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Particle radius (pm)

FIGURE 12.28 Particle surface area distributions in the stratosphere (a) before Mount Pinatubo eruption, (b) August 20, 1991, over California, and (c) May 7, 1993, over California (adapted from Russell et al. (1996) and Goodman et al. (1994)).

reaction (42) of HOC1 with HC1 increases by more than an order of magnitude from 2.8 X 105 to 4.9 X 106 L mol-1 s~' at 250 K as the weight percent sulfuric acid in a mixture with water increased from 49 to 67%. They propose that this is due to increasing protonation of the HOC1 to H2OCl+ in highly acidic solutions, followed by reaction of the protonated form with CI-.

Of these heterogeneous reactions, the hydrolysis of N2Os is particularly important in midlatitudes. For example, Fig. 12.30 shows the measured NOx (NOx = NO + N02) to NO>( (NO>( = NOx + HN03 + N205 + •••) ratio at different latitudes compared to the predicted ratio using a gas-phase model as well as to a model that incorporates the N2Os hydrolysis on aerosol particles (Fahey et al., 1993). Clearly, the inclusion of this reaction is necessary to bring the measurements

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