Regression analysis of biologically effective integrated irradiances versus ozone, clouds and geometric factors

Factors affecting UV radiation at the earth's surface include the solar zen ith angle, earth-sun distance, clouds, aerosols, altitude, ozone acid the g round's albedo, The variation of some factors, such as solar zenith angle a nd earth-sun distance, is well established. Total column ozone and UV radia tion are inversely related, but the presence of clouds may affect the resul ting UV in such a way that a depletion in the total column ozone may not al ways lead to an increase in the radiation at the earth's surface. The aim o f this paper is to determine the contribution to the variation of the biolo gically effective irradiance by geometric factors, clouds and ozone, jointl y and separately, in Ushuaia (54 degrees 49'S, 68 degrees 19'W, sea level), and the seasonal variation of this relationship, given the magnitude and s easonal distribution of the ozone depletion and the frequent presence of hi gh cloud cover in this site. For this purpose, multivariate and simple regr ession analyses of daily and monthly integrated irradiances weighted by the DNA damage action spectrum as a function of total column ozone and the int egrated irradiances in the band 337-342 nm (as a proxy for cloud cover and geometric factors) have been performed. For the analysed period (September 1989-December 1996) more than 97% of the variation of the DNA damage weight ed daily integrated irradiances is described by changes in ozone, clouds an d geometric factors. Simple regression analysis for daily integrated irradi ances, grouped by month, shows that most of this variation is explained by clouds and geometric factors, except in spring, when strong ozone depletion occurs intermittently over this area. When monthly trends are removed, sim ilar results are observed, except for late winter. (C) 2000 Elsevier Scienc e Ltd. All rights reserved.