In contrast to the ground-based point measurements, satellite data may provide us with global coverage at a moderate resolution by standard sensors that have been observing UV from space for more than 20 years. The EP/TOMS dataset can be used for monitoring long-term trends in total column ozone as well as the seasonal
Figure 10.1 Map of the 37 USDA ground stations, 4 of which are circled for statistical analysis
Figure 10.1 Map of the 37 USDA ground stations, 4 of which are circled for statistical analysis chemical depletions in ozone that occur in both the southern and northern hemisphere polar springs. EP/TOMS also generates the erythemal exposure data product, which is an estimate of the daily integrated UV irradiance, calculated by using a model of the susceptibility of Caucasian skin to sunburn (erythema). This can be interpreted as an index of the potential for biological damage due to solar irradiation, and depends on the column ozone amount and cloud conditions on a given day.
The erythemal exposure was used in this study as a means for UV-B estimation from TOMS satellite data, and is mathematically defined by the following integral,
1 i*400 nm ftsr
es ss where des is the distance from the earth to the sun (A.U.); S is the solar irradiance incident on the top of the atmosphere at 1 A.U. (nW m nm ); W is the biological action spectrum for erythemal damage (B.D.); tsr and tss are the time of sunrise and sunset (radian), respectively; C is the cloud attenuation factor (unitless); rc1 is cloud optical thickness (unitless); 3 is the SZA that is a function of time (radian); F is the spectral irradiance at the surface under clear skies normalized to unit solar spectral irradiance at the top of the atmosphere (unitless); and Q is the total column ozone (DU).
According to McKinlay et al. (1987), the earth-sun distance, times of sunrise and sunset, and the dependence of the SZA on the time of day depend on the latitude and the time of year, and are calculated from standard formulae. The extraterrestrial solar irradiance incident at the top of the atmosphere when the earth is at a distance of 1 A.U. from the sun was measured over the wavelength interval of interest by the UARS/SOLSTICE instrument. The weighting function used to approximate the wavelength-dependent sensitivity of Caucasian skin to erythema-causing radiation followed the model proposed by McKinlay and Diffey (1998a, b) and was adopted as a standard by the Commission Internationale de l'Éclairage (CIE).
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