The new MISR research aerosol retrieval algorithm: a multi-angle, multi-spectral, bounded-variable least squares retrieval of aerosol particle properties over both land and water
Authors: Limbacher James A. , Kahn Ralph A. , Lee Jaehwa 
Published: November 2022

Abstract. Launched in December 1999, NASA's Multi-angle Imaging
SpectroRadiometer (MISR) has given researchers the ability to observe the
Earth from nine different views for the last 22 years. Among the many
advancements that have since resulted from the launch of MISR is progress in
the retrieval of aerosols from passive space-based remote sensing. The MISR
operational standard aerosol (SA) retrieval algorithm has been refined
several times over the last 20 years, resulting in significant
improvements to spatial resolution (now 4.4 km) and aerosol particle
properties. However, the MISR SA still suffers from large biases in
retrieved aerosol optical depth (AOD) as aerosol loading increases. Here, we
present a new MISR research aerosol (RA) retrieval algorithm that utilizes
over-land surface reflectance data from the Multi-Angle Implementation of
Atmospheric Correction (MAIAC) to address these biases. This new
over-land and over-water algorithm produces a self-consistent aerosol and surface
retrieval when aerosol loading is low (AOD <0.75); this is combined
with a prescribed surface algorithm using a bounded-variable least squares
solver when aerosol loading is elevated (AOD >1.5). The two
algorithms (prescribed + retrieved surface) are then merged as part of our
combined surface retrieval algorithm. Results are compared with AErosol
RObotic NETwork (AERONET) validation sun-photometer direct-sun + almucantar inversion retrievals. Over land, with AERONET AOD (550 nm) direct-sun observations as the
standard, the root mean squared error (RMSE) of the MISR RA combined
retrieval (n=11563) is 0.084, with a correlation coefficient (r) of 0.935
and expected error of ±(0.20×[MISRAOD]+0.02). For MISR RA retrieved AOD >0.5 (n=664), we report an Ångström
exponent (ANG) RMSE of ∼0.35, with a correlation coefficient
of 0.844. Retrievals of ANG, fine-mode fraction (FMF), and single-scattering
albedo (SSA) improve as retrieved AOD increases. For AOD >1.5
(n=66), FMF RMSE is <0.09 with correlation >0.95, and
SSA RMSE is 0.015 with a correlation coefficient of ∼0.75. Over water, comparing AERONET AOD to the MISR RA combined retrieval
(n=4596), MISR RA RMSE is 0.063 and r is 0.935, with an expected error of
±(0.15×[MISRAOD]+0.02). ANG sensitivity is excellent when MISR
RA reported AOD >0.5 (n=188), with an RMSE of 0.27 and
r=0.89. Due to a lack of coincidences with AOD >1 (n=21),
our conclusions about MISR RA high-AOD particle property retrievals over
water are less robust (FMF RMSE =0.155 and r=0.94, whereas SSA
RMSE =0.010 and r=0.50). In general, better aerosol particle property constraints can be made at
lower AOD over water compared to our over-land retrievals. It is clear from
the results presented that the new MISR RA has quantitative sensitivity to
FMF and SSA (and qualitative sensitivity to non-sphericity) when retrieved
AOD exceeds 1, with qualitative sensitivity to aerosol type at lower AOD,
while also eliminating the AOD bias found in the MISR SA at higher AODs.
These results also demonstrate the advantage of using a prescribed surface
when aerosol loading is elevated.