The amount of UV radiation that reaches the Earth‚s surface from the Sun can be estimated using a combination of radiative transfer calculations and the measured amounts of ozone, cloud reflectivity or cloud-optical depth, aerosol optical depth, and known amounts of Rayleigh scattering. While complicated, the methods for obtaining the amount of UV irradiance between 290 and 400 nm striking the Earth‚s surface at any location are well developed and have been applied to TOMS data (Krotkov et al., 1998, 1999; Herman et al., 1996, 1999b; Kalliskota et al., 1999). The amounts calculated have been successfully compared to ground-based measurements made by broadband instruments and spectrometers.
The importance of identifying the regions of high UV irradiance and correlating them with human, plant, and animal health is well understood (UNEP, 1991). Regions such as Australia, the southwestern US, and most of the tropics are subject to high UV radiation levels. In Australia, the problem is recognized as a major public health problem (Green and Williams, 1993; Herlihy et al., 1994) as it is, to a lesser degree, in the US. The most common problems are increased incidence of skin cancer (de Gruijl and Van der Leun, 1993; Moan and Dahlback, 1993), eye cataracts (Zigman, 1993), and reduced yields in agricultural products (Bornman and Teramura, 1993; Teramura et al., 1990). An example of UV irradiance estimates possible from spacecraft observations and the correlation with skin cancer is shown in Fig. 21.
The difficulty with satellite estimates of UV irradiance has always been that the estimates are confined to the single time of the satellite overpass (usually near noon). The result has been that the variability of the cloud cover, and to a lesser extent the ozone variability, cannot be determined from the satellite data and compared with the ground measurements. With Triana-EPIC there will be measurements of ozone and aerosols once per hour, and measurements of cloud reflectivity every 15 minutes. This will put the spacecraft determination of UV irradiance on an equal basis when comparing with ground observing sites (e.g., Herman et al., 1999b, Correll et al., 1992; Weiler and Penhale, 1994; Zerefos et al., 1997).
The most important variables affecting the amount of UV irradiance reaching the ground are latitude, cloud cover, and ozone amount. When all other biological factors are equal, the regional differences in cloud cover are the most important factor in determining the health risk to UV exposure. An example of this is the effect of summertime UV exposure on the similar populations that originated in England and now live in Australia or the US at similar latitudes. While there is a small decrease in ozone amount between the Southern and Northern Hemispheres, at the same latitude, there is a major decrease in cloudiness. The reduced cloudiness causes almost double the noontime UV exposure in Australia compared to the US (see Figure 22 for January and July). A similar condition occurs at the equator during the equinoxes when there is much less cloud cover in March than at the same latitude in September, while the ozone amount is approximately the same.
Other less extreme cases may depend on knowledge of the difference between morning and afternoon cloudiness to understand the biological impact of UV exposure in a given region, and especially long-term changes in that exposure caused by ozone or climate change. This is why the global cloud measurements from Triana for the entire day will be important.
