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

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 (?) 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 ? = 550 nm, and the Ångström 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 reports with a minor modification. The effective aerosol optical thickness at ? = 550 nm and the Ångström coefficient were simultaneously retrieved from polarization measurements at ? = 443 and 865 nm by referring to the lookup table. The mean and standard deviation of the difference between the aerosol optical thickness retrieved from the polarization measurements and those determined from the direct solar radiation measurements were 0.00 and 0.02. The retrieved Ångström coefficients agreed well with those derived from the direct solar radiation measurements when the optical thickness exceeds 0.1, whereas the difference was about 0.5 when the aerosol optical thickness was only 0.05. Sensitivities of the retrieval algorithm to the size distribution parameters and the refractive indices of aerosol models are also examined.