Columnar physical and radiative properties of atmospheric aerosols in north central Spain

Direct solar irradiance spectra under clear skies have been measured in a rural station in the region of Castilla y León in north central Spain from March to November 1995 to determine the physical and radiative characteristics of atmospheric aerosols. About 300 spectra were measured with a LI‐COR Model LI‐1800 spectroradiometer, with a spectral resolution (full width at half maximum) of 6 nm from 300 nm to 1100 nm. This moderately high spectral resolution permitted the retrieval of the spectral aerosol optical depth (AOD) using a method based on spectral windows in nonabsorbing regions and modelling by the Ångström formula in the spectral range 370–1000 nm. These modelled Ångström AOD spectra were used to determine the columnar particle size distribution of atmospheric aerosols using Mie theory and a pseudo‐inversion method. We assumed a monomodal lognormal function with a fixed standard deviation, σ = 2.5, and two particle refractive indices (absorbing and nonabsorbing aerosols) based on climatological characteristics of the continental area of study. Physical parameters, such as the effective radius, the total vertical volume, and mass loading, have been derived. The effective radius ranged from 0.015 to 1 μm, and the values of the vertical aerosol volume, from 0.01 to 0.3 μm. Aerosol mass loading varied from 0.03 to 0.53 g m−2. Radiative properties represented by the asymmetry parameter g, the single‐scattering albedo ωo, and the phase function P were also evaluated for each of the retrieved particle size distributions as a function of wavelength. Because the radiative parameters show low wavelength dependence from 300 to 1000nm, we present their behavior at 500 nm. The parameter g varies from 0.45 to 0.75 (the average value is 0.6), and ωo varies from 0.75 to 0.94. Relationships of the effective radius and the asymmetry parameter with the Ångström turbidity parameter α have been found, which is of interest for the parametrization of aerosol properties. Finally, the phase function at 500nm, 120° (where it has a minimum) has also been evaluated and correlated with the α parameter, thus providing information about the size of aerosol particles. The uncertainties of all of these parameters are also estimated and discussed.