Derivation of aerosol properties from satellite measurements of backscattered ultraviolet radiation: Theoretical basis

We discuss the theoretical basis of a recently developed technique to characterize aerosols from space. We show that the interaction between aerosols and the strong molecular scattering in the near ultraviolet produces spectral variations of the backscattered radiances that can be used to separate aerosol absorption from scattering effects. This capability allows identification of several aerosol types, ranging from nonabsorbing sulfates to highly UV-absorbing mineral dust, over both land and water surfaces. Two ways of using the information contained in the near-UV radiances are discussed. In the first method, a residual quantity, which measures the departure of the observed spectral contrast from that of a molecular atmosphere, is computed. Since clouds yield nearly zero residues, this method is a useful way of separately mapping the spatial distribution of UV-absorbing and nonabsorbing particles. To convert the residue to optical depth, the aerosol type must be known. The second method is an inversion procedure that uses forward calculations of backscattered radiances for an ensemble of aerosol models. Using a look-up table approach, a set of measurements given by the ratio of backscattered radiance at 340-380 nm and the 380 nm radiance are associated, within the domain of the candidate aerosol models, to values of optical depth and single-scattering albedo. No previous knowledge of aerosol type is required. We present a sensitivity analysis of various error sources contributing to the estimation of aerosol properties by the two methods.