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. A validation study is conducted regarding aerosol optical size property retrievals from measurementsof 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 totalcolumn aerosol size distributions,assumed to be bimodal lognormal.In addition, a set of secondary integral parameters of aerosol size distribution andoptical 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 acquiredover 20 years (1997–2016)at 30 AERONET (Aerosol Robotic Network) sites.These validation sites have been selectedbased on known availability of an extensive data record,significant aerosol load variability throughout the year, wide worldwide coverage anddiverse 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 comparedto those retrieved by the spectral deconvolution algorithm (SDA) andby the AERONET aerosol retrieval algorithm.The size distribution properties obtained by the GRASP-AOD arecompared to their equivalent values from the AERONET aerosol retrieval algorithm.The analysis showed the convincing capacity of the GRASP-AOD approach tosuccessfully discriminate between fine- and coarse-mode extinctionto robustly retrieve τf(500).The comparisons of 2 million results of τf(500) retrieval by theGRASP-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 theAERONET aerosol retrieval algorithm agree slightly betterwith 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 showedagreement for the fine-mode median radius between the GRASP-AOD andAERONET aerosol retrieval algorithm resultswith an RMSE of 0.032 µm (or 18.7 % in relative terms) for the situationswhen τ(440)&gt;0.2 occur for more than 80 000 pairs of the study.For the cases where the fine mode is dominant (i.e., α&gt;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 ofthe coarse-mode volume median radius for different aerosol types observed at different locations.Nonetheless the GRASP-AOD retrieval provides reasonable agreement with theAERONET aerosol retrieval algorithm for overall coarse-mode properties withwith RMSE = 0.500 µm (RMSRE = 20 %) when τ(440)&gt;0.2.The values of effective radius and total volume concentrationcomputed from the GRASP-AOD retrieval have beencompared to those estimated by the AERONET aerosol retrieval algorithm.The RMSE values of the correlations were 30 % for the effective radius and25 % for the total volume concentration when τ(440)&gt;0.2.Finally, the study discussesthe 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.

atmospheric measurement techniques

Characterization of aerosol size properties from measurements of spectral optical depth: a global validation of the GRASP-AOD code using long-term AERONET data