Sensitivity metric approach for retrieval of aerosol properties from multiangular and multispectral polarized radiances

Abstract

Linearly polarized radiation is sensitive to the microphysical properties of aerosols, namely, to the particle-size distribution and refractive index. The discriminating power of polarized radiation increases strongly with the increasing range of scattering angles and the addition of multiple wavelengths. The polarization and directionality of the Earth's reflectances (POLDER) missions demonstrate that some aerosol properties can be successfully derived from spaceborne polarimetric, multiangular measurements at two visible wavelengths. We extend the concept to analyze the retrieval capabilities of a spaceborne instrument with six polarimetric channels at 412, 445, 555, 865, 1250, and 2250 nm, measuring approximately 100 scattering angles covering a range between 50 and 150 deg. Our focus is development of an analysis methodology that can help quantify the benefits of such multiangular and multispectral polarimetric measurements. To that goal we employ a sensitivity metric approach in a framework of the principal-component analysis. The radiances and noise used to construct the sensitivity metric are calculated with the realistic solar flux for representative orbital viewing geometries, accounting for surface reflection from the ground, and statistical and calibration errors of a notional instrument. Spherical aerosol particles covering a range of representative microphysical properties (effective radius, effective variance, real and imaginary parts of the refractive index, single-scattering albedo) are considered in the calculations. We find that there is a limiting threshold for the effective size (approximately 0.7 microm), below which the weak scattering intensity results in a decreased signal-to-noise ratio and minimal polarization sensitivity, precluding reliable aerosol retrievals. For such small particles, close to the Rayleigh scattering limit, the total intensity provides a much stronger aerosol signature than the linear polarization, inspiring retrieval when the combined signals of intensities and the polarization fraction are used. We also find a strong correlation between aerosol parameters, in particular between the effective size and the variance, which forces one to simultaneously retrieve at least these two parameters.

Capability of multi-viewing-angle photo-polarimetric measurements for the simultaneous retrieval of aerosol and cloud properties

“…6 has a much smaller fine mode effective radius than the European Background aerosol model used in Fig. 3, and for small particles it is harder to retrieve the refractive index (Miecznik et al, 2005).…”

Retrieval of aerosol properties over land surfaces: capabilities of multiple-viewing-angle intensity and polarization measurements

“…Kahn et al [14] investigated the information content over the ocean of multiple-viewing-angle intensity measurements as performed by MISR, and found that MISR allows one to distinguish two to four compositional groups based on refractive index, four size groups in the range of 0.1-2 m, and between spherical and nonspherical particles. Miecznik et al [15] used a principal component analysis in combination with simplified radiative transfer calculations to compare multiple-wavelength multiple-viewing-angle measurements of the relative Stokes parameters q and u with corresponding intensity measurements for all parameters of a monomodal size distribution. Their conclusions for a large part confirm the findings of Mishchenko and Travis [12], but they also found that for small soot particles the total intensity is a better discriminator for microphysical properties than the polarization fraction.…”

Near-infrared active polarimetric and multispectral laboratory demonstrator for target detection

“…The increasing capability in terms of image acquisition, data storage, and fast processing as well as the development of new sensors enables modern multidimensional imaging [1,2]. Hence the properties of the light backscattered by a given scene, including wavelength and polarization, can be analyzed and taken into account in order to reveal contrasts that are not seen in common intensity (INT) images [3][4][5][6][7][8][9][10]. Following this approach we have been investigating, in the past few years, the benefits of combining active polarimetric and multispectral imaging [11,12].…”