Surface UV radiation monitoring at two Italian Brewer stations (Rome and Ispra): a first comparison with OMI data

Currently there is still low spatial coverage of ground-based instruments measuring UV irradiance (spectral or broad band) and the length of time over which reliable UV observations have been made is mainly around 10 years. Solar spectral irradiance (from 290 to 325 nm at 0.5 nm wavelength step) has been measured at the stations of Rome (41.9 degrees N, 12.5 degrees E, 75 m a.s.l.) and Ispra(45.8 degrees N, 8.6 degrees E, 240 m a.s.l.), by means of Brewer single monocromator spectrophotometer since 1992. In this study a climatological characterization based on the time series of UV index (UVI) is presented. The mean of maximum UV indexes is (7.2 +/- 0.2) at Ispra and (8.9 +/- 0.4) at Rome under clear sky conditions. Low exposure category (UVI < 2) is persistent at Ispra and high exposure category (6 < UVI < 7) is more frequent at Rome. The seasonal behaviour is clearly evident, with higher values in summer (up to 8) and low ones (< 3) during winter in both sites. A comparison between these data sets show the UVI latitudinal dependence and the role of meteorological and atmospheric conditions at the two sites. The influence of Fohn events on UVI values at Ispra is also studied. Finally, results of a preliminary comparison between noontime surface UV radiation estimates provided by Ozone Monitoring Instrument (OMI) and Brewer UV data at Rome are shown.