Modeling Evolution in Secondary Organic Aerosol Formation Through Multiphase Reactions of Hydrocarbons in NO_x‐Rich Urbans During KORUS‐AQ

Abstract The UNIfied Partitioning‐Aerosol phase Reaction (UNIPAR) model predicts secondary organic aerosol (SOA) formation via the multiphase partitioning and aerosol‐phase reactions of explicitly predicted products from oxidations of various precursors. This UNIPAR was integrated into the comprehensive air quality model with extensions (CAMx) to predict SOA mass and species over the Korean Peninsula during the Korea‐US Air Quality (KORUS‐AQ) campaign in 2016. CAMx‐UNIPAR v1.4 included 10 aromatics, alkanes with different carbon lengths and branching ratios, naphthalene, isoprene, terpene, and sesquiterpene. Simulations indicated that nearly 70% of the organic aerosol mass in Seoul was associated with SOA, and more than 50% of SOA was anthropogenic. Alkane SOA was the most prominent (35%–40%), followed by terpene SOA (15%–16%) and aromatic SOA (13%). During the haze event, a large concentration of SOA formed at both ground sites and high altitudes as measured from aircraft. Aromatic SOA was somewhat sensitive to humidity. Isoprene SOA was susceptible to humidity, but its contribution was small in South Korea. In Seoul, where the NO x was rich, the simulated terpene SOA increased with increasing NO x levels, but the anthropogenic SOA formation declined slightly. A clear diurnal pattern appeared for terpene SOA, showing high SOA formation at nighttime owing to the terpene reaction with nitrate radicals to form low‐volatile organonitrates. The reduced mixing height at nighttime limited concentrations of precursors and radicals in the high altitude, and this condition suppressed the biogenic SOA formation. Anthropogenic SOA was produced mainly in the daytime when OH radicals were abundant.