FIGURE 12.30 Comparison of predicted ratio NO^/NO,. as a function of latitude at 19.5-20.5 km using only gas-phase chemistry (dotted line) or with the N205 hydrolysis on aerosol particles (solid line) compared to measured values shown as unfilled circles (from Fahey et a!., 1993).

There are a number of measurements documenting changes in NOx and NO,, in the stratosphere after the Mount Pinatubo eruption and which have been attributed to the removal of oxides of nitrogen due to reactions on aerosol particles. For example, a decrease in stratospheric N02 after the eruption followed by a return to normal levels has been reported (e.g., see Van Roozendael et al., 1997; and De Maziere et al., 1998). Similarly, NOx decreases of up to 70% were reported, as well as increases in gaseous HNO-, (much of that produced on the sulfate particles is released to the gas phase) (e.g., see Coffey and Mankin, 1993; Koike et al., 1993, 1994; David et al., 1994; Webster et al., 1994; and Rinsland et al., 1994).

Not only does this heterogeneous hydrolysis alter the NOx reactions that can lead to ozone destruction or formation, but it also changes the halogen cycles because less N02 is available to trap CIO as the nitrate. In addition, HOx levels are increased. Thus there is relatively more CIO and the C10x cycles leading to ozone destruction become more effective (e.g., McElroy et al., 1992; Avallone et al., 1993a, 1993b; Schoeberl et al., 1993b). Heterogeneous chemistry would be expected to shift the partitioning of chlorine away from HC1 toward more active forms. While increasing ratios of HCl/Cly subsequent to the Mount Pinatubo eruption have been reported in some studies, suggesting decreasing contributions from heterogeneous chemistry after the eruption (e.g., an increase in HC1 /C\y from 0.40 in late 1991 to 0.70 in 1996 based on in situ measurements; Webster et al., 1998), satellite data suggest smaller changes in HC1/C1 (16 ± 9% from 1992/1993 to f995/f996) (e.g., Dessler et al., 1997).

This effect can be seen in the midlatitude stratospheric measurements of Keim et al. (1996) shown in Fig. 12.31. In the tropopause region (shown by the

FIGURE 12.31 Aerosol surface area, NO, NOv, and CIO as a function of altitude (adapted from Keim et al., 1996).

dotted line), the aerosol surface area increases. A significant increase in CIO and decrease in NO is seen at the same time, while NOy increases. This was attributed to the heterogeneous reaction of C10N02 with HC1 to form HN03 on the aerosol particles. The CI 2 product generates CI atoms, which react with 03 to give enhanced CIO. Both C10N02 and particle HN03 are measured as NO>( so that conversion of one to the other should not lead to enhanced NOr The latter was attributed to highly efficient sampling of sulfate aerosol particles containing nitrate. Similar observations of enhanced CIO and suppressed NO have been reported in other studies as well (e.g., Fahey et al., 1993; Toohey et al., 1993). Dessler et al. (1993) invoked heterogeneous reaction to explain measured CIO concentrations that were larger than expected for gas-phase chlorine chemistry.

The hydrolysis reaction (46) of N2Os under many conditions in the atmosphere becomes limited by the rate of N205 formation, which only occurs at a significant rate at night (because of the rapid photolysis of the N03 precursor during the day). Hence under these conditions, reactions (39), (41), and (42) followed by photolysis of the chlorine-containing products become primarily responsible for the removal of gas-phase NOy and increase in CIO (Keim et al., 1996).

Figure 12.32 shows the results of model calculations of the effects of the increased aerosols for October 1986 at 43.5°N (Solomon et al., 1996). The calculated change in the odd-oxygen loss rate when the measured aerosol particle surface area is incorporated into the

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