PH

FIGURE 7.A.3 Sulfate production rate from the S(IV)-OH reaction as a function of pH for a mixing ratio of S02(g) = 1 ppb and [OH(aq)] = 1 x 10"12 M at 298 K.

The sulfate radicals SOJ and SOj participate in a series of additional reactions in ambient clouds that complicate even further the above chain and modify significantly the overall reaction rate. These reactions are discussed subsequently.

7.A.2 Oxidation of S(IV) by Oxides of Nitrogen

Nitrogen dioxide has limited water solubility and its resulting low aqueous-phase concentration suggests that the reaction

should be of minor importance in most cases. This reaction has been studied by Lee and Schwartz (1983) and was described as one that is first-order in both N02 and S(IV)

with a pH-dependent rate constant &7.a.15. At pH 5.0, &7.a.15 = 1-4 x 105M_1s_1 butatpH 5.8 and 6.4 only a lower limit, &7.a.15 = 2 x 106 M-1 s-1, could be determined.2 Whereas this reaction is of secondary importance at the concentrations and pH values representative of clouds, for fogs occurring in urban areas with high N02 concentrations this reaction could be a significant pathway for S(IV) oxidation, if the atmosphere has sufficient neutralizing capacity, for example, high NH3(g) concentration (Pandis and Seinfeld 1989b).

7.A.3 Reaction of Dissolved S02 with HCHO

HSOJ and SO2- in clouds and fogs react with dissolved formaldehyde to produce hydroxymethanesulfonate, H0CH2S03H (HMS) (Boyce and Hoffmann 1984),3

The elementary formaldehyde-S(IV) reactions have rate constants &7.a.17 = 7.9 x 102 and &7.a.18 = 2.5 x 107M_1 s"1, respectively (Boyce and Hoffmann 1984). Reactions (7.A. 17) and (7.A.18) involve HCHO and not its diol form, H2C(OH)2. HMS is a strong acid, dissociating completely in clouds to the hydroxymethanesulfonate ion (HMSA), HOCH2SOj:

HOCH2SOj can dissociate once more to ~OCH2SOj

2The evaluation of this rate expression was considered tentative by Lee and Schwartz, in view of evidence for the formation of a long-lived intermediate species.

3Sulfur in HMS is also in oxidation state IV and measurements of S(IV) in clouds include the HMS contribution. In this book we have not included HMS in the definition of S(IV).

FIGURE 7.A.4 HMSA aqueous-phase production rate as a function of pH for mixing ratios of S02(g) = 1 ppb and HCHO(g) = 1 ppb.

FIGURE 7.A.4 HMSA aqueous-phase production rate as a function of pH for mixing ratios of S02(g) = 1 ppb and HCHO(g) = 1 ppb.

but this second dissociation is weak, with /c7.a.20 = 2 x 10~12M, and, for all practical purposes HMS exists as HOCH2SO3 in the atmospheric aqueous phase (Sorensen and Anderson 1970). Therefore the HMSA formation rate Rf is given by

Rf = (fc7.a.i7[HS03-] + fc7.A.i8[S02-])[HCH0] (7.A.21)

and is shown as a function of pH in Figure 7.A.4. The reaction rate increases exponentially with pH, because of the increasing concentrations of HSOj and SO2-, and becomes appreciable above pH 5. The overall rate is more or less equal to that of the SO2" reaction for pH values higher than 3.

HMSA reacts with OH~ to reform SO2- and formaldehyde

HOCH2SOJ + OH- -> HCHO(aq) + SO^" + H20 (7.A.22)

with a rate given by

and a second-order rate constant &7.a.22 = 3.6 x 103M_1s_1 (Kok et al. 1986). The characteristic time for dissociation, 1/(&7.a.22[OH~]), is 770 h at pH 4, 7 h at pH 6, and 45 minutes at pH 7. The decomposition of HMSA can be neglected for acidic solutions but becomes appreciable as the solution approaches neutrality. Because several hours are generally necessary to achieve equilibrium between its formation and decomposition reactions, the HMSA concentration is usually not in equilibrium with HS03 and HCHO in atmospheric clouds (see Problem 7.8).

