Retrieval of columnar aerosol size distributions and radiative-forcing evaluations from sun-photometric measurements taken during the CLEARCOLUMN (ACE 2) experiment

Spectral measurements of direct solar irradiance were taken within 13 narro w-band channels in the 401-3676 nm wavelength range, using the IR-RAD sun-r adiometer at the Sagres station in southern Portugal during the CLEAR-COLUM N (ACE 2) experiment, from June 16 to 35 July 1997. The measurements perfor med on 21 clear-sky days were examined in terms of the Bouguer-Lambert-Beer law, following realistic correction procedures for Rayleigh scattering and absorption by minor atmospheric gases, to determine more than 2100 spectra l series of aerosol optical depth. All these spectral series were examined with the King inversion method to retrieve the columnar aerosol particle si ze distributions over the 0.07-10 mum radius range, for values of the real part of the particulate refractive index ranging between 1.43 and 1.50 and values of the imaginary part between 0.003 and 0.010. Using the well-known 6S computer code for all the columnar aerosol size-distribution curves dete rmined on ten "golden days" and both refractive index parts varying with wa velength, evaluations of the change DeltaF up arrow caused by aerosol parti cles in the outgoing solar radiation flux were made at solar zenith angles theta ranging between 15 and 76 degrees and for spectral albedo features of both clear water and green vegetation surfaces. The results show that the radiative forcing DeltaF up arrow assumes positive values (associated with cooling effects) for the clear water surface on all the measurement days an d at all the solar zenith angles, and mostly negative values (warming effec ts) for the green vegetation surface in the range theta < 60<degrees>. The present evaluations also give clear evidence of the close dependence of Del taF up arrow not only on the surface albedo, solar zenith angle and aerosol optical depth but also on the mean single scattering albedo of the columna r aerosols. (C) 2000 Elsevier Science Ltd. All rights reserved.