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29 August 2005 Circular polarization signal for aerosols and clouds
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Circular polarization of scattered solar radiation is essentially zero for almost all aerosol and cloud cases. Required conditions for non-zero circular polarization include multiple-scattering and large scatterer size relative to wavelength. The single-scattering of incident solar radiation can produce linearly polarized light but not circularly polarized light. A second scattering event can transform some of the linearly polarized light into circularly polarized light. Additional scattering events can both create and destroy circular polarization via the transformation process with linear polarization. The peak in circular polarization ratio magnitude occurs at the optical depth for which the multiplescattering processes have maximized its creation-to-destruction rate. Provided multiple-scattering has occurred, circular polarization can only exist for scatterers of large size relative to the wavelength. For aerosols, this implies desert dust or oceanic aerosols and short wavelength observations (i.e., less than 0.5μm). All cloud particles are considered large as they are roughly an order of magnitude larger than aerosols.
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Richard L. Slonaker, Yoshihide Takano, Kuo-Nan Liou, and Szu-Cheng Ou "Circular polarization signal for aerosols and clouds", Proc. SPIE 5890, Atmospheric and Environmental Remote Sensing Data Processing and Utilization: Numerical Atmospheric Prediction and Environmental Monitoring, 58900B (29 August 2005);

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