SENSITIVITY OF RETRIEVED AEROSOL PROPERTIES TO ASSUMPTIONS IN THE INVERSION OF SPECTRAL OPTICAL DEPTHS

The uncertainties of integral aerosol properties calculated using aero sol size distributions retrieved from multiwavelength observations of aerosol optical depth have been determined for a variety of typical at mospheric aerosol size distributions and refractive indices. The resul ts suggest that more information about the aerosol composition, as wel l as more information about the sizes that are less efficient, in the optical sense, is needed to improve the shape of the retrieved size di stributions. All the calculations in this paper assume spherical homog eneous particles. The sensitivity results refer to these conditions. T he moments of the retrieved size distributions are systematically unde restimated and errors can be as large as -82%, -30%, and -35% for the total number of particles, the total surface, and the total volume, re spectively. The errors in the mass scattering efficiency, the effectiv e radius, and the total volume depend Very much on whether the actual volume size distribution is monomodal or bimodal. Fora known refractiv e index, the total scattering coefficient, the hemispherical backscatt ering coefficient, and the extinction coefficient, as well as the hemi spheric backscattering to total scattering ratio and the asymmetry fac tor, are obtained with absolute Values for the average errors less tha n 4%. Similar behavior was expected for cases with uncertainty in the refractive index, especially for parameters defined by the ratio of tw o integral properties. However, it turns out that the hemispheric back scattering coefficient and the hemispheric backscattering to total sca ttering ratio were poorly retrieved, reaching errors of 29% in several cases, while the asymmetry factor was very well recovered with absolu te values of the average errors always under 7%. When the wavelength d ependence of the refractive index is included, the retrieved size dist ribution is very unrealistic, with average errors in the hemispheric b ackscattering coefficients and the hemispheric backscattering to total scattering ratio around 30% at some wavelengths. However, even in thi s case the errors in the retrieved asymmetry factor stay under 8%. Thu s, for spherical and homogeneous particles, the spectral optical depth data can be used to determine the asymmetry factor with little sensit ivity to the assumptions in the calculations. Furthermore, the retriev ed size distribution can be used as an intermediate step to extrapolat e one set of optical properties from another set of optical properties .