Advancement of the AMSSP for Ground-Based Measurements of the Complete Stokes Vector

Climate models which predict the Earth’s temperature and which are a basis for estimating climate change have large uncertainties induced by the lack of understanding of aerosol effects. Polarimetry is the most promising technique to gain information about aerosols and to understand their effect on the climate.

The airborne multispectral sunphotometer and polarimeter (AMSSP) can measure complete polarization information, including circular polarization for the visible spectral range. The transformation of intensity measurements measured by the AMSSP to polarization information is only possible with a sufficient calibration of the instrument. Laboratory calibration measurements resulted in calibration parameters that convert the intensity measurements to accurate polarization information with only small deviations.

On the basis of recent experiences during a previous field campaign, an improved ground-based polarimeter is going to be developed. A Pan-Tilt tracking system allows direct measurements of the sun which enables the optimization of the relative adjustments of the optical paths. The ground-based system allows a flexible measurement geometry within the upper hemisphere without additional mirrors. With this, the impact and uncertainty of the previously used mirror sytsem are eliminated. In addition, it is planned to optimize the optical components of the polarimeter.

The new setup will be tested during the measurement campaign ASKOS in Cape Verde in summer 2020.