So2 oh hso

SO3 + H20-» H2S04(g)

H2S04(g) + many H20-» H2S04(aq)

The sum of these reaction steps is

S02 + OH' + 02 + many H20-» HOO' + H2S04(aq)

When we include the return of HOO" to OH' via reaction with NO", the overall reaction is seen to be OH" -catalyzed co-oxidation of S02 and NO *:

S02 + NO* + O, + many HzO OH'caB'w> N02' + H2S04(aq)

For representative concentrations of the OH' radical in relatively clean air, a few percent of the atmospheric S02 is oxidized per hour by this mechanism. The rate is much faster for air masses undergoing photochemical smog reactions since the concentration of OH* there is much higher. However, generally only a small amount of sulfur dioxide is oxidized in cloudless air; the rest is removed by dry deposition before the reaction has time to occur.

Dissolved sulfur dioxide, S02, is oxidized to sulfate ion, S04i_, by trace amounts of the well-known oxidizing agents hydrogen peroxide, H202, and ozone, O3, that are present in the airborne droplets, as already discussed in Chapter 3. Indeed, such reactions currently are thought to constitute the main oxidation pathways for atmospheric SO?, except under clear sky conditions when the gas-phase homogeneous mechanism predominates. The ozone and hydrogen peroxide result mainly from sunlight-induced reactions in photochemical smog. Consequently, oxidation of S02 occurs most rapidly in air that has also been polluted by reactive hydrocarbons and nitrogen oxides. Since the smog reactions occur predominantly in summer, rapid oxidation of S02 to sulfate also is characteristic of the summer season.

Continue reading here: Systematics of Stratospheric Chemistry

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