Modeling Daytime and Nighttime Secondary Organic Aerosol Formation via Multiphase Reactions of Hydroxy-Aromatic Hydrocarbons from Wildfires

Atmospheric oxidations of phenol, o- cresol, o -catechol, and 3-methylcatechol were predicted using semiexplicit mechanisms for major oxidants (OH, O 3, and NO 3 ). Products formed under varying NO x levels were then classified using volatility and reactivity in aerosols and applied to the UNIfied Partitioning Aerosol Phase Reaction (UNIPAR) model, which predicts secondary organic aerosol (SOA) mass via multiphase reactions of hydrocarbons. UNIPAR was demonstrated using chamber data generated under varying environments. Daytime SOA formed dominantly by oxidations with OH radicals, and its order in SOA yields is as follows: 3-methylcatechol > o -cresol > o -catechol ≈ phenol. The NO x dependency of daytime SOA was weak and varied for each hydrocarbon. The methyl group on o -cresol and 3-methylcatechol increases SOA yields compared to phenol and o -catechol, respectively. With 50 ppb hydrocarbon under varying NO x levels, daytime o -cresol SOA yields are 0.46–0.68, which are 2-times larger than phenol, and those of 3-methylcatechol reach to 0.86, which is nearly 3-times higher than o -catechol. Nighttime oxidation of o -catechol (0.31) and 3-methylcatechol with O 3 or NO 3 produces considerable SOA yields 0.67 for 3-methylcatechol and 0.31 for o -catechol. OH-substituted aromatic HCs in wildfire smoke substantially form SOA through day and night oxidations in environments where O 3 and NO x are abundant and influence air quality.