Abstract
In this work, the impact of aerosols over ultraviolet-B at the Earth's surface (UVB) was studied. The estimations of aerosols radiative effects on UVB are significant to UVB estimations, air quality studies, as well as assessments of the impact of regional environmental change. The study used simultaneously hourly values of UVB, global solar radiation (G) at the horizontal surface and direct normal solar radiation (I-n). In addition, other meteorological parameters such as air temperature (T), relative humidity (R) and cloudiness were collected. These data were measured at Qena, Egypt (26.20 degrees N, 32.75 degrees E, and 96 m amsl) through the period from 2001 to 2004. To analyze the relationship between UVB and aerosols, the dimensionless parameter UVB transmission (K-tUVB) and Angstrom turbidity coefficient (beta) in cloudless conditions were considered. The results showed that there is no correlation between K-tUVB and beta in the wide range of SZA (3 degrees > SZA < 75 degrees). The change of K-tUVB to change beta was equal to -0.078 (the correlation coefficient, R = -0.22, i.e. 5% of the variability of K-tUVB was explained by beta). So, sensitivity analysis of aerosol effect in K-tUVB to SZA was employed. The relationship between K-tUVB and beta was determined for a narrow ranges of SZA (the range is equal to 1 degrees) and a linear regression was fitted for each range of SZA. The Delta K-tUVB/Delta beta, accordingly the correlation coefficient (R), increases with the increasing SZA, which means K-tUVB becomes more sensitive to beta as SZA increase. Datasets for each narrow range of SZA, which showed a correlation between K-tUVB and beta (-R > 0.50), were selected to quantify the relationship between both parameters. These selected datasets just show the effect of aerosols in K-tUVB when their UVB penetrating influence is more than the influence of other atmospheric factors such as ozone, i.e. the effect of aerosols is to induce a notable reduction in K-tUVB. For the selected datasets, Delta K-tUVB/Delta beta varied from -0.05 to -0.21 and its average value was equal to -0.12. The resulting regression analysis showed that the determination coefficients of linear fit vary from 0.25 to 0.77, i.e. 25% to 75% of the variance in K-tUVB was explained by beta. (C) 2013 Elsevier Ltd. All rights reserved.