Formation aqueous-phase oxidation pathway

Assessment of Aqueous-Phase Oxidation Pathway. The results of two modeling studies (7. 13) employing chemical reaction schemes that included aqueous-phase formic acid formation are summarized in Table III. The reaction mechanism used by Adewuyi et al. (7) included the aqueous-phase oxidation of formaldehyde by hydrogen peroxide and hydroxy radicals the mechanism of Chameides (13) included only oxidation by hydroxy radicals. Neither model included reactions for the formation of acetic acid. By comparing Table III with Table I, it can be seen that the concentrations of formic acid... 

Information obtained during the statistical analyses was used in evaluating aerosol scavenging and aqueous-phase oxidation as the dominant pathways for introducing formate and acetate ions into precipitation. The plausibility of homogeneous gas-phase production... 

An assessment of the plausibility of various pathways that could introduce the organic compounds into precipitation suggests aqueous-phase oxidation of aldehydes is probably not a major contributor because of the large atmospheric acetaldehyde concentration that must be postulated to produce the observed formate/acetate ratio. Alternatively, potential gas-phase formic and acetic acid... 

Calvert (2 ) has pointed out that gas-phase reactions of SO2 with ozone (O3), hydroxyl radical (OH ), and hydroperoxyl radical (HOp ) are too slow to account for the aforementioned rates of sulfate production. Consequently, the catalytic autoxidation of SO2 in deliquescent haze aerosol and hydrometeors has been proposed as a viable non-photolytic pathway for the rapid formation of sulfuric acid in humid atmospheres (30-35). In addition, hydrogen peroxide and ozone have been given serious consideration as important aqueous-phase oxidants of dissolved SO2 as discussed by Martin (35). Oxidation by H2O2 seems to be most favorable under low pH conditions (pH < 4) because of a rapid rate of reaction anc[ a negative pH-dependence that favors the facile conversion of HSO3 to sulfate. 

The majority of airborne acid sulfate appears to be formed in cloud droplets ("aqueous-phase oxidation"). SO2 dissolves to form the bisulfite anion (HSO3"), which then reacts with hydrogen peroxide (H2O2) to form acid sulfate. The lower the pH the faster this reaction proceeds. At pHs above 5.0, reaction between HS03 and ozone (O3) becomes appreciable and may become the dominant pathway for acid formation. ... 

Patiiway (c) is more speculative, but it has been previously suggested by Horikoshi et al., 2001, who investigated the OH oxidation of 2P in the aqueous phase, in the presence of Ti02. This pathway may occur for NMP under real tropospheric conditions. It proceeds via a ring-opening mechanism, leading to the formation of N-methyl-4-aminobutanoic acid. 

In addition to direct emission in combustion processes, most secondary formation pathways in the gas and aqueous phases depends on ozone. By contrast, there is only one way that is light independent (ozonolysis) and another way that is independent of oxidant precursors (aqueous phase electron transfer onto oxygen Eqs. 5.96 and 5.102). 

In recent years there has been intense interest in the formation and evolutiOTi of atmospheric particulate matter within the aqueous phase [170]. Such processes occur by dissolution of organics into a water droplet (deliquesced particle or cloud droplet) followed by oxidation by a dissolved oxidant (most likely OH). Studies of these pathways have been reviewed in detail very recently [118,171] and so will not be discussed here instead, as in the previous section, the focus here is on the relationship between partitiOTiing and aging chemistiy. 

Condensed-phase SOA formation water-soluble VOCs may dissolve into the aqueous phase of cloud droplets or wet aerosols. Subsequent aqueous-phase reactions (e.g., oxidation and/or oligomerization) can lead to the formation of low-volatility secondary organic material [82-87]. In particular, the dicarbonyl VOCs glyoxal and methylglyoxal have been studied as potential precursors for this SOA formation pathway. Recently, aqueous-phase reactions of isoprene-derived epoxydiols have also been shown to be efficient pathways to SOA formation in the aerosol aqueous phase [88-91]. 

Reaction Mechanisms. Our analysis of intermediates and reactions reported by other researchers leads to proposed reaction pathways describing the photocatalytic oxidation of 4-chlorophenol in TiOz aqueous suspensions. The photocatalytic oxidation reaction is brought about by OH radicals, which are formed mainly from water decomposition on the Ti02 surface upon UV light irradiation (9-13). The OH radicals can either directly react with the adsorbed organic species on the TiOa surface or diffuse to the solution and then react with the dissolved organic species in the solution phase. Both reactions lead to formation of hydroxylated products such as 4-chlorocatechol, hydroquinone, 4-chlororesorcinol, and hydroxyhydroquinone as the initial products (Figure 6). Eventually, the reaction will mineralize these interme-... 

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