Atmospheric Oxidation Mechanism of Biogenic Volatile Organic Compounds Initiated by OH Radical: Methyl 2-Hydroxybenzoate and Methyl Benzoate

Methyl 2-hydroxybenzoate, also known as methyl salicylate (MeSa), is a volatile organic compound emitted from green leaves into the atmosphere. The emission of MeSa into the atmosphere is comparable to that of benzene; however, information on its oxidation in the air is limited. Here, the atmospheric oxidation mechanism of MeSa, initiated by its reaction with the OH radical, is investigated using theoretical calculations. The oxidation mechanism of methyl benzoate (MeBz), a homologue to MeSa, is also studied. Oxidation of both MeSa and MeBz with OH starts with OH additions to the aromatic ring, forming MeSa/MeBz-nOH adducts (n denotes the addition site), and by hydrogen-atom abstraction (HAA) from the methyl group. MeSa-3OH, MeSa-5OH, and MeBz-3OH react with O2 similarly to substituted benzene-OH adducts, forming dicarbonyls through bicyclic and bicyclic peroxy radical intermediates, while MeSa-2OH reacts in a similar way as phenol-2OH, forming a substituted cyclohexadienone compound. For MeSa-6OH, MeBz-2OH, and MeBz-4OH, they react with O2 mainly by direct HAA, forming methyl 2,6-dihydroxybenzoate, methyl 2-hydroxybenzoate (MeSa), and methyl 4-hydroxybenzoate, respectively. Under atmospheric conditions, MeSa-6OH and MeBz-4OH react with O2 slowly at pseudo-first-order rate coefficients of ∼102 s–1, allowing their reactions with NO2 in polluted regions and forming the notorious aromatic nitrate compounds.