PH

FIGURE 7,15 Second-order reaction rate constant for the S(IV)-0, reaction defined according to d[S(VI)]/ift = i[Oj(aq)] [S(tV)l, as a function of solution pH at 298 K. The curve shown is the three-component expression of (7.80} and the symbols are the corresponding measurements by a number of investigators.

Hoffmann and Calvert (1985) proposed that the S(IV)-Oj reaction proceeds by nucleophilic attack on ozone by S02 • H2Q, HSOj, and Considerations of nucleophilic reactivity indicate that SO2"" should react more rapidly with ozone than HSOj, and HSO< should react more rapidly in turn than S02 H20. This order is reflected in the relative numerical values of ko, ku and ki. An increase in the aqueous-phase pH results in an increase of [HSOj ] and [SO2 ] equilibrium concentrations, and therefore in an increase of the overall reaction rate. For an ozone gas-phase mixing ratio of 30 ppb, the reaction rate varies from less than 0.001 jiM h 1 (ppb SO2)"1 at pH 2 (or less than 0.01% S02(g)h"1 (gwater/m3 air) ')to 3000pMh"1 (ppb S02)_1 at pH 6 (7000% S02(g) h"1 (gwater/m air)-1) (Figure 7.16). A typical gas-phase S02 oxidation rate by OH is on the order of 1 % h-1 and therefore S(IV) heterogeneous oxidation by ozone is significant for pH values greater than 4. The strong increase of the reaction rate with pH often renders this reaction self-limiting; production of sulfate by this reaction lowers the pH and slows down further reaction. The ubiquitousness of atmospheric ozone guarantees that this reaction will play an important role both as a sink of gas-phase S02 and as a source of cloudwater acidification as long as the pH of the atmospheric aqueous phase exceeds about 4.

The S(iV)-03 reaction rale is not affected by the presence of metallic ion traces: Cu2-, Mn2+, Fe2+, Fe3+, and Cr2* (Maahs 1983; Martin 1984; Lagrange et al. 1994).

Effect of Ionic Strength Lagrange et al. (1994) suggested that the rate of the '

S(IV)—O3 reaction increases linearly with the ionic concentration of the solution.

The ionic strength of a solution / is defined as

FIGURE 7.16 Rate of aqueous-phase oxidation of S(IV) by ozone {30 ppb) and hydrogen peroxide (t ppb), as a function of solution pH at 298 K. Gas-aqueous equilibria are assumed for all reagents, Rji,so, represents the aqueous phase reaction rate per ppb of gas-phase S02. R/L represents rate of reaction referred to gas-phase S02 pressure per (gra 3) of cloud liquid water contcrtt.

FIGURE 7.16 Rate of aqueous-phase oxidation of S(IV) by ozone {30 ppb) and hydrogen peroxide (t ppb), as a function of solution pH at 298 K. Gas-aqueous equilibria are assumed for all reagents, Rji,so, represents the aqueous phase reaction rate per ppb of gas-phase S02. R/L represents rate of reaction referred to gas-phase S02 pressure per (gra 3) of cloud liquid water contcrtt.

where n is the number of different ions in solution, m, and z, arc the molality (solute concentration in mol kg_: of solvent) and number of ion charges of ion i, respectively. Experiments at I8°C supported the following ionic strength correction

where F is a parameter characteristic of the ions of the supporting electrolyte, I is the ionic strength of the solution in units of mol L"1, and R and Rq are the reaction rates at ionic strengths / and zero, respectively. The measured values of F were

Values for NaC104 and NH4C104 were smaller than 1.1. These results indicate that the ozone reaction can be 2.6 times faster in a seasalt particle with an ionic strength of 1 M than in a solution with zero ionic strength.

7.5.2 Oxidation of s(IV) by Hydrogen Peroxide

Hydrogen peroxide, H202, is one of the most effective oxidants of S(IV) in clouds and fogs (Pandis and Seinfeld 1989a). H202 is very soluble in water and under typical ambient conditions its aqueous-phase concentration is approximately six orders of magnitude higher than that of dissolved ozone. The S(IV)-H202 reaction has been studied in detail by several investigators (Hoffmann and Edwards 1975; Penkett et al. 1979; Cocks et al. 1982; Martin and Damschen 1981; Kunen et al. 1983; McArdle and Hoffmann 1983) and the published rates all agree within experimental error over a wide range of pH (0 to 8) (Figure 7.17). The reproducibility of the measurements suggests a lack of susceptibility of this reaction to the influence of trace constituents. The rate expression is (Hoffmann and Calvert 1985)

Was this article helpful?

0 0
How to Improve Your Memory

How to Improve Your Memory

Stop Forgetting and Start Remembering...Improve Your Memory In No Time! Don't waste your time and money on fancy tactics and overpriced

Get My Free Ebook


Post a comment