Free Radical Reactions in Clouds and Fogs

As might be expected based on our knowledge of the gas-phase chemistry, there are a variety of free 

Zafiriou (1983), for example, suggested that absorption of light by organics, followed by intersystem crossing (ISC) to the triplet state (T) as described in Chapter 3, could occur. The subsequent reaction of the organic in a triplet state with 02 could then give 02  

While energy transfer between triplets and 02 is well known, whether charge transfer can occur as shown in (21) is not clear. 

Production of superoxide can also occur via photolysis of iron-ligand (L) complexes, where L is oxalate, glyoxalate, or pyruvate, for example (Zuo and Hoigne, 1992, 1993 Zhu et al., 1993 Erel et al., 1993)  

Based on measurements of iron in both the (II) and (III) oxidation states and the anions in cloudwater and fogwater, Siefert et al. (1998) calculate that most of the Fe(III) is in the form of hydroxy species such as Fe(OH)2, with much smaller amounts ( 10%) in the form of oxalate complexes such as Fefoxalate),.  

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The solubility of SO2 is related to the pH of the aqueous phase, decreasing at lower values of pH. The oxidation of sulfur (IV) to sulfur (VI) is a complex process dependent on many physical and chemical factors. The main oxidants seem to be O2 (catalysed/uncatalysed), O3, H2O2, the oxides of nitrogen and free-radical reactions in clouds and fogs. For example, H2O2 is highly soluble in solution so even at relatively low gas-phase concentrations (typically ca. 1 ppbv) there is a... 

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