PNNL

Abstract

Atmospheric aerosols are a significant public health hazard and have substantial impacts on the climate. Secondary organic aerosols (SOA) have been shown to phase separate into a highly viscous organic outer layer surrounding an aqueous core. This phase separation can decrease the partitioning of semi-volatile and low-volatile species to the organic phase and alter the extent of acid-catalyzed reactions in the aqueous core. A new algorithm that can determine SOA phase separation based on their: glass transition temperature (Tg), Oxygen to Carbon (O:C) ratio, organic mass to sulfate ratio; and meteorological conditions was implemented into the Community Multiscale Air Quality Modeling (CMAQ) System version 5.2.1 and was used to simulate the conditions in the continental United States for the summer of 2013. SOA formed at the ground/surface level was predicted to be phase-separated with core-shell morphology i.e. aqueous inorganic core surrounded by organic coating 65.4% of the time during 2013 Southern Oxidant and Aerosol Study (SOAS) on average in isoprene-rich southeastern United States. Our estimate is in proximity to previously reported ~ 70% in literature. The phase states of organic coatings switched between semi-solid and liquid states, depending on the environmental conditions. The semi-solid shell occurring with lower aerosol liquid water content (western United States and at higher altitudes) has a viscosity that was predicted to be 100-1E12 Pa·s which resulted in organic mass being decreased due to diffusion limitation. Organic aerosol was primarily liquid where aerosol liquid water was dominant (eastern United States and at surface), with a viscosity