11th Conference on Atmospheric Radiation and the 11th Conference on Cloud Physics

Monday, 3 June 2002
Polarization remote sensing of trace aerosols
J. William Snow, MIT Lincoln Lab., Lexington, MA; and A. T. George, W. E. Bicknell, M. K. Griffin, and H. K. Burke
Polarization properties of radiation scattered by airborne particles are used to remotely detect and to identify physical characteristics of trace amounts of man-made aerosols. Scattered intensity from such aerosols, as a function of the scattering angle, conforms closely to the single-scattering phase function - the features of which constrain the values of the trace aerosol size parameter and index of refraction. Two particular properties of polarization are considered. The first is the percent linear polarization of scattered light, which has been employed for more than 50 years to infer aerosol characteristics in planetary atmospheres. The second property is the orientation of the polarization plane; in particular, the change in orientation from that of polarized incident light. This property is a useful indicator of the presence and generic composition of organic particles. A third property, the degree of elliptical (or circular) polarization, may not be helpful in detecting trace amounts of aerosol because it relies strongly on multiple scattering.

Polarization measurements of linearly polarized incident radiation redirected into a wide range of possible scattering angles provide information on the physical properties of trace man-made aerosols through analysis of the scattering phase matrix elements. Specifically, two parameters of the particle size distribution are retrieved, effective radius and effective variance, along with the real part of the index of refraction and a generic classification of the aerosol chemical composition. The atypical size distribution of certain man-made aerosols is crucial for their detection in the midst of background aerosols. The relatively small amount of the man-made aerosol, while limiting signal strength, does insure that the distinctiveness of single-scattering polarization features is preserved.

The practicability of a polarization aerosol remote detection system to detect the release of small amounts of man-made aerosols is determined. Details are given on the retrieval of specific aerosol physical properties from polarization features. Limits are established on detectable concentrations of such aerosols in the presence of typical background aerosols.

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