Simulated reductions in heterogeneous isoprene epoxydiol reactive uptake from aerosol morphology in the contiguous United States using the Community Multiscale Air Quality Model (CMAQv5.3.2)

Aerosol particles contain complex mixtures of polar and non-polar species that can undergo organic-inorganic phase separation. In phase-separated aerosol particles, the phase state of the outer organic coating can modulate heterogeneous chemistry. Heterogeneous chemistry leading to isoprene epoxydiol (IEPOX)-derived secondary organic aerosol (IEPOX-SOA) is encoded in the Community Multiscale Air Quality (CMAQ) model and has been the focus of previous aerosol phase separation and phase state work. In a previous study, a constant ratio of water in the organic coating ( w s ) was assumed in modeling phase separation and state. Recent studies, however, have highlighted w s as an important modulator of phase state. This work uses a later CMAQ version (version 5.3.2) with capabilities to model dynamic water uptake to the organic coating – to better predict w s and its impact on the organic coating phase state. In addition, new parameterizations for estimating organic aerosol phase state were encoded into CMAQ, and were compared with respect to their impacts on phase state and IEPOX-SOA predictions. These evaluations were completed simulating a summertime episode over the continental United States. Simulated diurnal profiles of aerosol phase state agreed within one standard deviation of observationally-derived field measurements. The implementation of phase separation and phase state parameterizations resulted in times and grid cells where IEPOX reactive uptake is completely suppressed. While modelled positive bias in 2-methyltetrol concentrations were decreased with phase separation and phase state updates, modelled methyltetrol sulfates and total IEPOX-SOA concentrations further underpredicted field observations in comparison to Base CMAQ.