In contrast to these ozone-induced changes in UVEry, the geographical and seasonal changes (as well as diurnal changes) are large. These geographical variabilities are usually derived from satellite-borne sensors that monitor solar UV radiation that is backscattered from the earth. It should be noted that estimates of UVEry from a satellite are sometimes too large in polluted locations because extinctions within the atmospheric boundary layer are not well probed by these sensors (McKenzie et al., 2001; 2003). Thus, extinctions by tropospheric clouds and aerosols, as well as variations in the profiles of ozone and temperature, can lead to errors in the retrieved surface UV irradiances.
Compared with noon intensities, the corresponding daily total doses of available UV radiation show much larger seasonal variabilities because of the longer hours of daylight in summer. Daily doses for the solstice months, as derived from the Ozone Monitoring Instrument (OMI) on the UARS satellite, are shown in Fig. 2.2. Generally, the daily doses of UV are a maximum at locations where the noon solar zenith angle (SZA) approaches zero, and tend to decrease rapidly in moving to locations where the noon SZA is larger. Thus, the highest daily doses of UV tend to occur in the tropics, and the lowest doses occur in polar regions where they fall to zero in mid winter for all latitudes within the Arctic or Antarctic circles (latitudes > 68°). There are two notable departures from this overall pattern, caused by the springtime Antarctic ozone hole and the effects of high altitudes, as discussed later.
At mid-latitudes (~40°- 50° in both hemispheres), the daily UVEry dose in summer is comparable with that in the tropics. However, in the winter, it is less than 10% of the summer dose. As the latitude increases, the seasonal swing becomes more and more marked.
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