Spectral measurements of aerosol particle extinction in the 0.4–3.7 μm 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 solar irradiance were performed at thirteen window-wavelengths in the 0.4–3.7 μm range, on 27 days. The measurements were performed using the IR-RAD sun-radiometer designed and manufactured at the Institute ISAO (FISBAT), Bologna (Italy), and carefully calibrated by applying the Langley plot method to the measurements performed on 24 October 1996, at the Schneefernerhaus Observatory on the Zugspitze (Germany). From these measurements, the values of the total atmospheric optical depth were obtained at the various wavelengths, from which the corresponding spectral values of aerosol optical depth were determined through accurate corrections for Rayleigh scattering and gaseous absorption. Such values were found to be mostly smaller than 0.1 during June and early July, presenting almost neutral spectral dependence characteristics, closely related to the prevailing oceanic origins of particulate matter. Higher values of aerosol optical thickness, mainly ranging between 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 contents of continental and anthropogenic aerosol particles arriving from polluted European regions. The King inversion method was applied to the spectral series of aerosol optical depth to determine the particle size-distribution curves in the 0.2–20 μm diameter range. For 2 of these cases (one chosen for a low atmospheric loading of marine aerosols and the other for a mean content of continental/anthropogenic aerosols), the changes in the outgoing solar radiation flux produced by aerosol particles were calculated, considering different surface albedo conditions. The results obtained in the second case show that changes of opposite sign can be caused by the same atmospheric aerosol loading when passing from oceanic to continental areas.