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

Smog chamber experiments have been conducted in which cyclic monoterpenes w ere 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 part icular phase corresponded to 0.5%-4.2% of the reacted carbon. The most abun dant 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-carboxy lic acids, and C-10-hydroxy-keto-aldehydes (pinonaldehyde, pinonic acid, hy droxy-pinonic acid isomers, and hydroxy-pinonaldehyde isomers from alpha -p inene; 3-caronaldehyde, 3-caronic acid, hydroxy-3-caronic acid isomers, and hydroxy-3-caronaldehyde isomers from 3-carene). The most abundant compound s 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, p inic acid, and hydroxy-pinonic acid isomers from beta -pinene; sabinaketone , sabinic acid and hydroxy-sabinonic acid isomers from sabinene). Decarboxy lated analogues of most of the compounds were present in SOA in minor conce ntrations, such as C-9-keto-carboxylic acids (norpinonic acid, nor-3-caroni c acid) and C-8-dicarboxylic acids (norpinic acid, nor-3-caric acid, norsab inic acid). In SOA from limonene, which contains an endocyclic as well as a n exocyclic C=C double bond, the most abundant compounds were a C-10-keto-a ldehyde and its oxo-derivative (limononaldehyde and keto-limonon aldehyde) together with hydroxy-derivatives of a C-10-keto-carboxylic acid (isomers o f hydroxy-limononic acid). Also a C-10-keto-carboxylic acid (limononic acid ) was present together with minor concentrations of a C-9-dicarboxylic acid s (limonic acid), its oxo-derivative (keto-limonic acid), and its decarboxy lated analogue (norlimonic acid). Mechanistic pathways for the formation of these products, some of which are identified here for the first time, are proposed.