FIGURE 13.12 Fits to percentage deviation in regionally averaged ozone in (a) North America, (b) Europe, and (c) the Far East after subtracting components due to the solar cycle, seasonal variations, the QBO, and atmospheric nuclear tests (adapted from Sto-larksi et al, 1992).

For example, analysis of a combined set of data from ground-based measurements using Dobson spectrometers and related approaches as well as ozonesondes and SBUV from 1979 to 1994 indicates that at midlatitudes in the Northern Hemisphere, the trend in total ozone is as much as -7% per decade in the winter and spring compared to - 3% per decade in the summer and fall; in the Southern Hemisphere at midlatitudes, there is less seasonal variation but negative trends in total ozone in the range —3 to — 6% per decade are observed (Harris et al., 1997). Trends in the tropics were not clearly statistically significant.

At Arosa, Switzerland, where there are records back to 1926 (the longest data record available), the trend in annual mean 03 has been determined to be —(2.3 + 0.6)% per decade. When contributions due to the solar cycle, temperature, and stratospheric aerosol concentrations are taken into account, the trend is —(1.9 + 0.6)% per decade. However, the total measured column 03 includes both stratospheric and a smaller tropo-spheric contribution, and the latter has been increasing (see Chapters 14 and 16). This would tend to mask part of a decrease in stratospheric 03. Applying an estimate of the increase in tropospheric ozone gives a trend in stratospheric 03 of -(3.0 + 0.6)% per decade at Arosa (Staehelin et al., 1998b).

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