Direct spectral measurements with a Brewer spectroradiometer: absolute calibration and aerosol optical depth retrieval

We present three different methods for the absolute calibration of direct spectral irradiances measured with a Brewer spectroradiometer, which are shown to agree to within +-2%. Direct irradiance spectra derived by Brewer and Bentham spectroradiometers agree to within 4 +- 3%. Good agreement was also found by a comparison of the aerosol optical depth and Angstrom exponent retrieved by the two instruments and a multifilter rotational shadowband radiometer. The spectral aerosol optical depth (300-365 nm) derived from six years of direct irradiance measurements at Thessaloniki shows a distinct seasonal variation, averaging to ~0.3 at 340 nm in winter and ~0.7 in summer.