Multiphase Reactions of Hydrocarbons Into an Air Quality Model With CAMx-UNIPAR: Impacts of Humidity and NOx on Secondary Organic Aerosol Formation in the Southern USA

Secondary organic aerosol (SOA) mass in the Southern USA during winter-spring 2022 was simulated by integrating Comprehensive Air quality Model with extensions (CAMx) with the UNIfied Partitioning-Aerosol phase Reaction (UNIPAR) model, which predicts SOA formation via multiphase reactions of hydrocarbons. UNIPAR streamlines multiphase partitioning of oxygenated products and their heterogeneous reactions by using explicitly predicted products originating from 10 aromatics, 3 biogenics, and linear/branched alkanes (C9-C24). UNIPAR simulations were compared with those using Secondary Organic Aerosol Partitioning (SOAP) model, which uses simple surrogate products for each precursor. Both UNIPAR and SOAP showed similar tendencies in SOA mass but slightly underpredicted against observations at given five ground sites. However, SOA compositions and their sensitivity to environmental variables (sunlight, humidity, NO_x, and SO₂) were different between two models. In CAMx-UNIPAR, SOA originated predominantly from alkanes, terpenes, and isoprene, and was influenced by humidity, showing high SOA concentrations with wet-inorganic salts, which accelerated aqueous reactions of reactive organic products. NO₂ was positively correlated with biogenic SOA because elevated levels of nitrate radicals and hygroscopic nitrate aerosol effectively oxidized biogenic hydrocarbons at night and promoted SOA growth via organic heterogeneous chemistry, respectively. Anthropogenic SOA, which formed mainly via daytime oxidation with OH radicals, was weakly and negatively correlated with NO₂ in cities. In CAMx-UNIPAR, the sensitivity of SOA to aerosol acidity (neutral vs. acidic aerosol at cation/anion = 0.62) was dominated by isoprene SOA. The reduction of NO_x emissions could effectively mitigate SOA burdens in the Southern USA where biogenic hydrocarbons are abundant.