Use of Polarimetric Measurements of the Sky over the Ocean for Spectral Optical Thickness Retrievals

Abstract

Abstract To investigate the feasibility of retrieving spectral aerosol optical thickness from polarization measurements, the degree of polarization of the sky radiation was measured at a 90° angle from the solar direction in the principal plane with a multichannel polarimeter from ships on the Pacific Ocean and on the Inland Sea of Japan in 1997. The direct solar radiation was also measured simultaneously. Both measurements were made at wavelengths (λ) of 443, 490, 565, 670, 765, and 865 nm. A lookup table of the degree of polarization with three parameters (the solar zenith angle, the aerosol optical thickness at λ = 550 nm, and the Angstrom coefficient) was created from radiative transfer calculations. The aerosol model over the ocean is assumed to be composed of the externally mixed “oceanic” and “water soluble” components, whose size distributions are expressed by the lognormal functions. The size distribution parameters and refractive indices are adopted from the International Radiation Commission re...