ESTIMATION OF THE AEROSOL PHASE FUNCTION IN BACKSCATTER FROM SIMULTANEOUS SATELLITE AND SUN-PHOTOMETER MEASUREMENTS

The backscatter part of the aerosol phase function P-A(chi), where chi is the scattering angle, is difficult to measure from the ground. Exp erimental data for chi > 120 degrees are not reported in the literatur e. Customarily, P-A(chi) is calculated from Mie theory using an aeroso l size distribution either prescribed or estimated by inversion of spe ctral or almucantar/aureole measurements. These results clearly requir e validation using direct measurements. In this paper, an empirical ph ase function of atmospheric aerosol over the ocean is estimated in bac kscatter (chi > 130 degrees) from coincident measurements of upward ra diance in channel 1 (0.63 mu m) of the Advanced Very High Resolution R adiometer (AVHRR) on board National Oceanic and Atmospheric Administra tion satellites and sun-photometer aerosol optical thickness, delta(SP )(A). This study uses 31 sun-photometer measurements, collected during two oceanic cruises over the North Atlantic in 1989 and 1991. The acc uracies of both satellite radiances and sun-photometer delta(SP)(A) ar e well documented. The Linearized form of the single-scattering approx imation for the radiative transfer equation is used, with some adjustm ents to account approximately for multiple scattering effects, The new ly estimated empirical phase function shows variability from one point to another, but on the average, is close to that expected for maritim e aerosols as found in the literature. The results of the present stud y may be used to constrain the range of variability of the aerosol pha se function in real marine atmospheres, which is important for aerosol retrieval from historical Coastal Zone Color Scanner (CZCS), present (AVHRR), and future satellite sensors Moderate-Resolution Imaging Spec troradiometer (MODIS), Multiangle Imaging Spectroradiometer (MISR), an d Sea-Viewing-Wide-Field-of-View-Sensor (SeaWiFS).