Ben Veihelmann

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

[ 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 similar to 0.3 ( flattened spheroids) to similar to 3.0 ( elongated spheroids) and for 41 narrow size bins covering the size parameter range from similar to 0.012 to similar 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 >= similar to 1 mu m), the nonsphericity of aerosol particles can be detected as part of AERONET retrievals. The retrieval results indicate that nonspherical particles with aspect ratios similar to 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.

Retrieval of aerosol optical properties from OMI radiances using a multiwavelength algorithm: Application to western Europe

The Ozone Monitoring Instrument (OMI) multiwavelength algorithm has been developed to retrieve aerosol optical depth using OMI-measured reflectance at the top of the atmosphere. This algorithm was further developed by using surface reflectance data from a field campaign in Cabauw (The Netherlands), a new cloud-screening method, and a global aerosol database derived from the aerosol transport model TM5. The first results from an application of this algorithm over western Europe are presented. The OMIretrieved aerosol optical depth is evaluated by comparison with both ground-based measurements from Aerosol Robotic Network (AERONET) and Moderate Resolution Imaging Spectroradiometer (MODIS) satellite data. The various aerosol optical depth values compare favorably, except in situations where large changes occur in the surface properties, which is illustrated over the Iberian peninsula. OMI and MODIS aerosol optical depth are well correlated (with a correlation coefficient of 0.66 over land and 0.79 over sea), although the multiwavelength algorithm appears to overestimate the aerosol optical depth values with respect to MODIS. The multiwavelength algorithm performs better over sea than over land. Qualitatively, the multiwavelength algorithm well reproduces the expected spatial aerosol optical depth gradient over western Europe

Light reflected by an atmosphere containing irregular mineral dust aerosol

We simulate polarimetric satellite observations of sunlight reflected by a turbid atmosphere over the ocean. We determine the sensitivity of these observations with respect to the optical thickness and the single-scattering albedo of irregular mineral aerosol. Simulated results indicate that both quantities can be retrieved from simultaneous polarization and intensity measurements. Aerosol scattering is modelled using a true measured scattering matrix of irregularly-shaped mineral dust aerosol. We study the suitability of various approximations of the particle shape used for the numerical calculation of scattering matrices. Approximations with spheres or spheroids with a distribution of moderate axis ratios can lead to large errors of the simulated light intensities. A spheroidal approximation including extreme axis ratios is found to be the most appropriate one for simulations of light scattering by irregular mineral aerosol particles, if measured scattering matrices are not available.