Spectral discrimination of coarse and fine mode optical depth

Abstract

The recognition that the aerosol particle size distribution (PSD) is effectively bimodal permits the extraction of the fine and coarse mode optical depths (τf and τc) from the spectral shape of the total aerosol optical depth (τa = τf + τc). This purely optical technique avoids intermediate computations of the PSD and yields a direct optical output that is commensurate in complexity with the spectral information content of τa. The separation into τf and τc is a robust process and yields aerosol optical statistics, which are more intrinsic than those, obtained from a generic analysis of τa. Partial (optical) validation is provided by (1) demonstrating the physical coherence of the simple model employed, (2) demonstrating that τc variation is coherent with photographic evidence of thin cloud events and that τf variation is coherent with photographic evidence of clear sky and haze events, and (3) showing that the retrieved values of τf and τc are well‐correlated, if weakly biased, relative to formal inversions of combined solar extinction and sky radiance data. The spectral inversion technique permitted a closer scrutiny of a standard (temporally based) cloud‐screening algorithm. Perturbations of monthly or longer‐term statistics associated with passive or active shortcomings of operational cloud screening were inferred to be small to occasionally moderate over a sampling of cases. Diurnal illustrations were given where it was clear that such shortcomings can have a significant impact on the interpretation of specific events; (1) commission errors in τf due to the exclusion of excessively high‐frequency fine mode events and (2) omission errors in τc due to the inclusion of insufficiently high‐frequency thin homogeneous cloud events.