Comparison of satellite-derived UV irradiances with ground-based measurements at four European stations

Satellite-derived ultraviolet (UV) irradiances may form the basis for establishing a gloval UV climatology, provided that their accuracy is confirmed against ground-based measurements of known quality. In this study, quality-checked spectral UV irradiance measurements from four European stations (Sodankyla, Finland; Bilthoven, Netherlands; Ispra, Italy; and Thessaloniki, Greece) are compared with those derived from TOMS, based on the (version 8) data set. The aim of this study is to validate the TOMS UV irradiances and to investigate the origin of disagreements with ground-based data. Comparisons showed that TOMS overestimates summertime noon CIE-weighted irradiances from 6.6% at the high-latitude site of Sodankyla up to 19% for the three other sites. The influence of clouds and aerosols on the observed differences was investigated. For the other three sites (Bilthoven, Ispra, and Thessaloniki), TOMS overestimates the irradiance at 324 nm by almost 15% even under conditions with cloud optical depth of less than 5. For cloud-free days at Ispra and Thessaloniki, differences ranging between 3% and 20% are well correlated with aerosol optical depth. Copyright 2006 by the American Geophysical Union