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

Abstract

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.

Retrieval of aerosol properties using relative radiance measurements from an all-sky camera

“…We also intend to quantify the uncertainty of the retrieved properties. To this end, and following the methodology of Torres et al (2017), seven different aerosol models (types), obtained from the climatology reported by Dubovik et al (2002), are chosen. These models correspond to the next sites: Goddard Space Flight Center (GSFC; Maryland, USA), Mexico city (MEXI; Mexico), African Savanna (ZAMB; Zambia), Solar Village (SOLV; Saudi Arabia), Bahrain (BAHR; Persian Gulf), Lanai (LANA; Hawaii, USA), INDOEX (MALD; Maldives).…”

“…A new inversion algorithm that allows for a similar retrieval of aerosol properties is the GRASP code (Generalized Retrieval of Atmosphere and Surface Properties 2 ; Dubovik et al, 2014). This algorithm is capable of retrieving aerosol properties by inverting sun-sky photometer measurements, but its versatility also allows for inversion of other kinds of measurements like lidar/ceilometer signal plus AOD and sky radiances (Lopatin et al, 2013;Benavent-Oltra et al, 2017;Román et al, 2018;Titos et al, 2019;Herreras et al, 2019;Molero et al, 2020;Tsekeri et al, 2017), satellite measurements (Chen et al, 2020;Wei et al, 2021), nephelometer data (Espinosa et al, 2017), and even only AOD measurements (Torres et al, 2017).…”

A Comprehensive Description of Multi-Term LSM for Applying Multiple a Priori Constraints in Problems of Atmospheric Remote Sensing: GRASP Algorithm, Concept, and Applications

“…The upper panel of Figure 8 illustrates the size distribution by GRASP-AOD from AOD observations by lunar-photometers. The lower panel of Figure 9 illustrates the improvements in the aerosol retrieval by inverting radiances measured in solar aureole together with AOD data compare to inversion of AOD data only (Torres and Fuertes, 2020).…”

Estimation of Aerosol Columnar Size Distribution from Spectral Extinction Data in Coastal and Maritime Environment

“…The common element which links together all AOD-based estimations of SD [8][9][10][11][12] is that they contain an AOD fitting component, which is a comparison of AODs measured at several wavelengths with modeled AODs calculated from estimated SDs. Although there are several ways for performing the AOD fitting, we outline only one of them in this section.…”

“…It has been argued for more than four decades that it is possible to estimate the SD reasonably well from AODs measured at several wavelengths [8][9][10][11][12]; however, a complete practical solution still escapes us. Such a possibility has been illustrated, for example, by Sayer et al [11], for a marine environment.…”