RETRIEVING CCN COLUMN DENSITY FROM SINGLE-CHANNEL MEASUREMENTS OF REFLECTED SUNLIGHT OVER THE OCEAN - A SENSITIVITY STUDY

The Twomey effect is an increase of the cloud albedo with increasing c oncentration of tropospheric aerosols serving as cloud condensation nu clei (CCN). Confirmation and quantification of this effect on a global basis requires accurate satellite retrievals of CCN concentrations. W e present a theoretical study of the ability of passive satellite remo te sensing techniques to provide reliable estimates of tropospheric ae rosol column densities over the ocean. We show that a retrieval algori thm based on single-channel single-viewing-angle radiance measurements is incapable of accurately determining CCN column densities and that an algorithm based on multiangle radiance measurements provides much b etter retrievals. However, even for the latter algorithm the errors in the retrieved CCN column densities can exceed a factor of 5. The poor performance of single-channel radiance-only algorithms is explained b y the strong dependence of the extinction cross section and weak depen dence of the phase function on aerosol effective radius. In contrast, high-precision multiangle polarization measurements, which are much mo re sensitive to aerosol microphysics, are capable of constraining CCN column densities to within a few tens of percent.