Global Simulations of Phase State and Equilibration Timescales 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: 378
Working Group: Remote and Regional Atmospheric Aerosol
Abstract
The phase state of secondary organic aerosols (SOA) can be liquid, amorphous semi-solid, or glassy solid, which is important to consider as it influences various multiphase processes including SOA partitioning, size distribution dynamics, heterogeneous chemistry as well as cloud condensation and ice nucleation. Most chemical transport models typically employ instantaneous equilibrium partitioning for SOA formation by implicitly assuming that SOA adopts a low viscous liquid phase state. In this study we simulate the glass transition temperatures (Tg) of SOA species and particle viscosities over the globe using the global chemical transport model, GEOS-Chem. The simulated spatial distributions show that SOA at the surface exists as liquid over equatorial regions, solid at the mid-latitude continental regions with low RH, and semisolid over the remaining lands. In the free troposphere, the predicted frequency of semisolid and solid particles becomes much greater than that of liquid particles. Using the derived bulk diffusivities, we simulate spatial distributions of the effective mass accommodation coefficient (αeff). We also calculate equilibration timescales (τeq) with the KM-GAP model which considers gas and bulk diffusion of oxidation products into the particle phase. Our results show that SOA particles over Western US, northern Africa, and Western China do not equilibrate within the chemical time step of GEOS-Chem of 20 min with αeff less than 0.1; hence, kinetically limited non-equilibrium growth would need to be considered for these regions in future large-scale model studies. The addition of an SOA phase state prediction scheme and the change to SOA partitioning could impact the predicted aerosol evolution in the atmosphere and its projected impacts on air quality and climate.