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As a result, dissolved S02 really includes three chemical species: hydrated S02 (S02 • H20), the bisulfite ion (HSO3-), and the sulfite ion (SO2 ). While HSO, is commonly referred to as bisulfite, it should be noted that it can exist in two possible structures: HOSO^~ or

HSO^. Recent calculations indicate that the sulfonate form, HSO^, is the most stable form (Brown and Barber, f995). However, Hoffmann (1986) argues that the chemical reactivity of bisulfite in aqueous solutions is consistent with H0-S02, being the major reactive automer, which is in rapid equilibrium with HSO^~. While the formation of an S02 • H20 complex similar to that shown in (11, -11) can, in principle, occur in the gas phase as well, the binding energy is low 1.8-3.5 kcal mol"1; e.g., see Li and McKee, 1997; and Bishenden and Donaldson, 1998) and hence this complex is not important in the atmosphere.

The predominant form dissolved in solution depends on the acidity of the solution in which S02 dissolves. Figure 8.7 shows the concentrations of the three species as a function of pH; over the pH range typical of atmospheric droplets, 2-6, most of the dissolved S02 is in the form of bisulfite ion (HSO^).

Because of the different forms in which dissolved S02 exists in solution, the oxidation state (i.e., +4) is often used to denote all these forms of S02 taken together, that is,

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