title: Direct spectral measurements with a Brewer spectroradiometer: absolute calibration and aerosol optical depth retrieval.
abstract: 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 approximately 0.3 at 340 nm in winter and approximately 0.7 in summer.
authors.affiliation: Laboratory of Atmospheric Physics, Aristotle University of Thessaloniki, P.O. Box 149, Thessaloniki 54124, Greece. skaza@skiathos.physics.auth.gr
authors.forename: Stelios
authors.initials: S
authors.lastname: Kazadzis
authors.forename: Alkiviadis
authors.initials: A
authors.lastname: Bais
authors.forename: Natalia
authors.initials: N
authors.lastname: Kouremeti
authors.forename: Evangelos
authors.initials: E
authors.lastname: Gerasopoulos
authors.forename: Katerina
authors.initials: K
authors.lastname: Garane
authors.forename: Mario
authors.initials: M
authors.lastname: Blumthaler
authors.forename: Barbara
authors.initials: B
authors.lastname: Schallhart
authors.forename: Alexander
authors.initials: A
authors.lastname: Cede