Alkoxy Radical Bond Scissions Explain the Anomalously Low Secondary Organic Aerosol and Organonitrate Yields from α-Pinene + NO₃

Oxidation of monoterpenes (C₁₀H₁₆) by nitrate radicals (NO₃) 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 + NO₃ are low, while those from Δ³-carene + NO₃ are high. Using computational methods, we suggest that bond scission of the nitrooxy alkoxy radicals from Δ³-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 (C₁₀H₁₆O₂), thereby limiting organonitrate and SOA formation. This hypothesis is supported by laboratory experiments that quantify products of the reaction of α-pinene + NO₃ under atmospherically relevant conditions.