Abstract. Proven by multiple theoretical and practical studies,
multi-angular spectral polarimetry is ideal for comprehensive retrieval of
properties of aerosols. Furthermore, a large number of advanced space
polarimeters have been launched recently or planned to be deployed in the
coming few years (Dubovik et al.,
2019). Nevertheless, at present, practical utilization of aerosol products
from polarimetry is rather limited, due to the relatively small number of
polarimetric compared to photometric observations, as well as challenges in
making full use of the extensive information content available in these
complex observations. Indeed, while in recent years several new algorithms
have been developed to provide enhanced aerosol retrievals from satellite
polarimetry, the practical value of available aerosol products from
polarimeters yet remains to be proven. In this regard, this paper presents
the analysis of aerosol products obtained by the Generalized Retrieval of
Atmosphere and Surface Properties (GRASP) algorithm from POLDER/PARASOL
observations. After about a decade of development, GRASP has been adapted
for operational processing of polarimetric satellite observations and
several aerosol products from POLDER/PARASOL observations have been
released. These updated PARASOL/GRASP products are publicly available (e.g.,
http://www.icare.univ-lille.fr, last access: 16 October 2018, http://www.grasp-open.com/products/, last access: 28 March 2020); the dataset used in the current study is
registered under https://doi.org/10.5281/zenodo.3887265 (Chen
et al., 2020). The objective of this study is to comprehensively evaluate the GRASP aerosol
products obtained from POLDER/PARASOL observations. First, the validation of
the entire 2005–2013 archive was conducted by comparing to ground-based
Aerosol Robotic Network (AERONET) data. The subjects of the validation are
spectral aerosol optical depth (AOD), aerosol absorption optical depth
(AAOD) and single-scattering albedo (SSA) at six wavelengths, as well as
Ångström exponent (AE), fine-mode AOD (AODF) and coarse-mode AOD
(AODC) interpolated to the reference wavelength 550 nm. Second, an
inter-comparison of PARASOL/GRASP products with the PARASOL/Operational,
MODIS Dark Target (DT), Deep Blue (DB) and Multi-Angle Implementation of
Atmospheric Correction (MAIAC) aerosol products for the year 2008 was
performed. Over land both satellite data validations and inter-comparisons
were conducted separately for different surface types, discriminated by bins
of normalized difference vegetation index (NDVI): < 0.2, 0.2 ≤
and < 0.4, 0.4 ≤ and < 0.6, and ≥ 0.6. Three
PARASOL/GRASP products were analyzed: GRASP/HP (“High Precision”),
Optimized and Models. These different products are consistent but were
obtained using different assumptions in aerosol modeling with different
accuracies of atmospheric radiative transfer (RT) calculations.
Specifically, when using GRASP/HP or Optimized there is direct retrieval of
the aerosol size distribution and spectral complex index of refraction. When
using GRASP/Models, the aerosol is approximated by a mixture of several
prescribed aerosol components, each with their own fixed size distribution
and optical properties, and only the concentrations of those components are
retrieved. GRASP/HP employs the most accurate RT calculations, while
GRASP/Optimized and GRASP/Models are optimized to achieve the best trade-off
between accuracy and speed. In all these three options, the underlying
surface reflectance is retrieved simultaneously with the aerosol properties,
and the radiative transfer calculations are performed “online” during the
retrieval. All validation results obtained for the full archive of PARASOL/GRASP
products show solid quality of retrieved aerosol characteristics. The
GRASP/Models retrievals, however, provided the most solid AOD products, e.g.,
AOD (550 nm) is unbiased and has the highest correlation (R ∼ 0.92) and the highest fraction of retrievals (∼ 55.3 %)
satisfying the accuracy requirements of the Global Climate Observing System
(GCOS) when compared to AERONET observations. GRASP/HP and GRASP/Optimized
AOD products show a non-negligible positive bias (∼ 0.07) when
AOD is low (< 0.2). On the other hand, the detailed aerosol
microphysical characteristics (AE, AODF, AODC, SSA, etc.) provided by
GRASP/HP and GRASP/Optimized correlate generally better with AERONET than do
the results of GRASP/Models. Overall, GRASP/HP processing demonstrates the
high quality of microphysical characteristics retrieval versus AERONET.
Evidently, the GRASP/Models approach is more adapted for retrieval of total AOD,
while the detailed aerosol microphysical properties are limited when a
mixture of aerosol models with fixed optical properties are used. The results of a comparative analysis of PARASOL/GRASP and MODIS products
showed that, based on validation against AERONET, the PARASOL/GRASP AOD (550 nm) product is of similar and sometimes of higher quality compared to the
MODIS products. All AOD retrievals are more accurate and in good agreement
over ocean. Over land, especially over bright surfaces, the retrieval
quality degrades and the differences in total AOD products increase. The
detailed aerosol characteristics, such as AE, AODF and AODC from
PARASOL/GRASP, are generally more reliable, especially over land. The global
inter-comparisons of PARASOL/GRASP versus MODIS showed rather robust
agreement, though some patterns and tendencies were observed. Over ocean,
PARASOL/Models and MODIS/DT AOD agree well with the correlation coefficient
of 0.92. Over land, the correlation between PARASOL/Models and the different
MODIS products is lower, ranging from 0.76 to 0.85. There is no significant
global offset; though over bright surfaces MODIS products tend to show
higher values compared to PARASOL/Models when AOD is low and smaller values
for moderate and high AODs. Seasonal AOD means suggest that PARASOL/GRASP
products show more biomass burning aerosol loading in central Africa and
dust over the Taklamakan Desert, but less AOD in the northern Sahara. It is
noticeable also that the correlation for the data over AERONET sites are
somewhat higher, suggesting that the retrieval assumptions generally work
better over AERONET sites than over the rest of the globe. One of the
potential reasons may be that MODIS retrievals, in general, rely more on
AERONET climatology than GRASP retrievals. Overall, the analysis shows that the quality of AOD retrieval from
multi-angular polarimetric observations like POLDER is at least comparable
to that of single-viewing MODIS-like imagers. At the same time, the
multi-angular polarimetric observations provide more information on other
aerosol properties (e.g., spectral AODF, AODC, AE), as well as additional
parameters such as AAOD and SSA.
