Modeling the Aerosol Extinction versus Backscatter Relationship for Lidar Applications: Maritime and Continental Conditions

A model to derive functional relationships linking extinction (alpha) and backscatter (beta) of continental and maritime aerosol at 532 nm is presented and tested. These relationships are needed to solve the single-wavelength lidar equation, where both (alpha) and (beta) appear as unknown variables. To obtain the investigated relationships, the extinction and backscatter coefficients of a large number (40 000) of continental and maritime aerosol size distributions and compositions have been computed. Each computation is performed randomly choosing the aerosol microphysical parameters within a variability range fixed according to data available in the literature. An altitude (z) dependence of aerosol microphysical parameters is included in the model so that z-dependent values (alpha) = (beta)(z) and (alpha) = (beta)(z) are obtained. By fitting the scatterplots of the (beta) versus ((alpha), z) points, three analytical expressions (alpha) = f(beta, z) are then obtained corresponding, respectively, to maritime and continental aerosol computations and to their combination. Inversion of the lidar signal by means of these relationships allows the quantitative estimation of the aerosol extinction profiles, therefore of the aerosol optical thickness (AOT). To test the model results applicability, sun photometer AOT measurements are compared to coincidental, lidar-derived AOT obtained employing both the modeled relationships and fixed (alpha)/(beta) ratios (alpha)/(beta) = 30, 60 sr). Performed in both mixed maritime–continental and typical continental conditions, these comparisons show the best lidar–sun photometer accordance when employing, respectively, the combined maritime–continental and the continental relationship. In this latter case, results are comparable to the ones obtained fixing (alpha)/(beta) = 60 sr.