CHANGES IN ULTRAVIOLET-RADIATION REACHING THE EARTHS SURFACE

The quality and quantity of UV measurements have increased greatly in the last few years. Variations among measurements from different instr uments are diminishing toward the 5% level. Long-term trend detection is still a problem, with little historical data available for baseline estimations. Enhanced UV levels are clearly associated with the Antar ctic springtime ozone reductions. Measurements show that maximum UV le vels at the South Pole are reached well before the summer solstice, an d DNA-damaging radiation at Palmer Station, Antarctica (64 degrees S) during the springtime ozone depletion can exceed maximum summer values at San Diego, USA (32 degrees N). UV increases at mid-latitudes are s maller. However, increases associated with the record low ozone column of 1992/93 in the Northern Hemisphere are evident when examined on a wavelength-specific basis. Measurements in Argentina, Chile, New Zeala nd, and Australia show relatively high UV levels compared to correspon ding Northern Hemispheric latitudes, with differences in both stratosp heric ozone and tropospheric pollutants likely to be playing a role. T ropospheric ozone and aerosols can reduce global UV-B irradiances appr eciably. At some locations, tropospheric pollution has increased since pre-industrial times, leading to decreases in surface UV radiation. H owever, recent trends in tropospheric pollution probably had only mino r effects on UV trends relative to the effect of stratospheric ozone r eductions. Global ozone measurements from satellites over the period 1 979-1993 imply significant UV-B increases at high and mid-latitudes of both hemispheres, but only small changes in the tropics. Such estimat es however assume that cloud cover and tropospheric pollution have rem ained constant over this time period. Under the current CFC phase-out schedules, global UV levels are predicted to peak around the turn of t he century in association with peak loading of chlorine in the stratos phere and the concomitant ozone reductions. The recovery to pre-ozone depletion levels is expected to take place gradually over the next 50 years.