Aeronet Polarization Measurements
Since 1995, a polarized version of the CIMEL sunphotometer is operated on a number of AERONET sites. Those polarized AERONET sunphotometers provide daily measurements of the linear polarization in the principal plane, in addition to the standard solar transmission and total radiance measurements. The polarized version of the instrument is equipped with three polarizers at the same wavelength (870nm), scanning the principle plane horizon to horizon in 5 degrees intervals on scattering angle. The polarized network is designed to investigate the feasibility of retrieving aerosol properties from ground-based polarization data and to prepare the validation of remote sensing polarimetric experiments. Theoretical computations clearly show the sensitivity of polarization to particle properties, especially to their size and refractive indexi. Field measurements confirm that sensitivity. Adding polarization measurements to radiance measurements in retrieval algorithms provide a better assessment of aerosol properties. We present the AERONET network of polarized instruments and describe the polarization calibration method and accuracy. We show typical polarization measurements collected in various geographical regions and atmospheric conditions. We describe the method used to correct the measurements for the effects of multiple scattering, Rayleigh scattering , and ground influence and we consider aerosol properties retrieved from the angular distribution of the sky radiance and its polarization. Advantages of radiance and polarization measurement in the principal plane to retrieve aerosol properties are demonstrated. Specific issues associated with that retrieval technique are discussed.