FIGURE 7.11 Formation of NO, in solutions of nitrite at pH 4.0 and either 25°C (right axis) or —21°C (left axis). Note the different scales for the rate of formation of nitrate. (Adapted from Takenaka et al., 1992.)

oxidation of sulfur compounds in a cloud chamber (Finnegan et al., 1991; Finnegan and Pitter, 1991; and Chapter 8). Takenaka et al. (1996) propose that this acceleration is primarily due to concentration of nitrite and H+ in solution during the freezing process, by a factor of 2.4 X f03 at -3°C, followed by the known oxidation by 02. This is an area that clearly needs further investigation, as it has significant implications for the chemistry of freezing cloud and fog droplets in the atmosphere.

Free radical oxidation of nitrite by OH, for example, can also occur. This reaction (which will occur during daylight hours when photolytic sources of OH are present) and the chemistry of associated nitrogen oxides in solution have been studied by L0gager and Sehested (1993):

Uptake of HONO onto ice and H2S04-H20 solutions has been observed (e.g., Zhang et al., 1996; Fenter and Rossi, 1996). The uptake onto ice at temperatures from f 80 to 200 K occurs with a mass accommodation coefficient (a) of ~1 X 10~3 and is reversible. Uptake onto sulfuric acid solutions depends on the concentration of the solution, with a varying from ~f0"4 at 55 wt% H2S04 to -10"' at 95 wt% H2S04. Interestingly, in the presence of HC1, HONO undergoes a heterogeneous reaction on these surfaces to form gaseous C1NO. The reaction probability for HONO with frozen HC1 solutions was measured to be ~0.1 (Fenter and Rossi, 1996). The reaction was slower on sulfuric acid solutions; Fenter and Rossi (1996) report that the reactive uptake is less than approxi-

mately 10"3 even under optimized conditions of 60 wt% H2S04, whereas Zhang et al. (1996) report values about an order of magnitude larger, in the range of

0.01-0.02, for 60-71 wt% H2S04. Whether or not these heterogeneous reactions of HONO are important in the upper troposphere or stratosphere is not clear.

It is noteworthy that a similar reaction of HONO with HC1 occurs, apparently, mainly in the gas phase (Wingen et al., 1999):

Although it is relatively slow (k ~ 10"19 cm3 molecule-1 s"1 at room temperature measured in a 560-L chamber), it has proven useful as a titration for HONO in order to measure absolute concentrations of this species.

As discussed in 7.B.3b above, deposition can also be a significant removal process for HONO.

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