Inhibition of atmospheric aqueous phase autoxidation of sulphur dioxide by volatile organic compounds: mono-, di- and tri-substituted benzenes and benzoic acids

Like many other volatile organic compounds (VOCs), the methoxy derivatives of benzene and benzoic acid are found in the atmosphere and, therefore, looking to their possible intervention in aqueous phase atmospheric oxidation of the major acid rain precursor, SO2, by oxygen, the kinetics of SO2 [hereafter referred to as S(IV)] autoxidation have been studied in the presence of methoxybenzene (anisole) and disubstituted 1,2-dimethoxybenzene, 1,3-dimethoxybenzene and 1,4-dimethoxybenzene. The effects of benzoic acid and its 2,6-dimethoxy, 3,4-dimethoxy, 2,3,4-trimethoxy and 2,4,5-trimethoxy derivatives have also been examined, as have the influences of 4-hydroxybenzoic acid, benzanilide and o-sulfobenzimide. As the methoxy group is known to influence the reaction SO4- center dot + organics (kinh)(->) SO42- + non-chain products, to understand the degree of influence of the methoxy group on the inhibition of this reaction, this series of methoxy compounds was selected. The kinetics were first-order in S(IV). Most of the VOCs inhibited S(IV) autoxidation, except benzanilide and o-sulfobenzimide, in accordance with k(obs) = k(0)/(1+B [Inh]), where k(obs) and k(0) are the first-order rate constants in the presence and absence of VOCs respectively and B is the inhibition parameter. The most interesting feature of this work is that, despite the high k(inh) values, the B values are unexpectedly quite low and benzanilide and o-sulfobenzimide showed no effect. The B values appear to be independent of kinh, which is opposite to that found in the case of a series of hydroxyl compounds. It appears probable that additional steps involving peroxy intermediates are involved.