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<D°"(308 nm) = 0.98 <t>°"(351 nm) = 0.96 <D°"(250 nm) = 1.8

<D°"(308 nm) = 0.07 <t>°"(351 nm) = 0.046 <D°"(280 nm) = 0.069 <t>°"(390 nm) = 0.022

<t>°(305 nm) = 1.1 X 10"3 $(305-313 nm) = 0.013

Gurol and Akata, 1996 Taube, 1957

Zellner et al., 1990; Zellner and Herrmann, 1995

Fischer and Warneck, 1996 Graedel and Weschler, 1981

Zellner et al., 1990 Zellner et al., 1990 Fischer and Warneck, 1996 Fischer and Warneck, 1996

Warneck and Wurzinger, 1988; Zepp et al., 1987; Zellner et al., 1990; Zellner and Herrmann, 1995

Treinin, 1970

" These are effective quantum yields, that is, those for photolysis and escape of the species from the solvent cage.

' Estimated.

As expected based on our knowledge of gas-phase chemistry, in addition to the Fenton type chemistry involving iron, photolysis of 03, H202, HONO, and HNO3 are all potential OH sources in clouds and fogs. In addition, the photolysis of nitrite, nitrate, and H02 in aqueous solutions can also form OH. in short, there are many potential sources of OH in clouds and fogs.

Given that there are large sources of OH in atmospheric suspended droplets, oxidation of S(IV) by OH is expected. The proposed mechanism (see, for example, McElroy, 1986; Huie and Neta, 1987; Huie, 1995; and Buxton et al., 1996) is as follows (rate constants are in units of L mol s):

Initiation

OH + HSO,2 -» H20 + S03 , (31) kM = (2.7-4.5) X 109 (Buxton etal., 1996; Huie and Neta, 1987)

Propagation

SO^ + 02 -» SO^ (peroxymonosulfate radical), (33) k33 = (1.5-2.5) X 109 (Huie and Neta, 1984; Buxton etal., 1996)

(Buxton etal., 1996; Huie and Neta, 1987)

kMh = (0.034-7.5) X 104 (Buxton etal., 1996; Huie and Neta, 1987)

*35 = (0.68-2) X 109 (Buxton etal., 1996; Wine etal., 1989; Huie and Neta, 1987) S04" + S032" -» SO," + S042" , (36) *3fi = 5.5 X 108 (Deister and Warneck, 1990) SO^ + SO^ -> 2S04~ + 02, (37a)

*37 = 2.2-6) X 10x (Buxton etal., 1996; Huie and Neta, 1987)

S04~ + H20 -» OH + HS04~ , (38) fc3f![H20] = 440 s"1 (Bao and Barker, 1996) SOf + S032" HSOf + SO^, (39a) ky)a = 3.6 X 105 (Buxton et al., 1996) ^S04-+S02", (39b)

FIGURE 8.18 Summary of initiation, propagation, and termination steps in the free radical oxidation of S(IV) in solution. (Adapted from J. Atmos. Chem. 20, Sander R., Lelieveld, J., and Crutzen, P. J. "Modelling of the Nighttime Nitrogen and Sulfur Chemistry in Size Resolved Droplets of Orographic Cloud," Fig. 7, pp. 89-116. Copyright 1995, with kind permission from Kluwer Academic Publishers.)

FIGURE 8.18 Summary of initiation, propagation, and termination steps in the free radical oxidation of S(IV) in solution. (Adapted from J. Atmos. Chem. 20, Sander R., Lelieveld, J., and Crutzen, P. J. "Modelling of the Nighttime Nitrogen and Sulfur Chemistry in Size Resolved Droplets of Orographic Cloud," Fig. 7, pp. 89-116. Copyright 1995, with kind permission from Kluwer Academic Publishers.)

(Buxton etal., 1996; Huie and Neta, 1987)

(McElroy and Waygood, 1990)

(Waygood and McElroy, 1992)

It can be seen that there are some significant discrepancies in the rate constants measured for the individual reactions in this complex system, which affects how important this oxidation cycle is in atmospheric droplets. For example, the rate constant for the key propagation reactions (34a) and (34b), which occur in the pH range most commonly encountered in atmospheric droplets, was reported recently to be even smaller than shown above, at k34 = (k34.d + k34b) = 3.6 X 103 Lmoh1 s~ ' (Yermakov et al., 1995). Clearly, additional work is needed in this area.

In addition, some of the key, free radical intermediates have other potential reactions that can impact the chemistry. The sulfate radical, S04~, for example, reacts rapidly with aromatics in solution (e.g., Herrmann et al., 1995; Huie, 1995), removing S04~ from the sequence shown.

The aqueous-phase free radical oxidation of S(IV) to S(VI) is summarized in Fig. 8.18 (Sander et al., 1995) and Table 8.10. Oxidation is initiated by attack on the bisulfite ion, HSO^, or the sulfite ion, SO2-, by various species. The sulfite radical (SO^), sulfate radical (S04~), and peroxymonosulfate radical (S05 ) are the key intermediates involved in the chain propagation. Formation of sulfate, HSO^, or S20^~ leads to chain termination.

Initiation with X = OH has been discussed earlier. Table 8.If summarizes some of the aqueous-phase HOx chemistry in which OH is generated and reacts in the atmosphere. (Note that the rate constants for some of the aqueous phase reactions shown in Tables 8.10—8.16 depend on such factors as ionic strength; see Chapter 5.D.) Involved with this chemistry is that of bicarbonate/carbonate, since OH reacts with these species as well (Table 8.12). It is interesting that, in contrast to the high reactivity of OH toward S(IV) in aqueous solutions, direct reactions of H02/02 with S(IV) do not appear to be important (Sedlak and Hoigne, 1994; Yermakov et al., 1995).

The aqueous-phase and gas-phase chemistries of HOx are sufficiently closely coupled that the chemistry shown in Tables 8.11 and 8.12 can affect gas-phase concentrations as well. For example, including the aqueous-phase chemistry in models of tropospheric ozone formation alters predicted 03 concentrations, although whether the perturbation is significant is subject to some controversy (e.g., see Lelieveld and Crutzen, 1990; Jonson and Isaksen, 1993; Walcek et al., 1997; Liang and Jacob, 1997).

Other species that can initiate this sulfur oxidation chemistry are N03 (discussed in Chapter 7.D.1) and CI 2- The latter radical anion is formed in sea salt particles when atomic chlorine is generated and reacts with chloride ion. In addition, Vogt et al. (1996) have proposed that oxidation of SO2- by HOC1 and HOBr in sea salt particles may be quite important. Table 8.13 summarizes the aqueous-phase chlorine chemistry that occurs in sea salt particles and Table 8.14 the oxidation of S(IV) by reactive chlorine and bromine species in solution.

C. OXIDATION OF SO, TABLE 8.10 Summary of Aqueous-Phase Chemistry of S(IV)

Reaction

Rate or equilibrium constant"

Initiation

Propagation

SO, + SO, s05" + I ISO , s04" + so?" SO4" + h2o so4" + nso, so4" + h2o2 so4" + no,

so42"

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