Retrieval of aerosol optical thickness over land surfaces from top‐of‐atmosphere radiance

Abstract

Aerosol remote sensing requires techniques enabling the determination of aerosol optical thickness (AOT) over land surfaces, because the most important sources (continental aerosols, anthropogenic aerosols, biomass burning, desert dust, volcano eruptions and others) are on continents. Here a retrieval method for the AOT over land surfaces from top‐of‐atmosphere (TOA) radiance using nadir looking instruments of the ocean color type (like Ocean Color and Temperature Sensor (OCTS), Sea viewing Wide Field Sensor (SeaWiFS), Moderate resolution Imaging Sensor (MODIS) or Medium Resolution Imaging Sensor (MERIS)) is presented. It is scheduled as an off‐line procedure for the ENVISAT radiometers SCIAMACHY and MERIS. The method is based on lookup tables (LUT) between the AOT and the aerosol reflectance for wavelength <0.67 μm. The aerosol reflectance is obtained from TOA reflectance accounting for Rayleigh path reflectance and the apparent spectral surface reflectance. Over land the surface reflectance is estimated by a mixing model of bare soil and green vegetation spectra, tuned by the normalized differential vegetation index (NDVI) of the satellite scene. The method has been tested and validated with SeaWiFS data and with aerosol properties of the closure experiment LACE‐98 (Lindenberg Aerosol Charactrization Experiment). For short wave channels (0.412–510 μm) an agreement between the retrieved and ground‐based data of 20% is achieved. Thus the method enables the investigation of AOT over land, yielding the regional turbidity situation as well as the identification of aerosol sources like large cities, large fire plumes, haze, small scale dynamical events and also thin cirrus clouds.