Effect of Volcanoes on Stratospheric Ozone

Volcanic injection of large quantities of sulfate aerosol into the stratosphere offers the opportunity to examine the sensitivity of ozone depletion and species concentrations to a major perturbation in aerosol surface area (Hofmann and Solomon 1989 Johnston et al. 1992 Prather 1992 Mills et al. 1993). The increase in stratospheric aerosol surface area resulting from a major volcanic eruption can lead to profound effects on C10 -induced ozone depletion chemistry. Because the heterogeneous reaction of N205 and water on the surface of stratospheric aerosols effectively removes N02 from the active reaction system, less N02 is available to react with CIO to form the reservoir species C10N02. As a result, more CIO is present in active CIO cycles. Therefore an increase in stratospheric aerosol surface area, as from a volcanic eruption, can serve to make the chlorine present more effective at ozone depletion, even if no increases in chlorine are occurring. 

20 pm2 cm 3 led to an increase in the CIO/Cly ratio from about 0.025 to about 0.05. Thus this increase in aerosol surface area from a large volcano renders the chlorine already present in the stratosphere twice as effective in ozone depletion as in the absence of the volcano. Even at a more modest scale, an increase of aerosol surface area from 1 to 5 pm2 cm 3 is estimated to increase CIO/Clv from 0.02 to 0.03 and thereby render the chlorine 50% more effective in ozone depletion. Thus one volcanic eruption, at least during the 2 years or so following the eruption when stratospheric aerosol levels are elevated, can produce the same ozone-depleting effect as a decade of increases in CFC emissions [e.g., see Tie et al. (1994)]. Conversely, in the absence of chlorine in the 

FIGURE 5.28 Contribution to total 03 loss rate by different catalytic cycles. [After Osterman et al. (1997) updated to current kinetic parameters.] 

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