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

[摘要] 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 NO 3 radicals. Here, daytime and nighttime patterns ofsecondary organic aerosol (SOA) originating from biogenic hydrocarbons werepredicted under varying environmental conditions (temperature, humidity,sunlight intensity, NO x levels, and seed conditions) by using theUNIfied Partitioning Aerosol phase Reaction (UNIPAR) model, which comprisesmultiphase gas–particle partitioning and in-particle chemistry. The productsoriginating from the atmospheric oxidation of three different hydrocarbons(isoprene, α -pinene, and β -caryophyllene) were predicted byusing extended semi-explicit mechanisms for four major oxidants (OH,O 3 , NO 3 , and O( 3 P)) during day and night. The resultingoxygenated products were then classified into volatility–reactivity-basedlumping species. The stoichiometric coefficients associated with lumpingspecies were dynamically constructed under varying NO x levels, and they were applied to the UNIPAR SOA model. The predictability ofthe model was demonstrated by simulating chamber-generated SOA data undervarying environments. For daytime SOA formation, both isoprene and α -pinene were dominated by the OH-radical-initiated oxidation showing a gradual increase in SOA yields with decreasing NO x levels. The nighttime isoprene SOA formation was processed mainly by the NO 3 -driven oxidation, yielding higher SOA mass than daytime at higher NO x level (isoprene  /  NO x    5 ppb C ppb −1 ). At a given amount of ozone, the oxidation to produce the nighttime α -pinene SOA gradually transited from the NO 3 -initiated reaction to ozonolysis as NO x levels decreased. Nighttime α -pinene SOA yields were also significantly higher than daytime SOA yields, although the nighttime α -pinene SOA yields gradually decreased with decreasing NO x levels. β -Caryophyllene, which rapidly produced SOA with high yields, showed a relatively small variation in SOA yields from changes in environmental conditions (i.e., NO x 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 α -pinene SOA in the presence of gasoline fuel, which can compete with α -pinene for the reaction with OH radicals in typical urban air, suggested more growth of α -pinene SOA by the enhanced ozonolysis path. We concluded that the oxidation of the biogenic hydrocarbon with O 3 or NO 3 radicals is a source of the production of a sizable amount of nocturnal SOA, despite the low emission at night.

[学科分类] 医学（综合）