A novel in-situ cloud and aerosol imaging polarimeter for atmospheric research - Bridging the gap between satellite data and in-situ measurements

The new-generation of satellites will revolutionize Earth observation with advanced sensors, including polarimetric observation capabilities like EUMETSAT's Metop-SG. Equipped with instruments like the Multi-Viewing Multi-Channel Multi-Polarisation Imaging (3MI), these satellites will offer unprecedented insights into the aerosol and cloud components of Earth's atmosphere. However, the indirect nature of satellite observations requires validation through in-situ measurements. In response, we are developing an in-situ cloud and aerosol imaging polarimeter, designed for operation in cloud chambers, at mountain-top stations as well as aboard research aircraft. This innovative approach aims to bridge the gap between satellite data and in-situ measurements, enhancing validation studies and ensuring the accuracy of next-generation satellite observations in climate change research.

Our polarimeter features an innovative design, highlighting a high-resolution camera chip combined with a wide-angle lens for capturing laser light scattered by cloud and aerosol particles at a high angular resolution of better than 0.05° and for a wide backscattering angular range from about 101° to 169°. A directed laser beam is imaged over several meters at a specific observation angle in an open path arrangement. Light scattered from particles transforms into a line on the camera chip, with each pixel corresponding uniquely to a different scattering angle. The optical design incorporates an adjustable polarization filter set-up within the imaging system, enabling seamless measurement of the full Stokes polarization vector of the angular light scattering function. This advanced imaging polarimeter concept offers high accuracy in measuring cloud drop size distribution, as well as linear and circular polarization and depolarization ratios. Notably, our in-situ polarimeter is "active," utilizing a polarized laser beam, distinguishing it from the "passive" approach of satellite polarimeters relying on sunlight.

The presentation will delve into the detailed concept of this innovative polarimeter, offering insights into its optomechanical design. Results from comprehensive optical simulations will showcase the expected measurement capabilities, concluding with findings from initial laboratory tests of the prototype instrument.