Retrieval of aerosol properties from Airborne Hyper Angular Rainbow Polarimeter (AirHARP) observations during ACEPOL 2017

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 15 instrument with the hyper-angular imaging capability of 60 along-track viewing angles at 670 nm, and 20 along-track viewing angles at other wavelengths 440, 550, 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 volume 20 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). AOD 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 correlation with HSRL2 ( = 0.940, | | = 0.062) and AERONET AOD (0.013 ≤ Mean Absolute Error ( ) ≤ 25 0.017, 0.013 ≤ | | ≤ 0.017) measurements is observed for the collocated points. Forest fire smoke intercepted during ACEPOL provided a situation with 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, 0.81 at wavelengths of 440 nm, 550 nm, 670 nm, and 870 nm, respectively. Finer particles with an average AE of 1.53, volume median radius of 0.157 µm and standard deviation of 0.55 µm for fine mode is observed for the same smoke plume. These results serve as a 30 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.

The Aerosol Characterization from Polarimeter and Lidar (ACEPOL) airborne field campaign

“…These HSRL-2 measurements are used to compute AOD as well as the aerosol extinction / backscatter ratio ("lidar ratio") at 355 and 532 nm. Rogers et al (2009) evaluated the HSRL extinction coefficient profiles and found that the HSRL extinction profiles are within the systematic errors of airborne in situ measurements at visible wavelengths. Derived products include, first, estimates of planetary boundary layer heights which use the vertically resolved profile measurements of aerosol backscatter to derive aerosol mixed-layer heights during the daytime (Scarino et al, 2014).…”

A Comprehensive Description of Multi-Term LSM for Applying Multiple a Priori Constraints in Problems of Atmospheric Remote Sensing: GRASP Algorithm, Concept, and Applications

“…Frontiers in Remote Sensing | www.frontiersin.org October 2021 | Volume 2 | Article 706851 Espinosa et al, 2019 W. R.;Puthukkudy et al, 2017;Schuster et al, 2019, etc.). A demonstration of one such retrieval study using measurements of polystyrene (PSL) spheres from Espinosa et al ( 2017) is shown in Figure 7.…”

“…There are also some efforts to adapt GRASP for processing the observations by the future DPC, Aerosol-UA, HARP2 missions. For example, Figure 17 illustrates the application of the GRASP algorithm for processing AirHARP (airborne Hyper Angular Rainbow Polarimeter) observations made during the NASA ACEPOL 2017 campaign (Puthukkudy et al, 2020a). GRASP retrieved AOD using the AirHARP data is validated using the collocated AERONET and HSRL2 measurements.…”

The Aerosol Characterization from Polarimeter and Lidar (ACEPOL) airborne field campaign

“…The figure also shows an example of data collected during the ACEPOL campaign, its second deployment. The instrument's maiden field campaign was the Lake Michigan Ozone Study (LMOS) in 2017(McBride et al, 2019. For ACEPOL, AirHARP collected data over defined targets, as the processing speed of the onboard data acquisition system precluded continuous data collection.…”

“…For level-2 processing (Fu et al, 2019), the SPEX airborne team has focused on aerosol retrievals, building further on the SRON aerosol retrieval algorithm previously used for POLDER-3 processing (Hasekamp et al, 2011, Lacagnina et al, 20162017) and groundSPEX (van Harten et al, 2014;di Noia et al, 2014). The algorithm has been extended from a bi-modal retrieval scheme to a multi-mode retrieval scheme for an arbitrary number of modes (Fu and Hasekamp, 2018).…”