The sun radiates energy in a wide range of wavelengths, and the non-ionizing part of the electromagnetic spectrum is ultraviolet radiation. Further, UV radiation is divided into three ranges: UV-A (315-400 nm), UV-B (280-315 nm) and UV-C (100-280 nm). Among these, the UV-C region (100-280 nm) is completely absorbed by the stratospheric oxygen/ozone and is not received at the earth surface. The UV-A region (315-400 nm) is not much attenuated by ozone (O 3 ), and more than 70% is received at the earth's surface. The UV-B region (280-315 nm) is largely absorbed by ozone, and about 20% reaches the earth's surface.
Although UV-B is a minor component of sunlight, its potential for causing biological damage has become a point of concern, due to its high energy. However, over the last few decades, UV-B in the biosphere has increased, due to substantial reduction of the stratospheric ozone layer in the upper atmosphere (McKenzie et al., 2011). Transmission of UV-B is mainly controlled by ozone, and ozone is depleted by gases such as CFCs (CFC-11, CFC-12, and CFC-113), with a high potential to deplete ozone and a half-life ranging from 50 to 150 years (Dentener et al., 2001;Kakani et al., 2003b).
However, some astronomical parameters, such as solar zenith angle, as well as physical characteristics of the earth's surface, like altitude, albedo and meteorological conditions, also affects the transmission of UV-B (Madronich et al., 1998;Porfirio et al., 2012). Catalytic destruction of ozone molecules by atomic halogens in the stratosphere can result in a decrease of UV radiation absorption, and each 1% reduction in ozone results in an increase of 1.3-1.8% in UV-B radiation reaching the biosphere (Caldwell and Flint, 1994;McKenzie et al., 2003). Though the Montreal Protocol is working, recovery of O 3 layer is not expected before 2070 (Caldwell et al., 2007).
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