On the optical thickness in the UV range: Analysis of the ground-based data taken at Belsk, Poland

[1] The longest time series of UV-B and short wavelength of UV-A aerosol optical depth has been constructed for the period 1992 - 2002 using the direct Sun irradiances obtained during standard calculations of total ozone by the Brewer spectrophotometer at Belsk (51degrees84N, 20degrees78E), Poland. The long-term variations appear to be trendless with only small year-to-year variations. The statistical analysis of the 1-day change of aerosol optical depth in the UV range shows that in 30% of cases the scale of the aerosol change is important for the next day UV index forecast. In the analysis of dependence of the aerosol optical depth on wavelength we combine the CIMEL Sun photometer and the Brewer spectrophotometer optical depth data. The Angstrom's exponent is calculated for the pairs of wavelengths in the visible, UV-A, and UV-A/UV-B ranges. It is found that Angstrom's exponent decreases for the shorter wavelength pair, and it is possible that in the UV-B/UV-A range the exponent is actually negative, yielding an increase of aerosol optical depth with wavelength. The Mie code simulations corroborate the hypothesis of possible negative values for the Angstrom's exponent in the UV-B/UV-A range.