Jacek Chowdhary

Retrieval of aerosol properties over the ocean using multispectral and multiangle Photopolarimetric measurements from the Research Scanning Polarimeter

The evaluation of the direct and indirect aerosol forcings of climate requires precise knowledge of the aerosol optical thickness, size distribution, chemical composition, and number density. Global monitoring of these parameters from satellites requires instruments that make full use of the information content of the scattered solar radiation. In this paper we analyze multispectral and multiangle photopolarimetric observations performed with an airborne version of the Earth Observing Scanning Polarimeter over the ocean and demonstrate their exceptional retrieval potential. Using the 0.865- and 2.250-µm channels, we are able to determine the parameters of a bimodal size distribution, identify the presence of water-soluble and sea salt particles, and retrieve the optical thickness and column number density of aerosols. We also demonstrate that less comprehensive measurements by existing instruments would fail to provide retrievals of comparable completeness and accuracy.

Case Studies of Aerosol Retrievals over the Ocean from Multiangle, Multispectral Photopolarimetric Remote Sensing Data

To evaluate the global effects of aerosols on the direct radiative balance, tropospheric chemistry, and cloud properties of the earth’s atmosphere requires high-precision remote sensing that is sensitive to the aerosol optical thickness, size distribution, refractive index, and number density. This study uses the multiangle 0.41-, 0.55-, 0.865-, and 2.25-mm channel data from the airborne Research Scanning Polarimeter to retrieve aerosol properties over the Pacific Ocean. It is shown that such photopolarimetric data are highly sensitive to the size distribution and refractive index of aerosol particles, which reduces the nonuniqueness in aerosol retrievals using such data as compared with less comprehensive datasets. Moreover, it is found that polarized reflectances obtained at the shorter wavelengths (0.41 and 0.55 mm) are significantly less sensitive to the contribution of the ocean’s upwelling light than total reflectance measurements, providing a natural tool for the separation between the estimation of oceanic and atmospheric scattering properties.

Modeling errors in diffuse-sky radiation: Vector vs scalar treatment

Radiative transfer calculations that utilize the scalar approximation of light produce intensity errors as large as 10% in the case of pure Rayleigh scattering. This modeling error, which arises primarily from second order scattering, is greatly reduced for flux and albedo results because of error cancellation brought about by integration over scattering angle. However, polarized light scattered from an underlying ocean surface, or from atmospheric aerosols, interacts with the pattern of Rayleigh scattered polarization to distort the error cancellation and thus incur larger flux and albedo errors. While addition of scattered radiation from clouds, aerosols or ground surface into the Rayleigh atmosphere tends to reduce the magnitude of scalar approximation intensity errors, the scalar errors in fluxes and albedos are not proportionately reduced, but are actually increased.

Retrieval of Aerosol Scattering and Absorption Properties from Photopolarimetric Observations over the Ocean during the CLAMS Experiment

The extensive set of measurements performed during the Chesapeake Lighthouse and Aircraft Measurements for Satellites (CLAMS) experiment provides a unique opportunity to evaluate aerosol retrievals over the ocean from multiangle, multispectral photometric, and polarimetric remote sensing observations by the airborne Research Scanning Polarimeter (RSP) instrument. Previous studies have shown the feasibility of retrieving particle size distributions and real refractive indices from such observations for visible wavelengths without prior knowledge of the ocean color. This work evaluates the fidelity of the aerosol retrievals using RSP measurements during the CLAMS experiment against aerosol properties derived from in situ measurements, sky radiance observations, and sunphotometer measurements, and further extends the scope of the RSP retrievals by using a priori information about the ocean color to constrain the aerosol absorption and vertical distribution. It is shown that the fine component of the aerosol observed on 17 July 2001 consisted predominantly of dirty sulfatelike particles with an extinction optical thickness of several tenths in the visible, an effective radius of 0.15 0.025 m and a single scattering albedo of 0.91 0.03 at 550 nm. Analyses of the ocean color and sky radiance observations favor the lower boundary of aerosol single scattering albedo, while in situ measurements favor its upper boundary. Both analyses support the polarimetric retrievals of fineaerosol effective radius and the consequent spectral variation in extinction optical depth. The estimated vertical distribution of this aerosol component depends on assumptions regarding the water-leaving radiances and is consistent with the top of the aerosol layer being close to the aircraft height (3500 m), with the bottom of the layer being between 2.7 km and the surface. The aerosol observed on 17 July 2001 also contained coarse-mode particles. Comparison of RSP data with sky radiance and in situ measurements suggests that this component consists of nonspherical particles with an effective radius in excess of 1 m, and with the extinction optical depth being much less than one-tenth at 550 nm.

