ABSTRACT
USDA Ground-Based Measurements ............................................... 414 17.7 Conclusions ................................................................................................... 419 Acknowledgment ................................................................................................... 420 References .............................................................................................................. 420
Changes in the earth’s atmosphere caused by anthropogenic and natural pollutants have led to a well-documented decline in stratospheric ozone (O3) and a corresponding increase in ultraviolet (UV) irradiance at higher latitudes (Kerr and Fioletov 2008). At lower latitudes (30.8°S to 30.8°N), however, UV radiation at the earth’s surface is highest during the summer season because smaller solar zenith angles minimize the atmospheric path length. In addition to the total ozone content that is interrelated with changes of solar radiation in the near ultraviolet wavelengths, the second largest cause of temporal and geographic variability of UV irradiance is clouds. UVA irradiances (320-400 nm) are not signižcantly affected by total ozone levels in the stratosphere because they are not strongly absorbed by ozone (Herman et al. 1997). However, the amount of UV radiation penetrating to the earth’s surface with wavelengths shorter than 320 nm (UVB) can be signižcantly reduced by stratospheric ozone absorption, aerosols, clouds, ground albedo, altitude, and Rayleigh scattering in the atmosphere (Herman et al. 1997).
