Advanced characterization of aerosol properties from measurements of spectral optical depth using the GRASP algorithm [Discussion paper]

Other literature type, Article English OPEN
Torres, Benjamín ; Dubovik, Oleg ; Fuertes, David ; Schuster, Gregory ; Cachorro, Victoria E. ; Lapyonok, Tatsiana ; Goloub, Philippe ; Barreto, África ; Mallet, Marc ; Toledano, Carlos ; Tanré, Didier (2016)
  • Publisher: European Geosciences Union
  • Journal: (issn: 1867-8548, eissn: 1867-8548)
  • Related identifiers: doi: 10.5194/amt-2016-334
  • Subject: Numerical simulations | Spectral optical depth | GRASP algorithm | Atmospheric aerosols | Aerosol properties

This study evaluates the potential of using aerosol optical depth (&tau;<sub>a</sub>) measurements to characterize the microphysical and optical properties of atmospheric aerosols. With this aim, we used the recently developed GRASP (Generalized Retrieval of Aerosol and Surface Properties) code for numerical testing of six different aerosol models with three different aerosol loads. We found that bimodal log-normal size distributions serve as useful input assumptions, especially when the measurements have inadequate spectral coverage and/or limited accuracy, such as lunar photometry. The direct numerical simulations indicate that the GRASP-AOD retrieval provides modal aerosol optical depths (fine and coarse) to within 0.01 of the input values. The retrieval of the fine mode radius, width, and volume concentration is stable and precise if the real part of the refractive index is known. The coarse mode properties are less accurate, but they are significantly improved when additional a priori information is available. In addition to these numerical studies, we used optical depth observations at six AERONET locations to validate our results with the standard AERONET inversion products. Differences in the fine mode volume median radii for the GRASP-AOD and AERONET inversions are less than 0.02&thinsp;&mu;m at sites dominated by the fine mode for all cases, although they are typically less than 0.01&thinsp;&mu;m when &tau;<sub>a</sub>(440)&thinsp;>&thinsp;0.3. The comparison of the coarse mode volume median radii shows larger differences than the fine mode at the same sites, with values typically between 0.2&ndash;0.3&thinsp;&mu;m. The comparison of coarse mode volume median radii between GRASP-AOD and AERONET improves for sites dominated by desert dust aerosol, with differences of less than 0.2&thinsp;&mu;m in most cases. The retrieved values of the fine-mode &tau;<sub>a</sub>(500) using GRASP-AOD are generally between those values obtained by the standard AERONET inversion and the values obtained by the advance AERONET Spectral Deconvolution Algorithm (SDA), with differences typically lower than 0.02 between GRASP-AOD and both algorithms. Finally, we present some examples of application of GRASP-AOD inversion using moon-photometry and the airborne PLASMA sun-photometer during ChArMEx summer 2013 campaign in the western Mediterranean.
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