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Introduction

The purpose of the present study is to determine the extent to which differences in tropospheric aerosols can affect the accuracy of satellite-derived UV at mid-latitude sites. This is achieved by comparing daily erythemal UV doses measured at a pristine southern hemisphere site, Lauder New Zealand, with more polluted northern hemisphere sites (Toronto Canada, Garmisch-Partenkirchen Germany, and Thessaloniki Greece).

These sites differ greatly in their aerosol optical depths. The 1 micron aerosol optical depth at Lauder, New Zealand is typically 0.02, whereas in unpolluted regions of the NH, corresponding values of around 0.1 to 0.2 are typically assumed. Other indicators of atmospheric pollution, such as black carbon, and SO₂ also differ by a factor of twenty between New Zealand and populated regions of the NH due to differences in anthropogenic contributions.. Model calculations show that these differences can have a large effect on UV irradiance . Observational studies comparing UV between Lauder NZ and Garmish-Partenkirchen have also demonstrated that tropospheric extinctions are an important contributor to the observed differences . For example, measurements revealed the sunburning UV in the New Zealand site to be approximately 40% more than at Garmisch-Partenkirchen.

Differences in surrounding topography can also have an influence on the satellite retrievals because they represent averages over areas of ~30 km x 30 km or more. This is a particular issue with data from Garmisch-Partenkirchen which is surrounded by high mountains. The Lauder site is also affected in this way, but to a lesser extent.

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