Polarized Imaging Nephelometer Scattering Measurements

Our lack of knowledge of aerosol optical and physical properties lies at the heart of the uncertainties in estimating aerosol direct radiative forcing and in deriving aerosol properties from space. No matter how many aerosol-retrieving satellite sensors we launch, these uncertainties will not diminish until we can constrain the particles' intrinsic optical characteristics by some powerful basic new measurement. Even proposed multi-wavelength, multi-angle imaging polarimeters will require validation of their retrievals from a sub-orbital instrument that is at least as capable as themselves. We offer here a new type of measurement that can be a partial solution that leads to better characterization of aerosol properties from space and reduces uncertainties in estimating climate change.

The Polarized Imaging Nephelometer (PI-Neph) is a novel in situ instrument that measures aerosol phase matrix components directly. Designed and built at the University Of Maryland Baltimore County (UMBC), the PI-Neph obtains two elements of the scattering phase matrix at high angular resolution spanning scattering angles 3 to 177 degrees at three wavelengths (473, 532 and 671nm). The two measured elements of the phase matrix, intensity (P11) and linear polarization (P12) contain extensive characterization of the scattering properties of the aerosol, which can be inverted to obtain particle size distribution and other properties. This is similar to the inversions applied to multi-wavelength, multi-angle scans of the sky radiance by AERONET and other sub-orbital sky-scanning radiometers. The difference with PI-Neph is the addition of polarization characterization to the retrievals, the broader angular range measured and the controlled environment of the sampled particles.

Two different versions of the PI-Neph have been deployed in four aircraft campaigns and two campaigns on the ground with the result of over 10^6 images of the scattering phase matrix ready for analysis. These data represent dust, smoke, various types of pollution and biogenics across the U.S. These data provide unprecedented insight into aerosol properties, linking different elements of the phase matrix to other measured variables and highlighting the added information provided by polarization. The data are organized into a web-based data display and analysis tool, accessible to the entire community.

Following in the PI-Neph tradition, a second type of imaging nephelometer, the Open-I-Neph is preparing for its first deployment this summer. The Open-I-Neph takes the concept out of an enclosed internal chamber and opens the device to the ambient air stream. Mounted beneath an aircraft wing, the Open-I-Neph will sample ambient aerosols outside the aircraft while the PI-Neph inside the aircraft measures particle properties under controlled conditions. Comparing these two measurements provides insight on how humidification and other differences between controlled and ambient conditions alter aerosol optical properties. The Open-I-Neph can also measure the optical properties of cloud ice crystals without shattering them.

We will describe the PI-Neph and Open-I-Neph, their role in different field campaigns and show examples from the data base that highlight the power of this new type of measurement.