Change in Ox loss rate (% per month)

FIGURE 12.32 Calculated change in loss rate for odd oxygen as a function of altitude in October 1986 at 43.5°N compared to the predictions for a constant particle concentration typical of winter 1978-1979 levels (adapted from Solomon et al., 1996).

model compared to the assumption of a constant particle concentration at the winter 1978/1979 levels is shown. Also shown are the contributions from the C10x and HOx + NO, cycles to the total. The effects of the HO, + NO, cycles on 03 destruction below about 20 km due to increased volcanic aerosols are negligible. At higher altitudes, removing oxides of nitrogen results in less ozone destruction because in this region NOx chemistry dominates (Fig. 12.8b). For example, Mickley et al. (1997a, 1997b) show that as oxides of nitrogen recovered (i.e., increased) at an altitude of ~28 km in the years after the Mount Pinatubo eruption, 03 decreased simultaneously due to the increased contribution of NOx to its loss. However, in the lower stratosphere where the CIO, cycles are important, the contribution of the chlorine cycles to ozone destruction is enhanced by the removal of oxides of nitrogen on aerosol particles. In short, the effects of volcanic aerosols on stratospheric ozone depend on altitude and, critically, on the halogen concentrations.

Indeed, modeling calculations (Tie and Brasseur, 1995) indicate that at the lower stratospheric halogen concentrations before 1980, the overall effect of increased aerosols due to volcanic eruptions would have been to increase stratospheric ozone due to the removal of N205 on aerosols. Interestingly, the predicted effect on ozone is not very sensitive to the amount of aerosol injected under these pre-CFC conditions because the rate of the N205 hydrolysis becomes limited by the rate of its formation in the N02 + N03 reaction (e.g., see Fahey et al., 1993; Tie et al., 1994; and Tie and Brasseur, 1996). However, after 1980, the halogen concentrations had increased to the point that the net effect was a decrease in total column ozone. The halogen effect on ozone is predicted to depend on the amount of volcanic aerosol injected because unlike N205, the hydrolysis of C10N02 on the particles is not limited by its rate of formation.

Based on modeling studies, it has been suggested that the depth of the Antarctic ozone hole may also be impacted in part by the presence of volcanic aerosols in addition to PSCs. For example, calculations by Portmann et al. (1996) have shown that the combination of increased halogens and volcanic aerosols may have been combined to give the dramatic reduction in 03 that was first reported by Farman and co-workers in 1985. They propose that there are four critical cycles in the formation of the ozone hole: (1) development of the concentrations of the important species, e.g., HC1 and C10N02, before winter; (2) conversion of these into active halogen forms during winter and denitrifi-cation and dehydration of the stratosphere; (3) continued conversion into active forms in the spring while ozone depletion is occurring, the so-called maintenance period; and finally, (4) termination of the ozone destruction cycles.

Model-predicted effects of continued activation of chlorine during ozone depletion and the effects of the extent of conversion of chlorine species into active forms on total column ozone at 75°S in 1990 are shown in Fig. 12.33 (Portmann et al., 1996). The calculated total column ozone is shown for the cases where HCl/Cly = 1.0, 0.4, and 0.0 at day 180 and for the case where this ratio is 0 but the heterogeneous chemistry that converts C10N02 to active forms ceases at day 220. A ratio of 0 for 11 CI /CI v corresponds to total conversion of HC1 into active forms, with a ratio of f corresponding to no such conversion. Figure 12.33 shows that the onset of ozone depletion would be significantly delayed if there has been no heterogeneous conversion of HC1 to active forms during the winter, but large ozone losses are still expected due to activation during the spring. If the conversion is assumed to be complete on day 180, but the heterogeneous chemistry ceases at day 220, the depth of the hole on October 1 (day 270) is seen to be much reduced. Hence the presence of aerosols in the absence of PSCs could provide a vehicle for continued heterogeneous chemistry during this "maintenance period." Temperature is another important factor due to its role not only in the formation of PSCs but also in determining the kinetics of both the heterogeneous and homogeneous reactions. Smaller effects on total column ozone are calculated due to denitrification and dehydration of the polar stratosphere (Portmann et al., 1996).

Support for the importance of aerosols in maintaining chlorine in an active form during the maintenance period is found in Fig. 12.34. This shows the satellite-derived average total 03 in the vortex as a function of

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