Contribution of water-leaving radiances to multiangle, multispectral polarimetric observations over the open ocean: bio-optical model results for case 1 waters

Multiangle, multispectral photopolarimetry of atmosphere-ocean systems provides the fullest set of remote sensing information possible on the scattering properties of aerosols and on the color of the ocean. Recent studies have shown that inverting such data allows for the potential of separating the retrieval of aerosol properties from ocean color monitoring in the visible part of the spectrum. However, the data in these studies were limited to those principal plane observations where the polarization of water-leaving radiances could be ignored. Examining similar potentials for off-principal plane observations requires the ability to assess realistic variations in both the reflectance for and bidirectionality of polarized water-leaving radiances for such viewing geometries. We provide hydrosol models for use in underwater light scattering computations to study such variations. The model consists of two components whose refractive indices resemble those of detritus-minerallike and planktonlike particles, whose size distributions are constrained by underwater light linear polarization signatures, and whose mixing ratios change as a function of particulate backscattering efficiency. Multiple scattering computations show that these models are capable of reproducing realistic underwater light albedos for wavelengths ranging from 400 to 600 nm, and for chlorophyll a concentrations ranging from 0.03 to 3.0 mg/m(3). Numerical results for spaceborne observations of the reflectance for total and polarized water-leaving radiances are provided as a function of polar angles, and the change in these reflectances with wavelength, chlorophyll a concentration, and hydrosol model are discussed in detail for case 1 (open ocean) waters.

Analysis of fine-mode aerosol retrieval capabilities by different passive remote sensing instrument designs.

Remote sensing of aerosol optical properties is difficult, but multi-angle, multi-spectral, polarimetric instruments have the potential to retrieve sufficient information about aerosols that they can be used to improve global climate models. However, the complexity of these instruments means that it is difficult to intuitively understand the relationship between instrument design and retrieval success. We apply a Bayesian statistical technique that relates instrument characteristics to the information contained in an observation. Using realistic simulations of fine size mode dominated spherical aerosols, we investigate three instrument designs. Two of these represent instruments currently in orbit: the Multiangle Imaging SpectroRadiometer (MISR) and the POLarization and Directionality of the Earths Reflectances (POLDER). The third is the Aerosol Polarimetry Sensor (APS), which failed to reach orbit during recent launch, but represents a viable design for future instruments. The results show fundamental differences between the three, and offer suggestions for future instrument design and the optimal retrieval strategy for current instruments. Generally, our results agree with previous validation efforts of POLDER and airborne prototypes of APS, but show that the MISR aerosol optical thickness uncertainty characterization is possibly underestimated.

Radiative transfer theory verified by controlled laboratory experiments

We report the results of high-accuracy controlled laboratory measurements of the Stokes reflection matrix for suspensions of submicrometer-sized latex particles in water and compare them with the results of a numerically exact computer solution of the vector radiative transfer equation (VRTE). The quantitative performance of the VRTE is monitored by increasing the volume packing density of the latex particles from 2% to 10%. Our results indicate that the VRTE can be applied safely to random particulate media with packing densities up to ∼2%. VRTE results for packing densities of the order of 5% should be taken with caution, whereas the polarized bidirectional reflectivity of suspensions with larger packing densities cannot be accurately predicted. We demonstrate that a simple modification of the phase matrix entering the VRTE based on the so-called static structure factor can be a promising remedy that deserves further examination.

