinetically limited growth would need to be considered for these regions in future large-scale model studies. We implement a kinetic partitioning scheme using αeff directly into the GEOS-Chem model to treat dynamic partitioning of SOA to investigate effects of phase state on SOA mass loadings in the atmosphere.