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Introduction
An important consequence of stratospheric ozone depletion is the increased transmission of solar ultraviolet (UV) radiation to the Earth's lower atmosphere and surface. UV radiation is known to affect many biological and chemical processes, and is largely detrimental to individual organisms. Specific concerns include increases in the incidence of skin cancer, ocular damage, and other health effects in humans and animals; damage to terrestrial and oceanic vegetation; damage to some outdoor materials; changes in the chemistry of the lower atmosphere (the troposphere), e.g. photochemical smog formation; and alterations of the biogeochemical cycles of non-living organic and inorganic matter whose degradation depends on the exposure to ambient solar radiation (UNEP, 1991; 1994; 1998).

    Environmental UV radiation is highly variable. Some of these variations are easily quantified, such as those due to changes in the solar elevation with latitude, time of day and season. Variations in the atmospheric ozone column amount are of direct importance to surface UV radiation. Continuous ozone observations are available from ground-based stations since the 1950s for a number of locations, and near-global observations are available from satellite-based instruments for most of the period since 1979. Other factors, such as clouds, are much less predictable and their spatial and temporal distributions are still poorly characterized, especially on local scales and for short-term fluctuations. Additional localized perturbations may stem from surface elevation and reflections, and from variable atmospheric turbidity associated with air pollution.

    In this report, the dependence of UV radiation on atmospheric ozone is emphasized. Systematic reductions in ozone have been observed in the last two decades, a likely result of human activities and in particular the emission of halogen-containing compounds of mostly anthropogenic origin (WMO, 1994a; 1998). Observations confirm that ozone reductions lead to increased UV radiation levels at the Earth's surface, if all other factors that influence atmospheric transmission (e.g., clouds, pollutants) are constant. However, it is also necessary to view these ozone-related UV increases in the context of the natural UV variability, and to consider the possibility of long-term changes in other factors such as cloud cover and air pollution.


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