Modeling daytime and nighttime secondary organic aerosol formation via multiphase reactions of biogenic hydrocarbons

The daytime oxidation of biogenic hydrocarbons is attributed to both OH radicals and O-3, while nighttime chemistry is dominated by the reaction with O-3 and NO3 radicals. Here, daytime and nighttime patterns of secondary organic aerosol (SOA) originating from biogenic hydrocarbons were predicted under varying environmental conditions (temperature, humidity, sunlight intensity, NOx levels, and seed conditions) by using the UNIfied Partitioning Aerosol phase Reaction (UNIPAR) model, which comprises multiphase gas-particle partitioning and in-particle chemistry. The products originating from the atmospheric oxidation of three different hydrocarbons (isoprene, alpha-pinene, and beta-caryophyllene) were predicted by using extended semi-explicit mechanisms for four major oxidants (OH, O-3, NO3, and O(P-3)) during day and night. The resulting oxygenated products were then classified into volatility-reactivity-based lumping species. The stoichiometric coefficients associated with lumping species were dynamically constructed under varying NOx levels, and they were applied to the UNIPAR SOA model. The predictability of the model was demonstrated by simulating chamber-generated SOA data under varying environments. For daytime SOA formation, both isoprene and alpha-pinene were dominated by the OH-radical-initiated oxidation showing a gradual increase in SOA yields with decreasing NOx levels. The nighttime isoprene SOA formation was processed mainly by the NO3-driven oxidation, yielding higher SOA mass than daytime at higher NOx level (isoprene / NOx < 5 ppb C ppb(-1)). At a given amount of ozone, the oxidation to produce the nighttime alpha-pinene SOA gradually transited from the NO3-initiated reaction to ozonolysis as NOx levels decreased. Nighttime alpha-pinene SOA yields were also significantly higher than daytime SOA yields, although the nighttime alpha-pinene SOA yields gradually decreased with decreasing NOx levels. beta-Caryophyllene, which rapidly produced SOA with high yields, showed a relatively small variation in SOA yields from changes in environmental conditions (i.e., NOx levels, seed conditions, and sunlight intensity), and its SOA formation was mainly attributed to ozonolysis day and night. The daytime SOA formation was generally more sensitive to the aqueous reactions than the nighttime SOA because the daytime chemistry produced more highly oxidized multifunctional products. The simulation of alpha-pinene SOA in the presence of gasoline fuel, which can compete with alpha-pinene for the reaction with OH radicals in typical urban air, suggested more growth of alpha-pinene SOA by the enhanced ozonolysis path. We concluded that the oxidation of the biogenic hydrocarbon with O-3 or NO3 radicals is a source of the production of a sizable amount of nocturnal SOA, despite the low emission at night.

Automated summary

This study used the UNIPAR model to predict the diurnal and nocturnal variation patterns of secondary organic aerosol (SOA) generated from biogenic hydrocarbons. It was found that daytime oxidation was mainly driven by OH radicals and O-3, while nighttime chemistry was dominated by O-3 and NO3 radicals. Environmental factors such as temperature, humidity, sunlight intensity, NOx levels, and seed conditions were found to have an impact on SOA formation.