J V n

The fraction of NO* converted to HNO3 over a time period f is thus

TNjO, exp

'cm2cm

We are interested in t = 12h — 4.3 x 104 s. From before, tno, ~ 5.9 x 104 s. For Ap = 1 x 10_R cm2 cm-3, tNi0s = 3.2 x 105 s; for Ap = 10 x 10"" t^o, = 3.2 x 104 s. We find

/no^hnoj = 0.04 at Ap = 1 x I0_scm2cm"3 = 0.26 at

The conversion rate of NO* to HNO3

i-i through N2Os depends on the two

The lifetimes, tN[>, = (^[Qslf and iN=0i = [J(8CT/* ««.0,) ''% smaller these two characteristic times, the faster the conversion rate. The greatest sensitivity of the conversion is to T (ki is a strong function of temperature) and Ap\ the larger Tand Ap arc, the greater the conversion to nitric acid. The largest values of Ap occur when the stratospheric aerosol is perturbed by a volcano or by formation of PSCs in the polar stratosphere.

5.8.3 Effect of Volcanoes on Stratospheric Ozone

Volcanoes inject gaseous S02 and HC1 directly into the stratosphere. Because of its large water solubility, HC1 is rapidly removed by liquid water. The S02 remains and is converted to H2S04 aerosol by reaction with the OH radical. (We will discuss this reaction in Chapter 6 and the nucleation process that leads to fresh particles in Chapter 11.) Thus volcanic eruptions lead to an increase in the amount of the stratospheric aerosol layer (Figure 5.26). As we have just seen, "normal" stratospheric aerosol surface area concentrations lie in the range of 0.5-1.0 pm2cm' 3. Eruption of Mt, Pinatubo in the Philippines in June 1991, the largest volcanic eruption of the twentieth century, led to average midlatitude stratospheric aerosol surface areas of 20 pm2 cm-3. In the core of the plume, 5 months after the eruption, the aerosol surface area concentration was 35 cm2 cm-3 (Grainger el al. 1995). The time required for stratospheric aerosol levels to decay back

FIGURE 5.26 Time series of the abundance of stratospheric aerosols, as inferred from integrated backscatter measurements (WMO 2002). The data stream beginning in 1976 was acquired by ground-based lidar at Gartnisch, Germany (47.5°N). That beginning in 1985 is from the SAGE II satellite over the latitude band 40-50°N. Vertical arrows show major volcanic eruptions. The dashed line indicates the 1979 level. (Data from Garmisch provided courtesy of H. Jäger.)

FIGURE 5.26 Time series of the abundance of stratospheric aerosols, as inferred from integrated backscatter measurements (WMO 2002). The data stream beginning in 1976 was acquired by ground-based lidar at Gartnisch, Germany (47.5°N). That beginning in 1985 is from the SAGE II satellite over the latitude band 40-50°N. Vertical arrows show major volcanic eruptions. The dashed line indicates the 1979 level. (Data from Garmisch provided courtesy of H. Jäger.)

to "normal" levels following a volcanic eruption is about 2 years. With volcanoes erupting somewhere on Earth every few years or so, the stratosphere is seldom in a state totally uninfluenced by volcanic emissions.

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 CIO,-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.

Figure 5.27 shows in situ stratospheric data (solid circles) on NO,/NOj, and CIO/CI, ratios plotted as a function of stratospheric aerosol surface area concentration (Fahey et al. 1993). Figure 5.27 shows comparisons of measurements made in September 1991 in a region of the atmosphere that had not been strongly impacted by the Mt. Pinatubo aerosol with those made in March 1992 at a similar latitude and solar zenith angle where the effect of the eruption is clearly present. In the March data the aerosol surface area concentration was 20 pm2 cm-3, a factor of 40 over the "natural" level of 0.5 um2 cm"3 (vertical dashed line). The solid and dashed curves are model-calculated relationships for these two ratios for the September (solid) and March (dashed) data sets. The open circles on the NO,/NOv plot at a surface area of 20 pm2 cm-3 indicate calculations assuming gas-phase chemistry only; the crosses are calculations including heterogeneous hydrolysis of N205. The measured ratio of NO,/NOy decreased as aerosol surface area increased and the CIO/CI,, ratio increased. Both of these effects are expected as a result of the heterogeneous hydrolysis of N205 on aerosol surfaces. Increasing aerosol surface area from about 1 to

FIGURE 5.27 Measured ratios NO*/NOy and CIO/Cly (solid circles) shown as a function of stratospheric aerosol surface area (Fahey et al. 1993). The open circles represent simulations of the conditions of the measurements accounting for gas-phase chemistry only; the crosses are the corresponding simulations including the heterogeneous hydrolysis of N205. The vertical dashed line represents stratospheric background aerosol surface area (0.5 ^m2 cm-3). The curved lines represent the modeled dependence of the two ratios on aerosol surface area for the average conditions in the September (solid) and March (dashed) data sets. (Reprinted with permission from Nature 363, Fahey, D.W., et al. Copyright 1993 Macmillan Magazines Limited.)

FIGURE 5.27 Measured ratios NO*/NOy and CIO/Cly (solid circles) shown as a function of stratospheric aerosol surface area (Fahey et al. 1993). The open circles represent simulations of the conditions of the measurements accounting for gas-phase chemistry only; the crosses are the corresponding simulations including the heterogeneous hydrolysis of N205. The vertical dashed line represents stratospheric background aerosol surface area (0.5 ^m2 cm-3). The curved lines represent the modeled dependence of the two ratios on aerosol surface area for the average conditions in the September (solid) and March (dashed) data sets. (Reprinted with permission from Nature 363, Fahey, D.W., et al. Copyright 1993 Macmillan Magazines Limited.)

20 pm2 cm~3 led to an increase in the C10/Civ 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 |im2cm 3 is estimated to increase C10/C1V 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

03 Loss Rate, molecules cm-3 s~1

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

03 Loss Rate, molecules cm-3 s~1

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

stratosphere, stratospheric ozone would increase after a volcanic eruption as a result of the removal of active NO* by the heterogeneous N205 + H20 reaction.

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