Theoretical model on the formation possibility of secondary organic aerosol from OH initialed oxidation reaction of styrene in the presence of O2/NO

Understanding OH oxidation reaction is vital in understanding atmospheric pollution dynamics, and developing possible strategies for countering pollutant problems. This study used a theory-based approach to model the formation mechanisms of secondary organic aerosol (SOA) from styrene-OH oxidation reactions in the presence of O2/NO. As a comparative measure, the mechanisms in the absence of NO (representing a pollution-free environment) were also investigated. The results showed that styrene can be initially attacked by OH in two ways: OH-addition and H-abstraction. The OH-aliphatic-addition pathway occurs easily; the H-abstraction pathway may be ignored given atmospheric conditions. It was found that IMaddβ (C6H5CHCH2OH) was the main intermediate, and could be transformed to a peroxyl radical in the presence of O2. In the NO-free atmosphere, the peroxyl radical was decomposed to recycling-OH and aldehydes. In the NO-polluted atmosphere, it could be degraded to organic nitrate (RO-NO2) which plays an important role in the production of SOA. Besides, the percent of organic nitrate in the particulate phase was calculated within the range of 2.4%–6.3% in Guangzhou city, and organic nitrates may constitute an important fraction of the total organic aerosol. The kinetic data calculated using canonical variational transition state theory with the small-curvature tunneling correction showed that, in the NO-polluted/unpolluted atmospheres, the styrene-OH oxidation reaction easily occurred across an altitude range of 0–12 km. Especially, peroxyl radical lifetime was 10−3 s in the high NO-polluted atmosphere, indicating that the styrene-OH oxidation reaction could significantly contribute to SOA formation in the NO-polluted atmosphere. The current results informed possible approaches for forming SOA from volatile organic compound (VOC) oxidation reactions, and could help evaluate regional air quality, especially in high NO-polluted atmospheres.