Polarization impacts on the water-leaving radiance retrieval from above-water radiometric measurements

Above-water measurements of water-leaving radiance are widely used for water-quality monitoring and ocean-color satellite data validation. Reflected skylight in above-water radiometry needs to be accurately estimated prior to derivation of water-leaving radiance. Up-to-date methods to estimate reflection of diffuse skylight on rough sea surfaces are based on radiative transfer simulations and sky radiance measurements. But these methods neglect the polarization state of the incident skylight, which is generally highly polarized. In this paper, the effects of polarization on the sea surface reflectance and the subsequent water-leaving radiance estimation are investigated. We show that knowledge of the polarization field of the diffuse skylight significantly improves above-water radiometry estimates, in particular in the blue part of the spectrum where the reflected skylight is dominant. A newly developed algorithm based on radiative transfer simulations including polarization is described. Its application to the standard Aerosol Robotic Network-Ocean Color and hyperspectral radiometric measurements of the 1.5-year dataset acquired at the Long Island Sound site demonstrates the noticeable importance of considering polarization for water-leaving radiance estimation. In particular it is shown, based on time series of collocated data acquired in coastal waters, that the azimuth range of measurements leading to good-quality data is significantly increased, and that these estimates are improved by more than 12% at 413 nm. Full consideration of polarization effects is expected to significantly improve the quality of the field data utilized for satellite data validation or potential vicarious calibration purposes.

Polarimetric remote sensing of aerosols over land surfaces

Writing from the S.S. Narkunda, near Aden, [1921] that using a Nicol prism ‘serves to cut off a great deal of the blue atmospheric “haze” which usually envelops a distant view, and mostly consists of polarized light. ’ Although the reason for the color and polarization of the sky had been explained some time before by [1871], later Lord Rayleigh, and the neutral points, where the polarization of the sky becomes zero, had already been named after their discoverers Arago [Barral, 1858], [1840] and [1842], this simple observation of Raman’s was still considered noteworthy, because of the difference between the behavior of the object being observed and the haze. The reason for this difference is that light scattered by molecules and small aerosol is strongly polarized in a plane perpendicular to the scattering plane (the plane defined by the sun, the object being viewed and the observer) while light scattered by surfaces is only weakly polarized. Thus, when Raman oriented the polarizer to transmit light in the plane parallel to the scattering plane the contributions from light scattered by aerosols and molecules were suppressed while the lighthouse was made more visible (had more contrast). This difference between the polarizing properties of aerosols and molecules as compared to surfaces is used by modern polarimetric remote sensing instruments to determine the amount, size and type of aerosols that are present above the surface.

The relationship between upwelling underwater polarization and attenuation/absorption ratio

The attenuation coefficient of the water body is not directly retrievable from measurements of unpolarized water-leaving radiance. Based on extensive radiative transfer simulations using the vector radiative transfer code RayXP, it is demonstrated that the underwater degree of linear polarization (DoLP) is closely related to the attenuation-to-absorption ratio (c/a) of the water body, a finding that enables retrieval of the attenuation coefficient from measurements of the Stokes components of the upwelling underwater polarized light field. The relationship between DoLP and the c/a ratio is investigated for the upwelling polarized light field for a complete set of viewing geometries, at several wavelengths in the visible part of the spectrum; for varying compositions of the aquatic environment, whose constituents include phytoplankton, non-algal particles, and color dissolved organic matter (CDOM); and for varying microphysical properties such as the refractive index and the slope of the Junge-type particle size distribution (PSD). Consequently, this study reveals the possibility for retrieval of additional inherent optical properties (IOPs) from air- or space-borne DoLP measurements of the water-leaving radiation.