Since changes in cloud, plus aerosol, reflectivity have been small over the 1979 to 2008 period, except in some local regions (Herman et al., 2009b), the zonal average change in UVB radiation is dominated by changes in ozone amounts. For this purpose, the changes can be estimated from Eq. (5.1) or (5.3), or including the observed LER change term as in Eq. (5.2). The ozone data set is based on a combination of satellite data from Nimbus-7/TOMS, the SBUV-2 series of satellite data, EP/TOMS, and AURA/OMI organized into monthly average and zonal average time series. The time of the day is taken as local noon, so that the SZA is the latitude L± 5 (the solar declination angle 5) depending on the day of the year T. When t = 12 (noon), SZA = L -5 from Eq. (5.12).
cos(SZA) = cos(L) cos(J) cos[n(t-12)/12] + sin(L) sin(J) (5.12)
A useful, but moderate, accuracy estimate for 5 (degrees) as a function of the day of the year T (days from 1 to 365) is given in Eq. (5.13). Knowledge of SZA is needed to evaluate Eqs. (5.1) to (5.4) to estimate UV or VIS changes from satellite data discussed in Section 5.3.8.
a = 0.5a(2 + b 2)T + 2absin(T) + 0.25ab2sin(2T) a = 0.017,202,166 b = 0.016,75 c = -0.397,68 d = 0.177,84
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