Estimation of the Aerosol Phase Function in Backscatter from Simultaneous Satellite and Sun-Photometer Measurements

Abstract

The backscatter part of the aerosol phase function PA(?), where ? is the scattering angle, is difficult to measure from the ground. Experimental data for ? > 120° are not reported in the literature. Customarily, PA(?) is calculated from Mie theory using an aerosol size distribution either prescribed or estimated by inversion of spectral or almucantar/aureole measurements. These results clearly require validation using direct measurements. In this paper, an empirical phase function of atmospheric aerosol over the ocean is estimated in backscatter (? > 130°) from coincident measurements of upward radiance in channel 1 (0.63 ?m) of the Advanced Very High Resolution Radiometer (AVHRR) on board National Oceanic and Atmospheric Administration satellites and sun-photometer aerosol optical thickness, δASP. This study uses 31 sun-photometer measurements, collected during two oceanic cruises over the North Atlantic in 1989 and 1991. The accuracies of both satellite radiances and sun-photometer δASP are well documented. The linearized form of the single-scattering approximation for the radiative transfer equation is used, with some adjustments to account approximately for multiple scattering effects. The newly estimated empirical phase function shows variability from one point to another, but on the average, is close to that expected for maritime aerosols as found in the literature. The results of the present study may be used to constrain the range of variability of the aerosol phase 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 Spectroradiometer (MODIS), Multiangle Imaging Spectroradiometer (MISR), and Sea-Viewing-Wide-Field-of-View-Sensor (SeaWiFS).