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FIGURE 13.13 Trends in annual average ozone (% per decade) as a function of latitude and altitude derived from SBUV data (adapted from Hollandsworth et al., 1995).

Figure 13.13 shows SBUV-derived trends in annual average ozone as a function of latitude and altitude; the magnitude of the trend is generally small in the equatorial regions and larger at higher latitudes and altitudes (Hollandsworth et al., 1995; Hilsenrath et al., 1992; Claude et al., 1994; Chandra et al., 1995, 1996; Harris et al., 1997; Bojkov and Fioletov, 1997).

Superimposed on long-term trends are "spikes," some of which may be related to the chemistry discussed in Chapter 12. For example, total column ozone was at a record low in 1992-1993, likely due to heterogeneous chemistry on aerosol particles from Mount Pinatubo (e.g., Gleason et al., 1993; Kerr et al., 1993; Bojkov et al., 1993; Hofmann et al., 1994; Komhyr et al., 1994). In January-February 1995, low ozone values were observed in the Northern Hemisphere and were as much as 25-35% below the long-term average over Siberia. This may be related to the displacement of the polar vortex to this location, accompanied by low stratospheric temperatures conducive to the formation of polar stratospheric clouds (Bojkov et al., 1995a). Figure 13.14, for example, shows the measured deviation of ozone from the long-term mean and the tem os

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