Modeling Kinetically-Limited IEPOX-SOA Uptake via Volatility based and Compositionally defined Glass Transition Temperatures

Abstract:

A major assumption in air quality models are that the organic and inorganic constituents found in aerosols are homogenously mixed. In recent years studies have shown that these aerosols can in fact form organic coatings with an inorganic core. This core-shell morphology has implications on human health, cloud condensation nuclei, and secondary organic aerosol formation and aging via multiphase chemical processes. A recent implementation in the Community Multi-Scale Air Quality Model (CMAQ), version 5.2, uses glass transition temperature (T_g) and composition of the bulk aerosol along with the relative humidity to determine phase state and organic-inorganic separation of the aerosol. When conditions were favorable for phase separation, the T_g of the bulk aerosol was used to predict the viscosity and thus diffusivity of the organic shell. The T_g of the bulk aerosol was determined via the aerosol liquid water (ALW) in the organic shell (assumed to be 10 percent of total ALW) and T_g of the individual species contributing to the organic shell - determined via their atomic oxygen-to-carbon (O/C) ratio and molecular weight. This study aims to implement the aforementioned phase state determinations and T_g equations into the latest iteration of CMAQ, version 5.3 - which parameterizes organic ALW based on hygroscopicity parameters. Furthermore, a renaissance of laboratory-based studies on organic aerosol T_g have developed new equations based on O/C ratios, saturation concentrations, and elemental compositions. Taking advantage of these new implementations, and new ALW estimates in CMAQ 5.3, this study evaluates various T_g parameterizations effect on phase state and multi-phase chemistry reactions of IEPOX-derived SOA. Model output for each parameterization will be compared to each other and to observational data collected from routine monitoring and intensive field campaigns. This research aims to support future model development of CMAQ and to inform future experimental work further investigating how oxygenated organic coatings can impact multiphase chemistry.