Linking dust observations from space-borne hyperspectral infrared sounders with alluvial source regions through particle size and dust mineralogy

Hyperspectral infrared sounders provide the potential to retrieve several properties of airborne mineral dust in addition to AOD. It is shown that by using advanced mathematical treatment also information about particle size and dust mineralogical composition can be extracted from the observed radiance spectra . Furthermore, the emission temperature of the dust layers can be converted to dust altitude using temperature profiles from numerical weather prediction with the WRF model. Methods of Bayesian inference are used to discern the potential contributions of airborne dust, ice crystals (cirrus clouds) and surface emissivity to the observed IASI spectra as well as to calculate the respective weights of seven representative particle size distributions and seven representative mineralogical mixtures from very different source areas of the global dust belt. It is shown that for dust outbreaks across North Africa and the Atlantic Ocean as well as over East Asia, the retrieved dust particle size and dust mineralogy provide information about active dust sources. In the presented cases the airborne dust is characterized by large particles and rather high feldspar content, as well as a reduced overall content of clay minerals. The relative contributions of feldspars and different clay minerals are different for North African and East Asian dust. Although particle size and mineralogical composition are not bound to each other in the retrieval, the retrieval algorithm manages to assign the clay minerals mainly to the smaller particles and the quartz and feldspar particles mainly to the larger effective radii. The low clay contribution together with higher fractions of quartz and feldspars can be assumed to point towards freshly eroded material from alluvial dust sources instead of older (more clay-rich) soils responsible for background desert dust. It can be seen that the dust mineralogy is not in alignment with highest dust AOD. Consequently it can be deduced that at the edges of the dust sweeps originating from alluvial sources the airborne dust mixes with dust from desert background sources (potentially lifted by saltation induced from the larger particles within the dust storm).