On the Information Content of Hyperspectral Infrared Observations with Respect to Mineral Dust

Abstract

Desert dust is characterized by strong silicate absorption bands located within the atmospheric window region in the terrestrial infrared (TIR) between 8 µm and 12 µm. These absorption bands and the corresponding optical properties (extinction efficiency, single scattering albedo, scattering phase function) have very specific spectral shapes for different silicate minerals, modulated by the particle size and shape. The asphericity of desert dust particles strongly affects the absorption band characteristics, for example due to surface wave modes for small particles. The use of the correct particle shape model significantly increases the spectral correlation between simulated dust optical properties for typical minerals and corresponding laboratory measurements for single minerals as well as for bulk dust from desert samples. The presence of absorption peaks and the spectral shape of the extinction signal carry dust information, which can be exploited for remote sensing purposes. With hyperspectral infrared methods it is thus possible to infer information beyond dust optical depth, that is to acquire information about dust particle size, composition and also vertical information. Examples of such information are shown for the Infrared Mineral Aerosol Retrieval Scheme (IMARS) which has been developed for the Infrared Atmospheric Sounding Interferometer (IASI) on board the European Metop satellite series. Another strong advantage of the hyperspectral signal from satellite instruments is the capability to minimize the influence of disturbing gas absorption lines within these bands. The probabilistic IMARS approach also directly provides the number of independent signals (variables) for each observation. For desert dust this number typically ranges from 2.5 to 4.0 depending on the characteristics of the observed dust plume. Consequently a lot more information beyond Aerosol Optical Depth (AOD) can be retrieved from these measurements.