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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
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  • Yujin Jo,
  • Myoseon Jang,
  • Azad Madhu,
  • Jiwon Choi,
  • Jin-soo Park
Yujin Jo
Department of Environmental Engineering Sciences, Engineering School of Sustainable Infrastructure and Environment, University of Florida
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Myoseon Jang
Department of Environmental Engineering Sciences, Engineering School of Sustainable Infrastructure and Environment, University of Florida

Corresponding Author:[email protected]

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Azad Madhu
Department of Environmental Engineering Sciences, Engineering School of Sustainable Infrastructure and Environment, University of Florida
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Jiwon Choi
Department of Environmental Engineering Sciences, Engineering School of Sustainable Infrastructure and Environment, University of Florida
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Jin-soo Park
National Institute of Environmental Research
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Abstract

Secondary organic aerosol (SOA) mass in the Southern USA during winter-spring 2022 were 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 in different carbon lengths. UNIPAR simulations were compared with those using the partitioning-based model (SOAP), 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 its sensitivity to environmental variables (sunlight, humidity, NOx, and SO2) were different between two models. In CAMx-UNIPAR, SOA was predominated by alkane, terpene, and isoprene, and was notably influenced by humidities 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 nitrate radicals effectively oxidized biogenic hydrocarbons at night and increased hygroscopic nitrate aerosol promoted SOA growth via organic heterogeneous chemistry. Anthropogenic SOA, which formed mainly via the 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 decline of NOx emission benefits the mitigation of SOA burdens in the Southern USA where biogenic hydrocarbons are abundant.