Circular polarization in atmospheric aerosols

[摘要] Recent technological advances have demonstrated thefeasibility of deploying spaceborne optical detectors with full polarimetriccapabilities. The measurement of all four Stokes coefficients openssignificant new opportunities for atmospheric aerosol studies andapplications. While considerable amounts of attention have been dedicated tosensors with sensitivity to the total intensity and linear polarization(represented by Stokes coefficients I , U , Q ), there has been less attentionto the additional information brought by measuring circular polarization(coefficient V ). This report fills this gap in knowledge by providing anoverview of aerosol sources of circular polarization in the atmosphere anddiscusses possible remote sensing signatures. In this paper, circularly polarized radiation that results from theinteraction of incident unpolarized radiation is considered in threephysical settings: optical activity originating in biogenic aerosols,alignment of non-spherical particles in the presence of electrical fields(such as dust, smoke, and volcanic ash), and aerosol multiple scatteringeffects. Observational and theoretical evidence of, and the settings andconditions for, non-zero aerosol circular polarization generated fromincident unpolarized radiation are here gathered and discussed. In addition,novel radiative transfer simulations are shown to illustrate notablespectral and other features where circular polarization may provideadditional information that is possibly independent from total intensity andlinear polarization-only observations. Current techniques for the detection of aerosol composition (also referred asaerosol type) from space provide limited information. Remote identificationof aerosols such as smoke, volcanic ash, and dust particles can only beaccomplished with some degree of confidence for moderate to highconcentrations. When the same aerosols are found at lower concentrations(but still high enough to be of importance for air quality and cloudformation), these methods often produce ambiguous results. The circularpolarization of aerosols is rarely utilized, and we explore its value forimproved determination aerosol composition. This study is presented as anoverview with a goal to provide a new perspective on an overlooked opticalproperty and to trigger interest in further exploration of this subject.

[学科分类] 医学（综合）