Retrieval of the aerosol optical depth in the UV-B at Uccle from Brewer ozone measurements over a long time period 1984-2002

Since 1984, Brewer spectrophotometer #16 has measured the ozone column at Uccle near Brussels in Belgium (50degrees48'N, 4degrees21'E, 100 m) from the direct sun observations at five isolated wavelengths in the UV-B: 306.3 nm, 310.1 nm, 313.5 nm, 316.7 nm and 320.1 nm. We have used the Langley Plot Method (LPM) to retrieve the aerosol optical depth (AOD) from these observations over a long time period: from 1984 to November 2002. A seasonal variation of the AOD is clearly observed with mean AOD values of about 0.4 and 0.9 at 306.3 nm in winter and in summer respectively. The magnitude of these AODs is comparable to the AODs measured by AERONET sunphotometers at other places. We succeeded to demonstrate the impact of the eruption of Mount Pinatubo on the retrieved AOD in the UV-B at Uccle in winter. Trend analysis on the mean annual AOD gives significant negative trends at the 2sigma level only from 1989 to 2002 at all wavelengths and not over the whole period 1984-2002. The annual trend amounts to -2.46 +/- 0.37%/year at 306.3 nm over 1989-2002. Significant negative seasonal trends at the 2sigma level are only found in winter at all wavelengths (-2.66 +/- 0.65%/year at 306.3 nm for example) and in summer at 306.3 nm (-1.24 +/- 0.55%/year). In summer, in autumn and in spring at the other 4 wavelengths the trends are not significant at the 2sigma level.