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Conclusions

• Satellite-derived erythemal UV doses often represent the observed day-to-day variabilities due to changing cloud and ozone very well.
• However, the distribution of errors about the mean is rather large. The 2-sigma errors in ratios occur at ~0.8 and 2.0. The greatest deviations occur at Garmisch-Partenkirchen and the smallest occur at Lauder.
• At the unpolluted NZ site there is good agreement in absolute terms. However, the agreement may be fortuitous.
• At the three more polluted Northern Hemisphere sites, the satellite overestimates the UV relative to that measured at the ground.
• On a monthly basis there is excellent agreement in the patterns of variability at each site, but there are small systematic seasonal differences at the Northern Hemisphere sites.
• The differences observed here are consistent with those reported previously.
• It appears that air pollution effects such as from absorbing aerosols that attenuate UV irradiance at the surface are rather ubiquitous over large areas of the planet. To understand the effects of these better, measurements of the single scattering albedo of aerosols are required.
• In the case of Garmisch-Partenkirchen, the high-altitude surrounding terrain and the possible snow cover may compensate the aerosol-induced deviation in the satellite data.
• Sites that represent broad geographical regions and are not influenced by snow, or complex topography may reduce the complexity for scientific analysis, but they do not necessarily represent reality.
• The stated accuracy of satellite retrievals of UV (± 12%) is overly optimistic. In three of the four sites selected, differences are larger than this.
• Despite the increasing availability of satellite-derived products, there remains a need for high quality ground-based measurements.
• The use of higher resolution accuracy of satellite-derived UV may improve the accuracy of satellite retrievals.

A more detailed analysis of this data set is in preparation, and will be submitted for publication [McKenzie et al., 2000]

Acknowledgements: We thank Jay Herman and Ed Celarier at NASA Goddard Space Flight Center for providing the TOMS overpass data used in this study. For quality control of the ground-based data sets we thank Mike Kotkamp (Lauder), Tim Martin (Garmisch-Partenkirchen), Tom Grajnar and Vitali Fioletov (Toronto).

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