Aerosol phase function derived from CIMEL measurements

An approach to simulate the signal at the top of the atmosphere (TOA) in the visible and near-infrared regions is based on the exploitation of CIMEL sky radiances in three spectral bands (i.e. 440 nm, 675 nm and 870 nm). An iterative method, developed at the Laboratoire Interdisciplinaire des Sciences de l'Environnement (LISE, Wimereux, France), allows the aerosol phase function to be extracted from these ground-based measurements. Sky radiances are corrected for the multiple scattering based on the use of a radiative transfer tool and the aerosol phase function is derived from the primary scattering approximation. In order to cover the largest range of scattering angles, only the sky radiances acquired at low solar elevations are employed in this retrieval. These extreme geometrical conditions impose to adapt the radiative transfer code and to check its performances. Limits, performances and accuracy of this inverse method are discussed and illustrated both from the radiative transfer computations and from the CIMEL measurements. Moreover, thanks to the fact that the Aerosol Robotic Network (AERONET) also proposes CIMEL derived aerosol phase functions, the latter have been compared with our results. The substantial discrepancies that appear between the two sets are explained by the different objectives used in the two retrieval algorithms.     

MERIS atmospheric correction over coastal waters: validation of the MERIS aerosol models using AERONET

Standard aerosol models (SAMs) are used for the Medium-Resolution Imaging Spectrometer (MERIS) level-2 processing over water, first to remotely sense the aerosols in the near-infrared and secondly to perform the atmospheric correction for ocean colour analysis. However, are these SAMs still suitable over coastal areas? The present work was intended to answer that question through the use of the Aerosol Robotic Network (AERONET) by selecting CIMEL radiometers operating over the sea surface or near the coastline. The current official MERIS algorithm overestimates aerosol optical thickness (AOT) over coastal waters at 865 nm. This can be related either to incorrect assumptions of the underlying surface assumption or to the assumptions of the aerosol properties (e.g. phase function). This study looks at the importance of aerosol modelling and confirms that the improved aerosol models must be used in the retrieval chain. Extinction measurements were first used to derive the aerosol optical thicknesses (AOTs). The spectral dependency of the AOTs between 670 nm and 865 nm allowed the selection of a standard aerosol model. The ability of the standard aerosol models to retrieve the AOTs at 440 nm was then analysed as a key element in the extrapolation of the aerosol path radiance from the near-infrared to the blue spectral range. The two outputs of this analysis are systematic biases in this retrieval process and accordingly they are an estimation of the dispersion. The first output can be defined as a corrective factor in the aerosol path radiance at 440 nm and the second output can be used for error analysis. A radiative transfer code was used to simulate the sky radiance in the principal plane of acquisition. Comparisons at 870 nm illustrated the ability of the standard aerosol models to retrieve the aerosol path radiances with a direct impact on the AOT retrieval from satellite observations at 865 nm.     

Validation of atmospheric scattering functions used in atmospheric correction over the ocean

A new approach has been developed to validate atmospheric correction (AC) over the ocean. The latter has been applied to the ground-segment data from the Medium Resolution Imaging Spectrometer (MERIS) on board the Environmental Satellite (Envisat) platform. An atmospheric validation database has been built up with a ground-based instrument, i.e. the Cimel radiometer from the Aerosol Robotic Network (AERONET). The aim of this work is to assess the atmospheric scattering functions needed to perform AC of remotely sensed data. The inputs to this new methodology were provided by AERONET, after inversion of radiometric measurements (i.e. solar direct extinctions and sky diffuse radiances) to get the inherent optical properties (IOPs) of the aerosols. The successive orders (SOs) of scattering code have been used as the radiative transfer tool in this study. This new concept for the validation of AC has been illustrated with the MERIS level-2 data extracted from the Meris Matchup In-situ Database (MERMAID) over the Acqua Alta Oceanographic Tower (AAOT, Venice – Italy). Results indicate first, an overestimate of the MERIS aerosol optical thickness (AOT) at 865 nm, and second, a marine reflectance affected by a negative bias of about 13% at 412.5 nm. This yields to an overestimate of the MERIS algal-1 pigment index, which may exceed 50%, over AAOT. The same trend is also observed in the determination of the algal-2 pigment index.     

A toroidal approximation of capillary forces in polydisperse granular assemblies

In this paper we propose a numerical procedure for the prediction of capillary forces in polydisperse granular assemblies at a degree of moisture content that corresponds to the so-called pendular regime. The capillary force model is adopted within the Laplace–Young framework with a toroidal approximation of the liquid bridge geometry. Governing equations are first derived in a form that highlights the role of intrinsic parameters such as inter-particle separation distance, ratio of particle radii and liquid volume. A proper scaling of these equations is adopted so that the solution applies to any particle pair configuration. Numerical integration algorithms are provided in a way that facilitates implementations in macroscopic procedures for computer simulations. A qualitative evaluation is undertaken to highlight model predictions of the effects on capillary force of various intrinsic parameters that characterise the particle pair and liquid bridge. The model is validated against the experimental results provided by Willet et al. (Langmuir 16:9396–9405, 2000) for a wide range of liquid volumes and particle-pair polydispersity.     

Natural variability of aerosol inherent optical properties: consequences for atmospheric correction over water

A good knowledge of the inherent optical properties (IOPs) of aerosols is a strong requirement for accurately performing atmospheric correction over the ocean. For several decades, IOPs have been computed using standard aerosol models (SAMs) that characterize the micro-physical properties of aerosols. These SAMs were used in the last generation of the Medium Resolution Imaging Spectrometer (MERIS) auxiliary data files (ADFs) to feed the atmospheric correction algorithm. Alternatively, Aerosol Robotic Network (AERONET) measurements can also provide IOPs. We built a database with the aerosol IOPs encountered over four AERONET stations in the North Sea plus one at the Acqua Alta Oceanographic Tower (AAOT, Venice, Italy). Several thousands of data sequences containing the aerosol IOPs were processed with filtering techniques and statistical methods to produce 16 classes of IOPs. An analysis of the dispersion of the IOPs within each class was conducted to evaluate the induced errors in the MERIS level-2 (L2) products in European coastal waters. We also investigated the reduction in the errors that can be achieved if there is access to auxiliary meteorological data (i.e. the relative humidity) or by using the bidirectionality in the satellite measurements, such as for advanced along-track scanning radiometer (AATSR) data.