Showing papers by "Ilya Slutsker published in 2006"

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 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.

Aeronet's Version 2.0 quality assurance criteria

Abstract: The AERONET inversion products provide powerful information for understanding column integrated aerosol properties particularly given the wide global distribution of sites and the 13 year record for some sites. Significant evolution of the instrument, data quality, ancillary input data and inversion algorithm has necessitated release of Version 2.0 and establishment of criteria for quality assured products. This paper documents version 1.0 quality assurance criteria and the analysis of the entire retrieval record available for the Version 2.0 to revise the quality assured criteria. The result is an improvement in the number and quality of aerosol inversion parameters for most sites through the entire AERONET data record.

A network for standardized ocean color validation measurements

Abstract: The Aerosol Robotic Network (AERONET) was developed to support atmospheric studies at various scales with measurements from worldwide distributed autonomous sunphotometers [Holben et al. 1998]. AERONET has now extended its support to marine applications through the additional capability of measuring the radiance emerging from the sea with modified sun-photometers installed on offshore platforms like lighthouses, navigation aids, oceanographic and oil towers. The functionality of this added network component called AERONET - Ocean Color (AERONET-OC), has been verified at different sites and deployment structures over a four year testing phase. Continuous or occasional deployment platforms (see Fig. 1) included: the Acqua Alta Oceanographic Tower (AAOT) of the Italian National Research Council in the northern Adriatic Sea since spring 2002; the Martha s Vineyard Coastal Observatory (MVCO) tower of the Woods Hole Oceanographic Institution in the Atlantic off the Massachusetts coast for different periods since spring 2004; the TOTAL Abu-Al-Bukhoosh oil Platform (AABP, shown through an artistic rendition in Fig. 1) in the Persian (Arabian) Gulf in fall 2004; the Gustaf Dal n Lighthouse Tower (GDLT) of the Swedish Maritime Administration in the Baltic Sea in summer 2005; and the platform at the Clouds and the Earth's Radiant Energy System (CERES) Ocean Validation Experiment (COVE) site located in the Atlantic Ocean off the Virginia coast since fall 2005. Data collected during the network testing phase, confirm the capability of AERONET-OC to support the validation of marine optical remote sensing products through standardized measurements of normalized water-leaving radiance, LWN, and aerosol optical thickness, a, at multiple coastal sites.

Ship‐based aerosol optical depth measurements in the Atlantic Ocean: Comparison with satellite retrievals and GOCART model

Abstract: [1] Aerosol optical depth measurements were made in October–December 2004 onboard the R/V Akademik Sergey Vavilov. The cruise area included an Atlantic transect from North Sea to Cape Town and then a crossing in the South Atlantic to Argentina. In the open oceanic areas not influenced by continental sources aerosol optical depth values were close to background oceanic conditions (τa ∼ 0.06–0.08). Spectral dependence, especially in the high latitude Southern Atlantic, can be considered as quasi-neutral (Angstrom parameter α was less than 0.4). Back-trajectory analysis allowed statistical division of the aerosol optical parameters and showed similar properties for the North Atlantic polar marine, South Atlantic subtropical marine and South Atlantic polar marine air. Ship-borne aerosol optical depth comparisons to GOCART model and satellite retrievals revealed systematic biases. Satellite retrieved optical depths are generally higher by 0.02–0.07 (depending on the sensor), especially in low τa conditions. GOCART model simulated optical depths correlate well with the ship measurements and, despite overall bias and a notable disparity with the observations in a number of cases, about 30% agree within ±0.01.

Local analysis of MISR surface BRF and albedo over GSFC and mongu AERONET sites

Abstract: We have developed an atmospheric correction algorithm to retrieve the surface bidirectional reflectance factor (BRF) and albedo from Multiangle Imaging SpectroRadiometer (MISR) measurements for small areas around Aerosol Robotic Network (AERONET) sunphotometer sites, using AERONET aerosol and column water vapor information. Our goal is to develop an indirect validation method for MISR surface reflectance products over heterogeneous land. Our algorithm makes independent retrievals with both the Li Sparse-Ross Thick kernel BRF model and the modified Rahman-Pinty-Verstraete BRF model used in the Moderate Resolution Imaging Spectroradiometer and MISR land algorithms, respectively. In this study, we report the first results of processing MISR Collection 4 data for 2003-2004 for two sites, Mongu, Zambia, and Greenbelt, MD. We found that MISR generally provides accurate retrievals of BRF and albedo in both clear and hazy atmospheric conditions, correctly reproducing the parameter time series and spatial distribution. We found that the MISR BRF, on average, is less anisotropic in the visible bands. The difference is greatest in the blue band, but decreases with increasing wavelength such that it is negligible in the near-IR band. This discrepancy originates in part in the MISR aerosol retrieval algorithm over heterogeneous land, which tends to select an aerosol model that favors spectrally invariant shapes of surface BRF. The other part of the discrepancy comes from the surface hemispherical-directional reflectance factor retrieval algorithm where the iteration loop that removes the diffuse atmospheric transmittance is currently turned off. Our initial results suggest that the MISR surface albedo is on average lower than our retrievals by about 0.005 in the green and red bands. In the near-IR, it agreed with our retrievals with the modified Rahman-Pinty-Verstraete model for the Mongu site, but was systematically lower over the Greenbelt site by about 0.016. When significant aerosol absorption is present (Mongu), the albedo discrepancy is additionally biased by the difference between the MISR and AERONET retrievals of aerosol absorption