Abstract
Abstract. Multi-angle polarimetric (MAP) imaging of Earth scenes can be used for the retrieval of microphysical and optical parameters of aerosols and clouds. The Airborne Hyper-Angular Rainbow Polarimeter (AirHARP) is an aircraft MAP instrument with a hyper-angular imaging capability of 60 along-track viewing angles at 670 nm and 20 along-track viewing angles at other wavelengths – 440, 550, and 870 nm – across the full 114∘ (94∘) along-track (cross-track) field of view. Here we report the retrieval of aerosol properties using the Generalized Retrieval of Aerosols and Surface Properties (GRASP) algorithm applied to AirHARP observations collected during the NASA Aerosol Characterization from Polarimeter and Lidar (ACEPOL) campaign in October–November 2017. The retrieved aerosol properties include spherical fraction (SF), aerosol column concentration in multiple size distribution modes, and, with sufficient aerosol loading, complex aerosol refractive index. From these primary retrievals, we derive aerosol optical depth (AOD), Angstrom exponent (AE), and single scattering albedo (SSA). AODs retrieved from AirHARP measurements are compared with the High Spectral Resolution LiDAR-2 (HSRL2) AOD measurements at 532 nm and validated with measurements from collocated Aerosol Robotic NETwork (AERONET) stations. A good agreement with HSRL2 (ρ=0.940, |BIAS|=0.062, mean absolute error (MAE) = 0.122) and AERONET AOD (0.010≤MAE≤0.015, 0.002≤|BIAS|≤0.009) measurements is observed for the collocated points. There was a mismatch between the HSRL2- and AirHARP-retrieved AOD for the pixels close to the forest fire smoke source and to the edges of the plume due to spatial mismatch in the sampling. This resulted in a higher BIAS and MAE for the HSRL2 AOD comparison. For the case of AERONET AOD comparison, two different approaches are used in the GRASP retrievals, and the simplified aerosol component-based GRASP/Models kernel which retrieves fewer number of aerosol parameter performed well compared to a more generous GRASP/Five mode approach in the low aerosol loading cases. Forest fire smoke intercepted during ACEPOL provided a situation with homogenous plume and sufficient aerosol loading to retrieve the real part of the refractive index (RRI) of 1.55 and the imaginary part of the refractive index (IRI) of 0.024. The derived SSAs for this case are 0.87, 0.86, 0.84, and 0.81 at wavelengths of 440, 550, 670, and 870 nm, respectively. Finer particles with an average AE of 1.53, a volume median radius of 0.157 µm, and a standard deviation (SD) of 0.55 for fine mode is observed for the same smoke plume. These results serve as a proxy for the scale and detail of aerosol retrievals that are anticipated from future space mission data, as HARP CubeSat (mission begins 2020) and HARP2 (aboard the NASA PACE mission with launch in 2023) are near duplicates of AirHARP and are expected to provide the same level of aerosol characterization.
Highlights
Aerosols play an important role in Earth’s climate (Boucher et al, 2013; Hobbs, 1993; Kaufman et al, 2002; Koren et al, 2004): they directly perturb Earth’s radiation budget and indirectly modify cloud properties, which in turn influences the planet’s energy and hydrological budgets (Lenoble et al, 2010; Penner et al, 2001)
Airborne Hyper-Angular Rainbow Polarimeter (AirHARP) polarimetric measurements, taken at the high angular and spatial resolution over a wide swath, combined with the Generalized Retrieval of Aerosols and Surface Properties (GRASP) algorithm allow for unprecedented spatial mapping of aerosol properties that are consistent with co-incident instrument retrievals
This wide swath will help in capturing more aerosol events globally compared to a narrow-swath multi-angle polarimeter when the Hyper-Angular Rainbow Polarimeter (HARP) concept is applied to space sensors
Summary
Aerosols play an important role in Earth’s climate (Boucher et al, 2013; Hobbs, 1993; Kaufman et al, 2002; Koren et al, 2004): they directly perturb Earth’s radiation budget and indirectly modify cloud properties, which in turn influences the planet’s energy and hydrological budgets (Lenoble et al, 2010; Penner et al, 2001). Which include the SRON multi-mode inversion algorithm for SPEX airborne (Fu et al, 2020; Fu and Hasekamp, 2018); Microphysical Aerosol Property from Polarimeters (MAPP) (Stamnes et al, 2018) and GISS/RSP algorithm (Knobelspiesse et al, 2011; Waquet et al, 2009) for RSP; and correlated multi-pixel and joint retrieval algorithm for AirMSPI developed at Jet Propulsion Laboratory (JPL) (Xu et al, 2017, 2019) This list is not complete, and for a comprehensive review of the polarimetric remote sensing of atmospheric aerosols based on MAPs, we encourage the readers to refer to several reviews in the literature (Dubovik et al, 2019; Kokhanovsky et al, 2010, 2015; Remer et al, 2019). One details the land and ocean surface models that are essential to the GRASP inversion of aerosol, and the second one describes the calculation of aerosol optical depth from retrieved aerosol particles
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