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A Laboratory Experiment for the Statistical Evaluation of Aerosol Retrieval (STEAR) Algorithms

1
NASA Langley Research Center, Hampton, VA 23681, USA
2
Department of Physics, University of Maryland Baltimore County, Baltimore, MD 21250, USA
3
NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA
4
California State University, San Bernardino, CA 92407, USA
5
Joint Center for Earth Systems Technology, University of Maryland Baltimore County, Baltimore, MD 21250, USA
6
Laboratoire d’Optique Atmosphérique, Université de Lille-1, CNRS, 59655 Villeneuve d’Ascq, France
7
GRASP-SAS, Remote sensing developments, LOA/Université de Lille-1, 59655 Villeneuve d’Ascq, France
*
Author to whom correspondence should be addressed.
Remote Sens. 2019, 11(5), 498; https://doi.org/10.3390/rs11050498
Received: 31 January 2019 / Revised: 19 February 2019 / Accepted: 23 February 2019 / Published: 1 March 2019
(This article belongs to the Special Issue Advances of Remote Sensing Inversion)
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Abstract

We have developed a method for evaluating the fidelity of the Aerosol Robotic Network (AERONET) retrieval algorithms by mimicking atmospheric extinction and radiance measurements in a laboratory experiment. This enables radiometric retrievals that use the same sampling volumes, relative humidities, and particle size ranges as observed by other in situ instrumentation in the experiment. We use three Cavity Attenuated Phase Shift (CAPS) monitors for extinction and University of Maryland Baltimore County’s (UMBC) three-wavelength Polarized Imaging Nephelometer (PI-Neph) for angular scattering measurements. We subsample the PI-Neph radiance measurements to angles that correspond to AERONET almucantar scans, with simulated solar zenith angles ranging from 50 to 77 . These measurements are then used as input to the Generalized Retrieval of Aerosol and Surface Properties (GRASP) algorithm, which retrieves size distributions, complex refractive indices, single-scatter albedos, and bistatic LiDAR ratios for the in situ samples. We obtained retrievals with residuals less than 8% for about 90 samples. Samples were alternately dried or humidified, and size distributions were limited to diameters of less than 1.0 or 2.5 μ m by using a cyclone. The single-scatter albedo at 532 nm for these samples ranged from 0.59 to 1.00 when computed with CAPS extinction and Particle Soot Absorption Photometer (PSAP) absorption measurements. The GRASP retrieval provided single-scatter albedos that are highly correlated with the in situ single-scatter albedos, and the correlation coefficients ranged from 0.916 to 0.976, depending upon the simulated solar zenith angle. The GRASP single-scatter albedos exhibited an average absolute bias of +0.023–0.026 with respect to the extinction and absorption measurements for the entire dataset. We also compared the GRASP size distributions to aerodynamic particle size measurements, using densities and aerodynamic shape factors that produce extinctions consistent with our CAPS measurements. The GRASP effective radii are highly correlated (R = 0.80) and biased under the corrected aerodynamic effective radii by 1.3% (for a simulated solar zenith angle of θ = 50 ); the effective variance indicated a correlation of R = 0.51 and a relative bias of 280%. Finally, our apparatus was not capable of measuring backscatter LiDAR ratios, so we measured bistatic LiDAR ratios at a scattering angle of 173 degrees. The GRASP bistatic LiDAR ratios had correlations of 0.71 to 0.86 (depending upon simulated θ ) with respect to in situ measurements, positive relative biases of 2–10%, and average absolute biases of 1.8–7.9 sr. View Full-Text
Keywords: aerosols; aerosol retrievals; AERONET; GRASP aerosols; aerosol retrievals; AERONET; GRASP
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This is an open access article distributed under the Creative Commons Attribution License which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited (CC BY 4.0).
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Schuster, G.L.; Espinosa, W.R.; Ziemba, L.D.; Beyersdorf, A.J.; Rocha-Lima, A.; Anderson, B.E.; Martins, J.V.; Dubovik, O.; Ducos, F.; Fuertes, D.; Lapyonok, T.; Shook, M.; Derimian, Y.; Moore, R.H. A Laboratory Experiment for the Statistical Evaluation of Aerosol Retrieval (STEAR) Algorithms. Remote Sens. 2019, 11, 498.

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