Oxidation by Oxides of Nitrogen

In aqueous solutions, nitrous acid reacts with S(IV) at a reasonable rate with a rate expresssion given by 

However, the levels of gaseous HONO observed in polluted ambient air (— 1—8 ppb) (see Chapter 11) taken with the Henry s law constant for HONO (Table 8.1) yield aqueous concentrations too low to contribute substantially to the aqueous-phase S(IV) oxidation. 

For example, with a Henry s law constant for HONO of 49 M atm-1, a gas-phase concentration of 1 ppb would result in a solution-phase concentration of only 4.9 X 10-8 mol L-1, compared to an anticipated H202 solution-phase concentration of 10-4 mol L-1 at the same gas-phase concentration. The rate constants also favor the H202 reaction at a pH of 3.0, that for oxidation of H202 is approximately a factor of 104 larger than that for reaction with HONO. Thus, the combination of concentrations and rate constants makes HONO unlikely to be a significant S(IV) oxidant in solution unless other oxidants such as O-, or H202 are absent. 

It is interesting, however, that the HONO-HSO-,-reaction has been shown to form a nitrene (HON ), which Mendiara and co-workers (1992) suggest could contribute to free radical formation in clouds and fogs. 

The reaction of NO, scavenged from the gas phase with S(IV) is discussed in Chapter 7 and, as seen there, may also be important (e.g., Chameides and Davis, 1983 Rudich et al., 1996, 1998). 

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