Nov Dec 1994

FIGURE 13.14 Percentage deviation of total ozone from long-term mean and stratospheric temperature at an altitude corresponding to 50 hPa near Siberia in late 1994 and early 1995 (adapted from Bojkov et al, 1995a).

perature in the stratosphere at a pressure of 50 hPa; the correlation between the two is striking. Both measurements (e.g., Solomon et al., 1998) and modeling studies (e.g., Tie et al., 1997) support this relationship between colder temperatures and increased 03 depletion due to increased chlorine activation through heterogeneous reactions.

A similar observation of record low total column ozone over Lauder, New Zealand, down to 222 DU compared to the 1985-1996 average of ~340 DU was reported by Brinksma et al. (f 998). They attributed the low ozone in part to a portion of the Antarctic polar vortex passing over this location at altitudes of ~ 25-35 km and in part to injection at lower altitudes (~22 km) of ozone-poor subtropical air.

In short, there is strong evidence for negative trends in total column ozone in mid and high latitudes, particularly during the winter and spring, superimposed on natural variations. However, the absolute magnitudes at a particular location and time have a high degree of uncertainty associated with them and large fluctuations from year to year are evident. Clearly, this is an area that warrants further attention.

3. Trends in Surface Ultraviolet Radiation

Because of the strong absorption of ultraviolet (UV) radiation starting at ~320 nm by 03, one of the major impacts of decreased stratospheric ozone is expected to be increased UV at the earth's surface, with associated effects such as increases in skin cancer and cataracts and damage to plants and other ecosystem components. It has therefore been of great interest to determine whether such a relationship can be detected and, if so, what the magnitude of the effect is. The latter is commonly expressed as an "amplification factor" (AF) or radiation amplification factor (RAF), defined as the fractional change in radiation (R) per fractional change in total column ozone (03):

Because it is the biological effects that are of interest, R may be weighted using the so-called "action spectrum," which is the wavelength dependence of the particular effect of concern (Brasseur et al., 1995). Figure 13.15, for example, shows the action spectra for damage to DNA, to plants, and for the induction of erythema, or reddening of the skin (Madronich, 1992).

The ultraviolet region is commonly divided into the UV-A region from 315 to 400 nm and the UV-B region from 280 to 315 nm. It is the UV-B region that is of greatest concern in terms of the impacts of UV radiation, and because of the strong absorption of light by

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