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–· + organics [Formula: see text] 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 kobs = k0/(1 + B [Inh]), where kobs and k0 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 kinh 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.

The automobile exhausts are one of the major sources of particulate matter in urban areas and these particles are known to influence the atmospheric chemistry in a variety of ways. Because of this, the oxidation of dissolved sulfur dioxide by oxygen was studied in aqueous suspensions of particulates, obtained by scraping the particles deposited inside a diesel truck exhaust pipe (DEP). A variation in pH showed the rate to increase with increase in pH from 5.22 to about ∼6.3 and to decrease thereafter becoming very slow at pH = 8.2. In acetate-buffered medium, the reaction rate was higher than the rate in unbuffered medium at the same pH. Further, the rate was found to be higher in suspension than in the leachate under otherwise identical conditions. And, the reaction rate in the blank reaction was the slowest. This appears to be due to catalysis by leached metal ions in leachate and due to catalysis by leached metal ions and particulate surface both in suspensions. The kinetics of dissolved SO2 oxidation in acetate-buffered medium as well as in unbuffered medium at pH = 5.22 were defined by rate law: k obs  = k 0 + k cat [DEP], where k obs and k 0 are observed rate constants in the presence and the absence of DEP and k cat is the rate constant for DEP-catalyzed pathway. At pH = 8.2, the reaction rate was strongly inhibited by DEP in buffered and unbuffered media. Results suggest that the DEP would have an inhibiting effect in those areas where rainwater pH is 7 or more. These results at high pH are of particular significance to the Indian subcontinent, because of high rainwater pH. Conversely, it indicates the DEP to retard the oxidation of dissolved SO2 and control rainwater acidification.

The oxidation of dissolved sulfur dioxide, sulfur(IV), by oxygen proceeds through the involvement of sulfoxy radicals among which sulfate radical anion is the main chain carrier. When organics are present, they inhibit the oxidation of sulfur(IV) via scavenging of SO4− radicals. In contrast to previous studies, which were limited mostly to aliphatic compounds, this paper presents the results of the effect of 13 new volatile organic compounds (VOCs) including aromatic and heterocyclic on uncatalyzed sulfur(IV) autoxidation at pH 8.2 and 25°C. In all cases, the kinetics was first order in the presence and absence of VOCs and experimental rate law was Eq. (1). trueright−d[normalSfalse( IV false)]/normaldt=k obs []normalSfalse( IV false)=ko[]normalSfalse( IV false)/{}1+Bfalse[ Inh false]where −d[S(IV)]/dt is the rate of sulfur(IV) disappearance, kobs is the first‐order rate constant in the presence of inhibitor, ko is the first‐order rate constant in the absence of inhibitor, [S(IV)] is concentration of sulfur(IV) at time, t, and B is an inhibition parameter. VOCs cause inhibition by scavenging sulfate radical anions, which propagate the autoxidation chain. An analysis of B (Eq. (1)) and kinh (Eq. (2)) values for 21 aliphatic, aromatic, acyclic, and heterocyclic organic compounds showed that these to be related by Eq. (3) for a subgroup and Eq. (4) for b subgroup. trueright SO 4−+ VOC 0.33em⟶k inh 0.33em SO 42−+ VOC a subgroup (benzamide, 2,2‐dimethyl‐1‐propanol, 1‐hexanol, methanol, ethanol, 1‐propanol, 2‐ propanol, 1‐butanol, 2‐butanol, ethylene glycol, rebaudioside A) truerightB=()0.87±0.21×10−30.16emk inh b subgroup (o‐toluic acid, m‐toluic acid, p‐toluic acid, 4‐hydroxybenzoic acid, 1‐heptanol, glycerol, sucralose, acesuifame K, glycine, 3‐pentanol) truerightB=()1.2±0.3×10−30.16emk inh

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