An improved estimation of daily clear-sky biologically EER from broadband global solar radiation.

To establish a relation between biologically effective erythemal radiation (EER) and global solar radiation, the hourly and daily clear-sky broadband (310-2,800 nm) global solar radiation (G) and spectral ultraviolet radiation incident on a horizontal surface at Esfahan, Iran (32 degrees 37'N, 51 degrees 40'E) were measured during the period 2001-2005. Good correlations at statistically significant levels between the daily values of EER and the daily G were found. The seasonal variability of EER/G is also discussed and the correction factors are determined for inclusion of vertical column ozone and solar zenith angle (SZA) cycles. The comparison of the estimated daily EER against the independent observed EER revealed that under clear sky conditions the estimations are accurate to 10% or better over SZA of 10-60 degrees and column ozone of 250-350 Dobson. The comparison of the results with the similar works that have used shorter period of experimental data showed more accurate estimates. The deduced relations could be used to a rough estimate of the daily EER from G in arid climate regions, where there is no measured UV radiation or there are instrumental and other difficulties encountered in measuring UV radiation.