An interesting feature of HMSA chemistry is that the high-pH conditions that are most conducive to its production (Figure 7.A.4) are not suitable for its preservation. However, if the cloud or fog pH is initially high and then decreases as a result of S(IV) oxidation, then high concentrations of HMSA can be attained and maintained in the cloud (Munger et al. 1986).

Potential pathways for the destruction of HMSA in cloudwater or fogwater include reactions with 03(aq), H202(aq), and the hydroxyl radical OH(aq). Hoigne et al. (1985) observed no direct reaction between ozone and HMSA. HMSA is also resistant to oxidation by H202 (Kok et al. 1986). The reaction between HMSA and OH results in the production of the SOj radical and links HMSA with the S(IV) radical oxidation chain discussed in the previous section. The rate of this reaction is pH independent and has a second-order rate constant £7.A.24 = 2.6 x 108 M_1 s_l (Olson and Fessenden 1992). The lifetime of HMSA due to the attack of OH, 1/(&7.a.24[OH]), varies from approximately lh for [OH(aq)] = 10"12 M, to 10 hours for [OH] = 10"13 M. Oxidation by OH is expected to be the main sink of HMSA during the daytime in typical clouds or fogs (Jacob 1986; Pandis and Seinfeld 1989a).

HMSA has been observed in different environments at concentrations as high as 300 pM near sources of S02 and HCHO (Munger et al. 1984, 1986). Its formation explains the relatively high S(IV) concentrations that have been reported in high-pH environments because, we recall, HMSA is a member of the S(IV) family. In such cases the lifetime of dissolved S02(HS03 and SO2-) should be extremely short (Munger et al. 1986), so the measured S(IV) was mostly HMSA and not HSOj or SO2-.

APPENDIX 7.3 AQUEOUS-PHASE NITRITE AND NITRATE CHEMISTRY 7.A.4 NO* Oxidation

Aqueous-phase oxidation of oxides of nitrogen is used by the chemical industry for the production of nitric acid. However, N02 aqueous-phase oxidation in water (Lee 1984a)

with a rate constant of 1 x 108 M_1 s_1, by NO (Lee, 1984a)

NO(aq) + OH(aq) N02 + H+ N02(aq) + OH(aq) NO J + H+

(rate constants 2 x 1010M_1 s"1 and 1.3 x 109M_1 s"1, respectively) all proceed far too slowly under ambient conditions to contribute either to the removal of these nitrogen oxides or to cloudwater acidification. For example, the aqueous-phase concentrations of NO and N02 are below 1 nM (Section 7.3.6) and therefore the production of nitrate from reaction 7.A.25 proceeds with rates less than 0.3 pM h~1, contributing a negligible amount to the aqueous-phase nitrate concentration.

7.A.5 Nitrogen Radicals

The N03 radical (either directly or as N205) is probably the most reactive nitrogen species in the aqueous phase during nighttime (it is negligible during daytime because of the rapid photolysis of N03(g)). N03 and N205 are both very soluble in water and are a potential source of nitrate. Jacob (1986) estimated a Henry's law coefficient of the order of 2.1 x 105Matm_1 for N03 and assumed that N205 is completely transferred to the aqueous phase at equilibrium. N205 reacts rapidly with water to produce nitrate4

while N03 is converted to nitrate by chloride ion

with a rate constant 1 x 108M_1 s_1 and produces the chlorine radical, CI (Ross and Neta 1979; Chameides 1986). For a chloride concentration of 10 pM, the lifetime of N03 in cloudwater as a result of reaction (7.A.30) is 1 ms, a value indicative of the highly reactive nature of N03. In environments with low chloride concentrations, N03 reacts with HSO^

and produces sulfate radicals, SOJ, and nitrate with/c7.a.3i = 1 x 108 M-1 s-1. Chameides (1986) calculated a typical nighttime continental cloud concentration of [N03] = 10"'2 M. Using this concentration and assuming £,so, = 1 ppb, one estimates that the reaction proceeds with a rate of 0.06 pMh-1 at pH 4 and 6pMh-1 at pH 6. Radicals produced during the N03 reactions participate in a number of additional reactions propagating the S(IV) oxidation radical chain (Pandis and Seinfeld 1989a).

4In Chapters 5 and 6 we considered

where H20(aq) is understood to be water in an atmospheric particle or droplet. Here we indicate the subsequent dissociation of HNO3.

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