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Introduction

The discovery of the Antarctic ozone depletion in 1985 has raised serious questions about the continued protection of the earth living system. Reductions in stratospheric ozone (O3) allow more solar ultraviolet-B radiation (UV-B, 280-315 nm) to reach the earth's lower atmosphere and surface because the stratospheric ozone column is its primary attenuator [1]. UV radiation affects many chemical and biological processes and its increase is of concern because of potential adverse effects on the biosphere and on quality and composition of tropospheric air [2].

The anticorrelation between the total ozone column and UV-B radiation is well established. However, several other atmospheric parameters such as clouds, haze, aerosols, albedo and absorbing gases largely influence the amount of UV-B radiation reaching the ground level due to scattering and absorption. Besides, the seasonal variation of the geometrical path length of solar radiation through the atmosphere plays a dominant role. It is also essential that the measurement instruments be well characterized and that a good calibration of instrumental response be maintained for a long period of time. Because of the combined effect of these parameters in controlling the UV-B levels, attempts to establish a long-term trend in UV-B radiation from existing data records have been until now very difficult, inconclusive or even controversial [3].

The objective of our research is to shed light on the influence of aerosols, clouds and the tropospheric pollutants on the UV-B radiation levels reaching the earth surface.