Information content of space-borne hyperspectral infrared observations with respect to mineral dust properties

In principle, observations from hyperspectral infrared (IR) sounders such as IASI (infrared atmospheric sounding interferometer) can be used to simultaneously retrieve dust aerosol optical depth (AOD) and properties such as dust particle size, composition, emission temperature and height. Starting from a compilation of “typical” mineral dust particle size distributions and mineralogical compositions, the information content of dust spectra from Mie simulations and from FTIR (Fourier-transform-infrared spectrometer) measurements (provided by the University of Iowa) is analysed. While the Mie spectra provide a higher number of degrees of freedom for signal (up to 6.7) than the FTIR spectra (up to 5.7), the Shannon information content is slightly lower (3.4) from Mie than from FTIR (3.5). The analysis shows that the spectra provide information on particle size and composition, but information about both cannot be extracted independently owing to the correlations between the different spectra. A dust retrieval approach for IASI probing the spectral shape of extinction has been updated using the Mie and FTIR spectra. Dust properties provided by the retrieval algorithm are: AOD (at 0.55μm and 10μm), effective radius, mass-weighted mean diameter, weight-fractions of mineralogical components, IR single scattering albedo and dust layer effective emission temperature. The retrieval uncertainty in each of these parameters is calculated for each IASI pixel. From the retrieved dust layer temperature the dust layer altitude is also inferred using temperature profiles from the WRF numerical weather prediction model.