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Relationship between Satellite Deviations and aerosol optical depth

The satellite-derived UV is not a direct measurement of UV at the surface. It is a product derived from a radiative transfer model, using UV radiation which is backscattered from the troposphere. Because the radiation does not penetrate fully through the boundary layer, there is a potential for the satellite product to overestimate UV, particularly in regions where aerosol and tropospheric ozone concentrations are greater so that the effective backscatter altitude is higher. The effect of this can be appreciable: Kylling et al., [1998] have shown that aerosols in Greece we can easily reduce the erythemal UV by 20% or even more; Mayer [1997] has shown that at Garmisch-Partenkirchen, aerosol reductions in global UV irradiance are typically 10%. The satellite deviations appear to be correlated to differences in 340 nm aerosol optical depths which are small at Lauder (0.03 in winter and 0.05 in summer ), but much larger at Thessaloniki (0.4 in winter to 0.5 in summer) and Toronto (0.25 in winter to 0.36 in summer).

At Garmisch-Partenkirchen, the situation is more complex. The 340 nm optical depths are intermediate (0.15) at this site, but the surrounding high terrain leads to a further overestimation of the satellite derived product. Although to the restricted horizons also reduce the field of view of the ground-based instrument, this effect is smaller than 2% as long as the direct sun is not obscured by the mountains. Measurements of optical depth in surrounding regions [Ingold et al., 2000] also suggest that those derived for Garmisch-Partenkirchen [Mayer, 1997] may be too small.

At Lauder, although the altitude errors are nearly as large, their effect is less important because of the low aerosol optical depths. It should also be noted that the satellite-derived ozone amounts are systematically more than ground-based measurements as this site [Bodeker et al., 2000]. The use of these larger ozone amounts tends to decrease the inferred UV (by up to 5%) and thus tend to cancel the errors due to altitude differences. However, it could be speculated that the agreement between satellite derived UV and ground-based measurments may still be fortuitous since the refelctances (e.g. from clouds and snow) over the measurement site are not typical of those over the satellite footprint. Typically, the reflectivity at Lauder is lower since it is snow free and less cloudy [Uddstrom et al., 2000]. The overestimated reflectivity from the satellite leads to reductions in the deduced UV. If these reductions were as large as 20%, the discrepancies at Lauder would then have been similar to those at the other sites.

Based on these intercomparisons we conclude that while the TOMS retrievals are in good agreement with ground based measurements at the pristine site in Lauder NZ, they systematically overestimate the UV doses in more polluted locations. Measurements from the two sites in Europe are 20-30% less than the satellite derived values, while measurements in Toronto are approximately 15% less than the satellite derived values. These findings are consistent with previously reported results from the as discussed previously .

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