Analysing cloud and ozone effects on biologically effective irradiances

The variation of UV radiation at the Earth's surface is a consequence of the variation of geometric factors (solar zenith angle, Sun-Earth distance, and altitude), atmospheric factors (clouds, aerosols and ozone) and the ground's albedo.

The variations of some parameters as the solar zenith angle, the Sun-Earth distance, and ozone annual cycle, and their effect on irradiances are well established, while the relative influence of daily changes in ozone total column and clouds has not totally been quantified.

In this paper we performed regression analysis of daily biologically effective integrated irradiances versus ozone total column, geometric factors and cloud changes, trying to quantify the influence of the last three parameters in the variations of the first one.

Multivariate and simple regression analysis of the integrated irradiances weighted by the DNA damage action spectrum versus ozone total column and the integrated irradiances in the band 337-342 nm (as an indicator of geometric factors and clouds variations) have been performed. The sites selected for the analysis are: Ushuaia (54o49'S, 68o19'W), Palmer (64o46'S, 64o03'W), Mc Murdo (77o51'S, 166o40'E), South Pole (90oS, 0) and San Diego (32o45'N, 117o11'W). All of them, except San Diego, suffer strong daily ozone variations during the spring because of the presence of the "Ozone Hole" and each site shows big differences in cloud cover conditions regarding to the others.

Ozone data for this study was provided by NASA Goddard Space Flight Center (Mc Peters and Beach, TOMS Nimbus-7 and Meteor-3, version 7 and Herman, TOMS ADEOS) and irradiances by Biospherical Instruments Inc. (NSF Polar Programs UV Spectroradiometer Network, Vol. 1-6, Booth, et al).

Simple regression analysis for daily-integrated irradiances showed that daily changes are driven by clouds effect, except for spring month, in the Antartic and Sub-Antartic sites, because of the strong modulation imposed by the intermittent effect of the "ozone hole" over these areas during those months.

The same analysis was performed with monthly values. In this case, ozone has the main role in the variation of the biologically effective integrated irradiances for all months.