Retrieval of aerosol properties from Airborne Hyper AngularRainbow 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 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 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 (𝜌 = 0.940, |𝐵𝐼𝐴𝑆| = 0.062) and AERONET AOD (0.013 ≤ Mean Absolute Error (𝑀𝐴𝐸) ≤ 0.017, 0.013 ≤ |𝐵𝐼𝐴𝑆| ≤ 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 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.