Spectral characteristics of the second Stokes' parameter of scattered atmospheric visible light measured by the Multi-angle Polarimetric Hyper-Spectrometer (MPHS)

Precise information on aerosol microphysical properties is necessary in order to understand various roles of the aerosols in the atmosphere. Since there are many different aerosol types and their temporal and spatial variations are considerably large, the retrieval of optical properties of the aerosols is difficult even with multiple constraints as provided by multi-spectral and multi-angle radiance measurements. Polarization measurements are highly sensitive to particle microphysics and can help to resolve this problem since they provide additional uncorrelated constraints for the retrieval. In this study, we established the Multi-angle Polarimetric Hyper-Spectrometer (MPHS) system. It measures the second Stokes parameter (Q) on the principal plane in the 400-700 nm wavelength domain with less than 1 nm spectral resolution and 0.5 degree field of view. The measurement uncertainty of Q is identified to be less than 0.5%. The measurements have been performed during the period of May - October 2012 which includes clear sky (AOD less then 0.1 with cloud free condition), cloudy sky, dust event, and haze events. The measurements were compared with aerosol optical properties from the Cimel sunphotometer. The measured Q has shown different spectral characteristics even with similar aerosol optical depth (AOD). The sensitivities of aerosol information on the measurements were also simulated with model calculations using the VLIDORT. The simulation results showed that the measurements of Q on the principal plane were highly sensitive to the aerosol size, real and imaginary refractive indices, AOD while the aerosol vertical distribution has shown negligible sensitivity on the measurements. Several inversion methods will be applied to these measurements in order to obtain more precise information on aerosols in the future study.