Characterization of aerosol size properties from measurements of spectral optical depth: a global validation of the GRASP-AOD code using long-term AERONET data
Authors: Torres Benjamin , Fuertes David 
Published: June 2021

Abstract. A validation study is conducted regarding aerosol optical size property retrievals from measurements
of the direct sun beam only (without the aid of diffuse radiation).
The study focuses on using real data to test the new GRASP-AOD application,
which uses only spectral optical depth measurements to retrieve the total
column aerosol size distributions,
assumed to be bimodal lognormal.
In addition, a set of secondary integral parameters of aerosol size distribution and
optical properties are provided:
effective radius, total volume concentration and fine-mode fraction of aerosol optical depth (AOD).
The GRASP-AOD code is applied to almost 3 million observations acquired
over 20 years (1997–2016)
at 30 AERONET (Aerosol Robotic Network) sites.
These validation sites have been selected
based on known availability of an extensive data record,
significant aerosol load variability throughout the year, wide worldwide coverage and
diverse aerosol types and source regions.
The output parameters are compared to those coming from the operational AERONET retrievals.
The retrieved fine-mode fractions at 500 nm (τf(500))
obtained by the GRASP-AOD application are compared
to those retrieved by the spectral deconvolution algorithm (SDA) and
by the AERONET aerosol retrieval algorithm.
The size distribution properties obtained by the GRASP-AOD are
compared to their equivalent values from the AERONET aerosol retrieval algorithm.
The analysis showed the convincing capacity of the GRASP-AOD approach to
successfully discriminate between fine- and coarse-mode extinction
to robustly retrieve τf(500).
The comparisons of 2 million results of τf(500) retrieval by the
GRASP-AOD and SDA showed high correlation with a root mean square error (RMSE) of 0.015.
Also, the analysis showed that the τf(500) values computed by the
AERONET aerosol retrieval algorithm agree slightly better
with the GRASP-AOD (RMSE = 0.018, from 148 526 comparisons)
than with the SDA (RMSE = 0.022, from 127 203 comparisons).
The comparisons of the size distribution retrieval showed
agreement for the fine-mode median radius between the GRASP-AOD and
AERONET aerosol retrieval algorithm results
with an RMSE of 0.032 µm (or 18.7 % in relative terms) for the situations
when τ(440)>0.2 occur for more than 80 000 pairs of the study.
For the cases where the fine mode is dominant (i.e., α>1.2),
the RMSE is only of 0.023 µm (or 13.9 % in relative terms).
Major limitations in the retrieval were found for the characterization of the coarse-mode details.
For example, the analysis revealed that the GRASP-AOD retrieval is not sensitive to the small variations of
the coarse-mode volume median radius for different aerosol types observed at different locations.
Nonetheless the GRASP-AOD retrieval provides reasonable agreement with the
AERONET aerosol retrieval algorithm for overall coarse-mode properties with
with RMSE = 0.500 µm (RMSRE = 20 %) when τ(440)>0.2.
The values of effective radius and total volume concentration
computed from the GRASP-AOD retrieval have been
compared to those estimated by the AERONET aerosol retrieval algorithm.
The RMSE values of the correlations were 30 % for the effective radius and
25 % for the total volume concentration when τ(440)>0.2.
Finally, the study discusses
the importance of employing the assumption of bimodal lognormal size distribution.
It also evaluates the potential of using ancillary data, in particular aureole measurements,
for improving the characterization of the aerosol coarse-mode properties.