Retrieving Aerosol Characteristics From the PACE Mission, Part 2: Multi-Angle and Polarimetry

Abstract

The Plankton, Aerosol, Clouds, ocean Ecosystem (PACE) mission presents new opportunities and new challenges in applying observations of two complementary multi-angle polarimeters for the space-based retrieval of global aerosol properties. Aerosol remote sensing from multi-angle radiometric-only observations enables aerosol characterization to a greater degree than single-view radiometers, as demonstrated by nearly two decades of heritage instruments. Adding polarimetry to the multi-angle observations allows for the retrieval of aerosol optical depth, Angstrom exponent, parameters of size distribution, measures of aerosol absorption, complex refractive index and degree of non-sphericity of the particles, as demonstrated by two independent retrieval algorithms applied to the heritage POLarization and Directionality of the Earth's Reflectance (POLDER) instrument. The reason why this detailed particle characterization is possible is because a multi-angle polarimeter measurement contains twice the number of Degrees of Freedom of Signal (DFS) compared to an observation from a single-view radiometer. The challenges of making use of this information content involve separating surface signal from atmospheric signal, especially when the surface is optically complex and especially in the ultraviolet portion of the spectrum where we show the necessity of polarization in making that separation. The path forward is likely to involve joint retrievals that will simultaneously retrieve aerosol and surface properties, although advances will be required in radiative transfer modeling and in representing optically complex constituents in those models. Another challenge is in having the processing capability that can keep Remer et al. Aerosol Characterization With PACE Polarimeters pace with the output of these instruments in an operational environment. Yet, preliminary algorithms applied to airborne multi-angle polarimeter observations offer encouraging results that demonstrate the advantages of these instruments to retrieve aerosol layer height, particle single scattering albedo, size distribution and spectral optical depth, and also show the necessity of polarization measurements, not just multi-angle radiometric measurements, to achieve these results.

A universal smartphone add-on for portable spectroscopy and polarimetry: iSPEX 2

“…In dual-beam mode, the modulations in I + (λ) and I − (λ) are exactly opposite for a uniform target, so the total radiance and modulation can be disentangled as shown in Eqs. (5) and (6). However, this is complicated in practice due to imperfections in the optical elements, misalignments, differences in transmission between the two beams, and nonnormal incidence.…”

Spatial distribution of cloud droplet size properties from Airborne Hyper-Angular Rainbow Polarimeter (AirHARP) measurements

“…Multi-angle polarimetric measurements have other advantages for cloud characterization beyond the retrieval of the two DSD parameters. We find that retrievals of cloud thermodynamic phase (Riedi et al, 2010;Goloub et al, 2000), ice crystal asymmetry (van Diedenhoven et al, 2013), aerosol above cloud (Waquet et al, 2013), and COT (Xu et al, 2018;Cornet et al, 2018) are considerably improved with the addition of polarized observations. At the time of this writing, only the Polarization and Directionality of the Earth's Reflectances (POLDER; Deschamps et al, 1994) instruments have demonstrated the polarized retrieval of cloud DSD properties from space, though several aircraft instruments, including the Airborne Multi-angle SpectroPolarimetric Imager (AirMSPI;Diner et al, 2013), the Research Scanning Polarimeter (RSP; Cairns et al, 1999), and the subject of this paper, the Airborne Hyper-Angular Rainbow Polarimeter (AirHARP; Martins et al, 2018), have demonstrated im- proved sampling schemes, resolution, and accuracy van Harten et al, 2018).…”

Retrieving Aerosol Characteristics From the PACE Mission, Part 1: Ocean Color Instrument

“…Focusing on passive remote sensing from radiometers, algorithms have extracted information from spectral measurements in the visible and shortwave infrared (SWIR) portions of the spectrum (i.e., MODIS), from spectral measurements in the ultraviolet (UV) portion of the spectrum (i.e., OMI and the earlier Total Ozone Mapping Spectrometer (TOMS) instrument from which OMI draws its heritage), and from multi-angular measurements in the visible and near-infrared (NIR) [i.e., MISR, Advanced Along Track Scanning Radiometer (AATSR) and POLDER]. Algorithms have also extracted information from multi-angular measurements of polarized reflectance in the visible (POLDER), but discussion of polarimetry is outside the scope of this review and will be covered in a companion paper by Remer et al (2019), hereafter referred to as "Part 2." The different information acquired by these different satellite sensors has enabled the community to produce reliable measures of total column ambient aerosol loading (aerosol optical depth -AOD) over a variety of land types and over the ocean (Boucher et al, 2013;Myhre et al, 2013), and even recently AOD in the column above clouds (Torres et al, 2012;Jethva et al, 2013Jethva et al, , 2014aJethva et al, ,b, 2016Meyer and Platnick, 2015;Sayer et al, 2016).…”

“…For retrieving these quantitative particle properties from space even more information is needed, such as with a multi-wavelength, multi-angle polarimeter of sufficient accuracy, wavelength range and resolution (Mishchenko et al, 2004;Cairns et al, 2010;Knobelspiesse et al, 2012;Kokhanovsky, 2015). Such instruments will be discussed more fully in Part 2 (Remer et al, 2019).…”

“…In Section "Advances Beyond Heritage, 2: Oxygen A-band Information Content on Aerosol Profiling, " we also broach the possibility of improved aerosol profile retrievals from the combination of multiangle observations in the O 2 A-band. Part 2 of this study (Remer et al, 2019) explores this in more detail. Finally in Section "Discussion and the Path Forward, " we discuss the results, identify the gaps that still exist before OCI's full potential as an aerosol instrument can be realized, and provide suggestions for the path forward.…”