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, O3, and NO3). Products formed under varying NOx 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 NOx 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 NOx 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 O3 or NO3 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 O3 and NOx are abundant and influence air quality.