Measurements of the atmospheric turbidity of the north-centre continental area in Spain:: Spectral aerosol optical depth and Angstrom turbidity parameters

Turbidity atmospheric conditions have been monitored during 1995, from March to November, in a rural station in the "Castilla y Leon" region in Spain, of continental climate type. Solar direct irradiance measurements carried out with a spectroradiomter in the 300-1100 nm range with a nominal spectral resolution of 6 nm have been used for this monitoring. The first method used to recover the aerosol optical depth from the irradiance measurements is a well-known technique that requires the knowledge of atmospheric ozone and water vapour contents (direct method). The second method makes only use of selected non-absorption spectral windows to determine the aerosol optical depth and models it by the Angstrom formula in a predetermined spectral range, obtaining at the same time the Angstrom parameters. More than 285 spectra under clear sky conditions show the variation of the aerosol optical depth at representative wavelengths by means of instantaneous, monthly mean values, and frequency histograms. Instantaneous values at 550 nm range from 0.04 to 0.5, with some records reaching values of 0.7, which have been classified as high turbidity episodes. The spectral features of the aerosol optical depth during the measured period are shown by means of representative spectra for different turbidity conditions. The variability and climatological behaviour of the Angstrom turbidity parameters are also analysed during the measured period in a way similar to that for the aerosol optical depth. The a parameter values range from - 0.5 to 2.6 while the beta parameter varies from 0.05 to 0.8, showing both parameters a high daily and day-to-day variability. Different correlations were analysed such as those between the aerosol optical depth at 550 and 1000 nm wavelengths; the alpha-beta parameters and that between the aerosol optical depth at 1000 nm and the beta parameters. (C) 2000 Elsevier Science Ltd. All rights reserved.