Abstract
Abstract. For the determination of aerosol optical thickness (AOT) Bremen AErosol Retrieval (BAER) has been developed. Method and main features on the aerosol retrieval are described together with validation and results. The retrieval separates the spectral aerosol reflectance from surface and Rayleigh path reflectance for the shortwave range of the measured spectrum of top-of-atmosphere reflectance for wavelength less than 0.670 μm. The advantage of MERIS (Medium Resolution Imaging Spectrometer on the Environmental Satellite – ENVISAT – of the European Space Agency – ESA) and SeaWiFS (Sea viewing Wide Field Sensor on OrbView-2 spacecraft) observations is the availability of several spectral channels in the blue and visible range enabling the spectral determination of AOT in 7 (or 6) channels (0.412–0.670 μm) and additionally channels in the NIR, which can be used to characterize the surface properties. A dynamical spectral surface reflectance model for different surface types is used to obtain the spectral surface reflectance for this separation. The normalized differential vegetation index (NDVI), taken from the satellite observations, is the model input. Further surface bi-directional reflectance distribution function (BRDF) is considered by the Raman-Pinty-Verstraete (RPV) model. Spectral AOT is obtained from aerosol reflectance using look-up-tables, obtained from radiative transfer calculations with given aerosol phase functions and single scattering albedos either from aerosol models, given by model package "optical properties of aerosol components" (OPAC) or from experimental campaigns. Validations of the obtained AOT retrieval results with data of Aerosol Robotic Network (AERONET) over Europe gave a preference for experimental phase functions derived from almucantar measurements. Finally long-term observations of SeaWiFS have been investigated for 11 year trends in AOT. Western European regions have negative trends with decreasing AOT with time. For the investigated Asian region increasing AOT have been found.
Highlights
Several applications in climate research and environmental control require a determination of spectral aerosol optical thickness
Like MERIS or SeaWiFS are single view instruments and the spectral surface contribution needs to be estimated from the observed spectral properties of top-of-atmosphere reflectance
Such aerosol retrievals can contribute to environmental control of air pollution by aerosols, especially, if additional aerosol products, like PM10 concentrations are derived from aerosol optical thickness (AOT), as it is done by von Hoyningen-Huene et al (2008)
Summary
Several applications in climate research and environmental control require a determination of spectral aerosol optical thickness. Like MERIS or SeaWiFS are single view instruments and the spectral surface contribution needs to be estimated from the observed spectral properties of top-of-atmosphere reflectance. Retrieval approaches for such sensors are provided by von Hoyningen-Huene et al (2003) and Hsu et al (2004). A retrieval of aerosol optical thickness (AOT) over land requires sufficient spectral information on top-of-atmosphere (TOA) reflectance in the blue, VIS and NIR channels, to estimate the surface contribution, its spectral properties and to correct them for suitable channels, where the surface contribution is weak. Since for MERIS no techniques are applied to avoid sun-glint, over ocean about 40% of the swath is contaminated by sun-glint in lower latitudes
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