Modelling UV-B irradiance in Canada

Abstract

Ultra-violet B (UVB) radiation (defined here as the 290325 nm waveband) constitutes less than 1% of the total irradiance reaching the ground but has important adverse biological effects. Life is shielded partially from this radiation by the ozone in the stratosphere which is equivalent to a depth of about 3mm or 300DU (Dobson units). This thin shield has been significantly attenuated by chlorine damage from halocarbons. Systematic ozone reductions of more than 5% have been found globally. Man-made chlorofluorocarbons are being phased out following the Montreal Protocol in 1987 and the ozone layer is expected to recover within about 60 years. Until then stratospheric ozone concentrations will be less than normal and UVB irradiance reaching the earths surface will be larger than normal. However, the UVB radiation band does not pose biological problems only during periods of reduced stratospheric ozone. Biological concerns for UVB damage existed before depletion of stratospheric ozone. This radiation damages DNA, immune systems, skin (erythema, skin cancers) plant growth and phytoplankton. Photobiologists have determined experimentally the spectral variation in effects in the form of action spectra or spectral weights which are applied to spectral irradiance measurements. However, spectral irradiance measurements are rare and must be supplemented with model calculations. In this chapter, we describe a model developed for Canada and show how it performs in calculating climatological estimates of spectral irradiance and biologically-weighted irradiance. Since there is little information on the vertical distribution of ozone and aerosol which Zeng et al. (1994) found important in successfully modelling short-term cloudless sky irradiances, we adopt a more climatological approach by seeking to model monthly averaged daily irradiances using readily available data. The model described may be used for shorter periods. It has been shown to produce instantaneous broadband irradiance estimates which compare with surface measurements as well as estimates from the inversion of satellite measurements of reflected UVB irradiances (Binyamin et al., submitted). © 2008 Springer-Verlag Berlin Heidelberg. All rights are reserved.