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

Abstract

Spectral measurements of direct solar irradiance were taken within 13 narrow-band channels in the 401–3676 nm wavelength range, using the IR–RAD sun-radiometer at the Sagres station in southern Portugal during the CLEARCOLUMN (ACE 2) experiment, from June 16 to 25 July 1997. The measurements performed 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 spectral series of aerosol optical depth. All these spectral series were examined with the King inversion method to retrieve the columnar aerosol particle size distributions over the 0.07–10 μm 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 determined on ten “golden days” and both refractive index parts varying with wavelength, evaluations of the change Δ F↑ caused by aerosol particles in the outgoing solar radiation flux were made at solar zenith angles θ ranging between 15 and 76° and for spectral albedo features of both clear water and green vegetation surfaces. The results show that the radiative forcing Δ F↑ assumes positive values (associated with cooling effects) for the clear water surface on all the measurement days and at all the solar zenith angles, and mostly negative values (warming effects) for the green vegetation surface in the range θ<60°. The present evaluations also give clear evidence of the close dependence of Δ F↑ not only on the surface albedo, solar zenith angle and aerosol optical depth but also on the mean single scattering albedo of the columnar aerosols.