Spectral measurements of aerosol particle extinction in the 0.4-3.7 mu m wavelength range, performed at Sagres with the IR-RAD sun-radiometer

During the CLEARCOLUMN campaign which took place at Sagres (Portugal) from 16 June to 25 July 1997, more than 2000 spectral measurements of direct sol ar irradiance were performed at thirteen window-wavelengths in the 0.4-3.7 mu m range, on 27 days. The measurements were performed using the IR-RAD su n-radiometer designed and manufactured at the Institute ISAO (FISBAT), Bolo gna (Italy), and carefully calibrated by applying the Langley plot method t o the measurements performed on 24 October 1996, at the Schneefernerhaus Ob servatory on the Zugspitze (Germany). From these measurements, the values o f the total atmospheric optical depth were obtained at the various waveleng ths, from which the corresponding spectral values of aerosol optical depth were determined through accurate corrections for Rayleigh scattering and ga seous absorption. Such values were found to be mostly smaller than 0.1 duri ng June and early July, presenting almost neutral spectral dependence chara cteristics, closely related to the prevailing oceanic origins of particulat e matter. Higher values of aerosol optical thickness, mainly ranging betwee n 0.1 and 0.5 at visible wavelengths and sharply decreasing with wavelength , were found during the rest of July in the presence of predominant content s of continental and anthropogenic aerosol particles arriving from polluted European regions. The King inversion method was applied to the spectral se ries of aerosol optical depth to determine the particle size-distribution c urves in the 0.2-20 mu m diameter range. For 2 of these cases (one chosen f or a low atmospheric loading of marine aerosols and the other for a mean co ntent of continental/anthropogenic aerosols), the changes in the outgoing s olar radiation flux produced by aerosol particles were calculated, consider ing different surface albedo conditions. The results obtained in the second case show that changes of opposite sign can be caused by the same atmosphe ric aerosol loading when passing from oceanic to continental areas.