The recent increase in the ultraviolet radiation that reaches the ground, mainly due to the decrease in stratospheric ozone, demands high quality measurements over the world. For this goal the use of broadband instruments to measure erythemal-weighted irradiance is widely extended due to their low cost and easy maintenance and use. Nevertheless considerable efforts in quality assurance and quality control (QA/QC) are requiredin order to obtain a homogenised eryhtemal radiation from different regional and national networks over whole Europe. The laboratories that provide the calibration to these networks must guarantee reliable methodologies. For this purpose, the WG4 (Quality Control) within the COST-726 European action (Long-term changes and climatology of the UV radiation over Europe) is promoting the inter-laboratory intercomparison and the adoption of common procedures. In this work the procedures for calibrating broadband radiometers adopted by the laboratory "El Arenosillo" in Huelva, Spain, are presented. The methodology used at INTA has been intercompared with the UV laboratory at PMOD/WRC (World Radiaton Centre) in Davos (Switzerland) and results are presented in this document.
Long-term ground-based UV radiometers and satellite UV spectrometers have been utilized for detecting trends in UV
radiation and for establishing its climatology. The aim of this work is to compare noon erythemal (CIE) UV irradiance
data from NASA Total Ozone Mapping Spectrometer (TOMS) with ground-based measurements from a Brewer
spectrophotometer. The Brewer instrument is located at the Atmospheric Sounding Station "El Arenosillo" (ESAt) in
Huelva, Southwestern Spain. ESAt, with a high number of cloud-free days per year, is particularly suitable for
atmospheric-radiation studies and satellite validation. The period of study covers the years 2000 to 2004. The effect of
clouds and aerosols on the satellite vs ground-based bias is evaluated under different atmospheric conditions regarding
aerosol load and cloudiness. It is found that under all sky conditions TOMS overestimates the noon CIE irradiance about
8%. This bias is even higher (about 12%) for cloud-free days, showing statistically significant correlation with the
aerosol optical depth at 440 nm as measured by a co-located CIMEL-AERONET sunphotometer. However, for thick
clouds (high TOMS reflectivity) the bias becomes negative. Regarding aerosols, the bias increases as the aerosol load
increases, showing the highest values during dust events. All these facts suggest the need to correct the TOMS UV
products mainly under dust conditions.