Alkoxy Radical Bond Scissions Explain the Anomalously Low Secondary Organic Aerosol and Organonitrate Yields From α-Pinene + NO.

Oxidation of monoterpenes (CH) by nitrate radicals (NO) constitutes an important source of atmospheric secondary organic aerosol (SOA) and organonitrates. However, knowledge of the mechanisms of their formation is incomplete and differences in yields between similar monoterpenes are poorly understood. In particular, yields of SOA and organonitrates from α-pinene + NO are low, while those from Δ-carene + NO are high. Using computational methods, we suggest that bond scission of the nitrooxy alkoxy radicals from Δ-carene lead to the formation of reactive keto-nitrooxy-alkyl radicals, which retain the nitrooxy moiety and can undergo further reactions to form SOA. By contrast, bond scissions of the nitrooxy alkoxy radicals from α-pinene lead almost exclusively to the formation of the relatively unreactive and volatile product pinonaldehyde (CHO), thereby limiting organonitrate and SOA formation. This hypothesis is supported by laboratory experiments that quantify products of the reaction of α-pinene + NO under atmospherically relevant conditions.