Gas-phase OH oxidation of monoterpenes: Gaseous and particulate products

Smog chamber experiments have been conducted in which cyclic monoterpenes were oxidised in the gas phase by OH. The evolved secondary organic aerosol (SOA) was analysed by LC-MSn and the gas-phase products were analysed by FT-IR. The concentrations of the identified compounds corresponded to carbon mass balances in the range of 40%-90%. The identified compounds in the particular phase corresponded to 0.5%-4.2% of the reacted carbon. The most abundant compounds in SOA from terpenes with an endocyclic C=C double bond were C-10-keto-aldehydes, C-10-keto-carboxylic acids, C-10-hydroxy-keto-carboxylic acids, and C-10-hydroxy-keto-aldehydes (pinonaldehyde, pinonic acid, hydroxy-pinonic acid isomers, and hydroxy-pinonaldehyde isomers from alpha -pinene; 3-caronaldehyde, 3-caronic acid, hydroxy-3-caronic acid isomers, and hydroxy-3-caronaldehyde isomers from 3-carene). The most abundant compounds in SOA from terpenes with an exocyclic C=C double bond were C-9-ketones, C-9-dicarboxylic acids, and C-10-hydroxy-keto-carboxylic acids (nopinone, pinic acid, and hydroxy-pinonic acid isomers from beta -pinene; sabinaketone, sabinic acid and hydroxy-sabinonic acid isomers from sabinene). Decarboxylated analogues of most of the compounds were present in SOA in minor concentrations, such as C-9-keto-carboxylic acids (norpinonic acid, nor-3-caronic acid) and C-8-dicarboxylic acids (norpinic acid, nor-3-caric acid, norsabinic acid). In SOA from limonene, which contains an endocyclic as well as an exocyclic C=C double bond, the most abundant compounds were a C-10-keto-aldehyde and its oxo-derivative (limononaldehyde and keto-limonon aldehyde) together with hydroxy-derivatives of a C-10-keto-carboxylic acid (isomers of hydroxy-limononic acid). Also a C-10-keto-carboxylic acid (limononic acid) was present together with minor concentrations of a C-9-dicarboxylic acids (limonic acid), its oxo-derivative (keto-limonic acid), and its decarboxylated analogue (norlimonic acid). Mechanistic pathways for the formation of these products, some of which are identified here for the first time, are proposed.