KCii = 4.9 X 104 L mol"1. Production of H202 can then occur via

7 2 7 1 fast z z k23 = 9.7 X 107 Lmoh1 s"1, or alternatively, as in the gas phase, by the self-reaction of H02:

2H02 -> H202 + 02, k1A = 8.3 X 105 L mol~1 s"1.

(The rate and equilibrium constants are from Bielski et al. (1978, 1985), which provides an excellent review of reactions of H02 and 02 in aqueous solutions.)

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):

Fe(III) — L + hv -> Fe(II) + L', (25) L'+ 02 -> 02'~ + Oxidized ligand. (26)

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)J, with much smaller amounts (<10%) in the form of oxalate complexes such as Fe(oxalate)^.

Indeed, iron (and probably other transition metals as well) appears to play a major role in the production of free radicals in clouds and fogs [see, for example, reviews by Faust (1994) and Hoigne et al. (1994)]. Graedel, Weschler, and co-workers (Graedel and Weschler, 1981; Graedel et al., 1985, 1986; Weschler et al., 1986) proposed that photolysis of Fe(III) complexes may be a major source of OH in atmospheric droplets during the day. Thus, the monohydroxy complex Fe(OH)2+, which is a major form of iron in solution at pHs commonly encountered in the troposphere (see Fig. 8.13), absorbs light in the 290- to 400-nm region (Fig. 8.17). It photolyzes to generate OH, with quantum yields of 0.f4 at 313 nm and 0.017 at 360 nm (Faust and Hoigne, 1990) and 0.31 and 0.07 at 280 and 370 nm, respectively (Benkelberg and Warneck, 1995):

The combination of these significant quantum yields in the actinic region with substantial absorption coefficients means that the lifetime of Fe(OH)2+ is quite short, of the order of minutes, during the day and that reaction (27) is expected to be a major source of OH in aqueous atmospheric droplets (Benkelberg and Warneck, 1995; Siefert et al., 1996).

Once formed, Fe2+ can be oxidized back to Fe3 + , for example, by the Fenton reaction involving H202, 02, or H02 (Siefert et al., 1996; Faust, 1994a,b; Arakaki and Faust, 1998), which again generate OH:

0 0

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