This paper discusses the influence of aerosols on UV erythemal dose over four metro cities in India. Tropospheric Emission Monitoring Internet Service (TEMIS), archived UV-index (UV-I), and UV daily erythemal dose obtained from SCIAMACHY satellite were used in this study during June 2004 and May 2005 periods covering four important Indian seasons. UV-Index (UV-I), an important parameter representing UV risk, was found to be in the high to extreme range in Chennai (8.1 to 15.33), moderate to extreme range in Mumbai and Kolkata (5 to 16.5), and low to extreme over Delhi (3 to 15). Average UV erythemal dose showed seasonal variation from 5.9 to 6.3？KJm？2 during summer, 2.9 to 4.4？KJm？2 during postmonsoon, 3 to 4.5？KJm？2 during winter, and 5.1 to 6.19？KJm？2 during premonsoon seasons over the four cities. To estimate the influence of aerosols on reducing UV dose, UV aerosol radiative forcing and forcing efficiency were estimated over the sites. The average aerosol forcing efficiency was found to be from to ？KJm？2 AOD？1 on different seasons. The study suggests that aerosols can reduce the incoming UV radiation dose by 30–60% during different seasons. 1. Introduction Ultraviolet radiation, in spite of its nominal presence in solar spectrum, is important in human health perspectives. The UV region of the spectrum mainly consists of UV-C (200–280？nm), UV-B (280–315？nm), and UV-A (315–400？nm). Out of these UV-B is the most important spectral range, as it directly influences the human health. Enhanced exposure to UV-B can damage both terrestrial and oceanic organisms and also results in increases in the incidences of cataracts and skin cancer in humans [1, 2]. Very high levels of UV radiation also are reported to result in the extinction of minute biological species [3]. The impact of UV irradiance on human skin is characterized with UV erythemal dose. UV-index (UV-I) serves as a primary indicator of impact of UV radiation on human health. The UV radiation reaching the earth surface is modulated by the ozone concentration, cloud cover, solar zenith angle, and aerosols. The influences of ozone on UV radiation are well documented [4]. Several studies report the influence of cloud cover on UV radiation. Reference [5] reported that, for a solar zenith angle of 50°, average UV-B transmission was observed to be 30% for overcast skies and found to be ranging between 61 and 79% according to cloud amount. The paper [6] has shown that the effect of cloud for UV wavelengths is less than that for the whole solar spectrum and less than that for the visible part of the
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