Sensitivity of multiangle, multispectral polarimetric remote sensing over open oceans to water-leaving radiance: Analyses of RSP data acquired during the MILAGRO campaign Jacek Chowdhary a, b, ⁎, Brian Cairns a, b , Fabien Waquet c , Kirk Knobelspiesse a, b , Matteo Ottaviani b , Jens Redemann d , Larry Travis b , Michael Mishchenko b a Department of Applied Physics and Applied Mathematics, Columbia University, 2880 Broadway, New York, NY 10025 USA b NASA Goddard Institute for Space Studies, 2880 Broadway, New York, NY 10025, USA c Laboratoire d'Optique Atmosphérique, Université des Sciences et Technologies de Lille, Villeneuve-d'Ascq, France d BAERI/NASA Ames Research Center, Moffett Field, CA 94035, USA abstract article info Article history: Received 22 June 2010 Received in revised form 18 October 2011 Accepted 23 November 2011 Available online 5 January 2012 Keywords: Remote sensing Polarization RSP MILAGRO Aerosol Hydrosol Ocean color Case-1 waters Bio-optics Chlorophyll a CDOM Plankton Scattering Radiative transfer For remote sensing of aerosol over the ocean, there is a contribution from light scattered under water. The bright- ness and spectrum of this light depends on the biomass content of the ocean, such that variations in the color of the ocean can be observed even from space. Rayleigh scattering by pure sea water, and Rayleigh–Gans type scattering by plankton, causes this light to be polarized with a distinctive angular distribution. To study the contribution of this underwater light polarization to multiangle, multispectral observations of polarized reflectance over ocean, we previously developed a hydrosol model for use in underwater light scattering computations that produces realistic variations of the ocean color and the underwater light polarization signature of pure sea water. In this work we review this hydrosol model, include a cor- rection for the spectrum of the particulate scattering coefficient and backscattering efficiency, and discuss its sensitivity to variations in colored dissolved organic matter (CDOM) and in the scattering function of marine particulates. We then apply this model to measurements of total and polarized reflectance that were acquired over open ocean during the MILAGRO field campaign by the airborne Research Scanning Polarimeter (RSP). Analyses show that our hydrosol model faithfully reproduces the water-leaving con- tributions to RSP reflectance, and that the sensitivity of these contributions to Chlorophyll a concentra- tion [Chl] in the ocean varies with the azimuth, height, and wavelength of observations. We also show that the impact of variations in CDOM on the polarized reflectance observed by the RSP at low altitude is comparable to or much less than the standard error of this reflectance whereas their effects in total re- flectance may be substantial (i.e. up to >30%). Finally, we extend our study of polarized reflectance var- iations with [Chl] and CDOM to include results for simulated spaceborne observations. © 2011 Elsevier Inc. All rights reserved. 1. Introduction The polarized intensity of light scattered by particles exhibits fea- tures as a function of wavelength and scattering angle that are distinctly different from those of the total scattered intensity (Coulson, 1988; Ulaby & Elachi, 1990; Videen et al., 2004). The polarized and total inten- sity features also exhibit very different sensitivities to particle proper- ties such as size relative to the wavelength, shape, and composition (Hansen & Travis, 1974; Mishchenko et al., 2002). Furthermore, both sensitivities vary with particle properties. For example, the polarized component of light scattered by particles having a refractive index close to that of the surrounding medium, such as particulates suspended in the ocean, shows fewer features with scattering angle and less varia- tion with shape and size than the corresponding features of light scat- tered by particles that have a strong refractive-index contrast with the surrounding medium such as atmospheric aerosols. Finally, the features in polarized intensity of singly scattered light are much less likely to be washed out by those of multiply scattered light than is the case for the features in total intensity (Hansen & Travis, 1974; Hovenier et al., 2004; Mishchenko et al., 2006; van de Hulst, 1980). This is because the magnitude of polarized intensity of light scattered n times decreases rapidly with n as compared with the magnitude of total intensity. These differences cause the retrieval of aerosol properties from remotely sensed polarization to be much more accurate than the corresponding retrievals from remotely sensed intensity, as has been demonstrated in theoretical studies (Hasekamp & Landgraf, 2005; Mishchenko & Travis, 1997a, 1997b). Analyses of actual polarimetric remote sensing data obtained by the Polarization and Directionality of the Earth's Remote Sensing of Environment 118 (2012) 284–308 ⁎ Corresponding author. Tel.: + 1 212 678 5643; fax: + 1 212 678 5552. E-mail address: jacek.chowdhary@nasa.gov (J. Chowdhary). 0034-4257/$ – see front matter © 2011 Elsevier Inc. All rights reserved. doi:10.1016/j.rse.2011.11.003 Contents lists available at SciVerse ScienceDirect Remote Sensing of Environment journal homepage: www.elsevier.com/locate/rse