Application of spheroid models to account for aerosol particle nonsphericity in remote sensing of desert dust

Abstract

[1] The possibility of using shape mixtures of randomly oriented spheroids for modeling desert dust aerosol light scattering is discussed. For reducing calculation time, look-up tables were simulated for quadrature coefficients employed in the numerical integration of spheroid optical properties over size and shape. The calculations were done for 25 bins of the spheroid axis ratio ranging from $0.3 (flattened spheroids) to $3.0 (elongated spheroids) and for 41 narrow size bins covering the size parameter range from $0.012 to $625. The look-up tables were arranged into a software package, which allows fast, accurate, and flexible modeling of scattering by randomly oriented spheroids with different size and shape distributions. In order to evaluate spheroid model and explore the possibility of aerosol shape identification, the software tool has been integrated into inversion algorithms for retrieving detailed aerosol properties from laboratory or remote sensing polarimetric measurements of light scattering. The application of this retrieval technique to laboratory measurements by Volten et al. (2001) has shown that spheroids can closely reproduce mineral dust light scattering matrices. The spheroid model was utilized for retrievals of aerosol properties from atmospheric radiation measured by AERONET ground-based Sun/sky-radiometers. It is shown that mixtures of spheroids allow rather accurate fitting of measured spectral and angular dependencies of observed intensity and polarization. Moreover, it is shown that for aerosol mixtures with a significant fraction of coarse-mode particles (radii ! $1 mm), the nonsphericity of aerosol particles can be detected as part of AERONET retrievals. The retrieval results indicate that nonspherical particles with aspect ratios $1.5 and higher dominate in desert dust plumes, while in the case of background maritime aerosol spherical particles are dominant. Finally, the potential of using AERONET derived spheroid mixtures for modeling the effects of aerosol particle nonsphericity in other remote sensing techniques is discussed. For example, the variability of lidar measurements (extinction to backscattering ratio and signal depolarization ratio) is illustrated and analyzed. Also, some potentially important differences in the sensitivity of angular light scattering to parameters of nonspherical versus spherical aerosols are revealed and discussed.Citation: Dubovik, O., et al. (2006), Application of spheroid models to account for aerosol particle nonsphericity in remote sensing of desert dust,

Motivation, rationale and key results from the GERBILS Saharan dust measurement campaign

“…Such a procedure provides data analogous to the almucantar scans from the Aerosol Robotic Network (AERONET; e.g. Dubovik et al, 2006). The Airborne Research Interferometer Evaluation System (ARIES) was also flown and has previously been used to measure the spectrally resolved impact of mineral dust in the long-wave region of the spectrum at a spectral resolution of approximately 0.5 cm −1 .…”

“…While a number of models have been developed based on measurements of the forward-scattered radiance from AERONET observations (e.g. Dubovik et al, 2006) the applicability of these models for backscattered radiances should be further tested. The Aerosol Polarimetry Sensor (APS) on the GLORY mission may assist in this regard (http://glory.giss.nasa.gov/aps) would have assisted in this regard as radiances and polarised radiances would have been measured over a wide range of scattering angles, but unfortunately the satellite failed to gain orbit on launch.…”

“…Osborne et al (2011) used aircraft orbits and SW spectral radiance measurements in close proximity to the Dakar AERONET site (Holben et al, 1998) to show that the aircraft SWS reproduced the AERONET sun photometer radiances as a function of the scattering angle during almucantar scans (e.g. Dubovik et al, 2006). Modelled radiances were then computed for spheres, spheroids, and irregular-shaped particles (polycrystals) with sharp edges using combinations of Mie scattering, T-matrix, and ray tracing diffraction on facet techniques (Havemann and Baran, 2001;Hesse, 2008).…”

Retrieving the height of smoke and dust aerosols by synergistic use of VIIRS, OMPS, and CALIOP observations

“…We take the Level 2.0 (for dust cases) or 1.5 (for smoke cases as Level 2.0 data are not available at the present time) Version 2 AERONET inversion data Dubovik et al, 2006] to compare the retrieved SSA. The AERONET data are averaged within 2 h temporal window of the VIIRS overpass time to be compared.…”

Impact of dust aerosols on the radiative budget, surface heat fluxes, heating rate profiles and convective activity over West Africa during March 2006

“…Recently, this algorithm was updated with several improvements (generating Version 2 retrievals used here). First, in order to account for aerosol non-sphericity, the coarse mode of desert dust is modeled as a mixture of randomly oriented spheroids (Dubovik et al, 2006). Second, the assumed surface reflectance model accounts for reflectance directionality and based on MODIS surface reflectance climatology (Sinyuk et al, 2007;Eck et al, 2008).…”