Simulations of SOA Formation Using CMAQ‐UNIPAR During the 2019 FIREX‐AQ Campaign

Abstract

Wildfire smoke contains a large amount of phenolic hydrocarbons (HCs) that react with atmospheric oxidants, producing secondary organic aerosol (SOA). In this study, the UNIfied Partitioning Aerosol‐phase Reaction (UNIPAR) model was integrated into the Community Multiscale Air Quality Modeling System (CMAQ) to predict SOA over the West Coast of the United States during the Fire Influence on Regional to Global Environments Experiment‐Air Quality (FIREX‐AQ) campaign. UNIPAR predicts SOA formation via multiphase reactions of explicitly predicted products from the oxidation of precursor HCs. In CMAQ‐UNIPAR, various phenolic HCs were surrogated with phenol, cresol, and catechol, and their parameters were established for three oxidation pathways (OH radicals, O ₃ , and NO ₃ radicals). Phenolic SOA reached nearly 80% of the total SOA in McCall, ID (remote area), followed by terpene SOA. Alkane SOA, driven by anthropogenic emissions, was minimal in remote areas, but still substantial in Sacramento, CA. Simulations showed that both partitioning and heterogeneous reactions of reactive organics were equally important for SOA formation. Phenolic SOA actively formed via reactions with OH radical in daytime. In a very low NO _x environment, phenolic SOA is also produced through the ozonolysis of catechol at nighttime, though not as abundantly as its daytime production. Under this circumstance, NO _x increments positively impacted phenolic SOA concentrations more than terpene SOA in remote areas. Ozone production increases with increasing NO _x in NO _x ‐limited regimes owing to high fluxes of smoke‐origin HCs; in turn, this ozone elevates OH radicals that rapidly react with phenolic HCs (catechol), effectively increasing SOA.