FIGURE 12.26 Correction factors for the measured uptake coefficient, ym, as a function of the ratio of the diffusoreactive length (/) to the droplet radius (a) (adapted from Hanson et al., 1994).

kinetics can be described by the usual aqueous-phase expressions.

There may be additional mechanisms that contribute to the generation of active chlorine in the stratosphere as well. For example, Burley and Johnston (1992a) have proposed that nitrosyl sulfuric acid (N0+HS04~) may be formed in the stratosphere and react with HC1 to give nitrosyl chloride, C1NO. De Haan et al. (1997) also suggest that the heterogeneous reaction of ClOOCl with HC1 may be important, depending on the fate of the product HOOQ. This would provide a means of removing the excess HQ remaining after the C10N02 + HQ reaction.

Most of the research to date has focused on aerosols and PSCs containing inorganic species such as nitric and sulfuric acids. While CH4 is the only hydrocarbon that is sufficiently unreactive in the troposphere to reach the stratosphere, it is oxidized to compounds such as HCHO that can be taken up into sulfuric acid particles (Tolbert et al., 1993). The effects of such uptake and subsequent chemistry are not well established.

In short, the heterogeneous chemistry that drives the Antarctic ozone hole can occur not only on solid surfaces but also in and on liquid solutions containing combinations of HN03, H2S04, and HzO. As discussed in the following section, it is believed that this is why volcanic eruptions have such marked effects on stratospheric ozone on a global basis.

6. Effects of Volcanic Eruptions

The finding that the heterogeneous chemistry that occurs on polar stratospheric clouds also occurs in and on liquid solutions in the form of liquid aerosol particles and droplets in the atmosphere provided a key link in understanding the effects of volcanic eruptions on stratospheric ozone in both the polar regions and mid-latitudes. As discussed herein, the liquid particles formed from volcanic emissions are typically 60-80 wt% H2S04-H20, and hence the chemistry discussed in the previous section can also occur in these particles (Hofmann and Solomon, 1989). We discuss briefly in this section the contribution of volcanic emissions to the chemistry of the stratosphere and to ozone depletion on a global scale. For a brief review of this area, see McCormick et al. (f 995).

Volcanic eruptions can be sufficiently energetic that they inject large quantities of gases and particles directly into the stratosphere (rather than by diffusion from the upper troposphere). The gases include S02, HQ, HF, and SiF4 (Mankin and Coffey, 1984; Symonds et al., 1988; Bekki, 1995; Francis et al., 1995). The particles can include inorganic mineral particles such as silicates, halide salts, and sulfates (e.g., see Woods et al., 1985; Snetsinger et al., 1987; and Pueschel et al., 1994).

Additional sulfates continue to form after the eruption as gaseous S02 is oxidized to sulfuric acid and sulfates. While we shall focus here on the effects of these sulfate particles on the heterogeneous chemistry of the stratosphere, there may be other important effects on the homogeneous chemistry as well. For example, model calculations by Bekki (1995) indicate that this oxidation of S02 by OH leads to reduced OH levels, which alters its associated chemistry.

Finally, there are a variety of effects associated particularly with the increased particle loading that we shall not discuss in detail. These include, for example, the potential for a change in actinic flux at the earth's surface [which could be either negative or even positive due to "trapping" of photons and multiple scattering between the aerosol layer and the surface (Miche-langeli et al., 1992; Minnis et al., 1993)], warming of the stratosphere due to absorption of solar and terrestrial radiation by aerosols (Angell, 1993a; Chandra, 1993), cooling of the troposphere (Dutton and Christy, 1992; see also Chapter 14), changes in stratospheric circulation (Tie et al., 1994), effects on cloud formation (Minnis et al., 1993), and altered rates of photolysis of 02 and 03 (Huang and Massie, 1997). For a more detailed discussion, see Fiocco et al. (1996).

The eruption of Mount Pinatubo in the Philipines in mid-June of 1991 is believed to be the largest volcanic perturbation on the stratosphere in this century, injecting 14-20 Mt of gaseous S02 and 2f-40 Mt of sulfuric acid/sulfate aerosol particles (Russell et al., 1996, and references therein). At many locations around the world, stratospheric particle concentrations were measured to increase by 1-2 orders of magnitude after this eruption (e.g., see Shibata et al., 1994; Ansmann et al., 1996; Anderson and Saxena, 1996; and Deshler et al., 1996). Figure 12.27, for example, shows one set of measurements of the aerosol optical depth during 1991, 1992, and part of 1993 at 19.5°N (Russell et al., 1996). The increase in light scattering due to the injection of volcanic aerosols is dramatic.

Figure 12.28 shows the particle surface area size distribution before the Mount Pinatubo eruption (Fig. f2.28a), inside the main aerosol layer several months after the eruption (Fig. 12.28b), and almost two years after the eruption (Fig. f2.28c). (See Chapter 9.A.2 for a description of how particle size distributions are normally characterized.) Prior to the eruption, the surface area distribution is unimodal, with typical radii of 0.05-0.09 /¿m and a number concentration of ~f-20 particles cm-3. In the main stratospheric aerosol layer formed after the eruption, the distribution is bimodal


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