Ground-based infrared retrievals of optical depth, effective radius, and composition of airborne mineral dust above the Sahel

Mineral dust aerosol optical depth ( AOD) at 11 mu m and effective radius are retrieved from ground-based spectral infrared ( 8-13 mu m) radiance observations collected in Niamey, Niger, in 2006 with the Atmospheric Radiation Measurement ( ARM) mobile facility. The algorithm used in this study has the ability to differentiate between two different mineral types, allowing the AOD and effective radius of each mineral to be retrieved as long as the minerals have significant and spectrally different absorption bands in the infrared. Kaolinite, gypsum, and quartz, common minerals found in African deserts, have different infrared absorption features and are used in this work. The mineral combination that yielded the best fit to the observations, after accounting for the increased degrees of freedom in the dual mineral fits, was identified as the radiatively significant'' composition of the dust. The results were analyzed as function of period ( premonsoon, early monsoon, late monsoon, and postmonsoon) as well as by back-trajectory direction. High-AOD events occurred in all four periods, with more high-AOD events occurring during the drier premonsoon and postmonsoon periods. Kaolinite is an important component to the dust, with 94% of the retrievals having a significant amount of this mineral. Kaolinite + gypsum was determined to fit the spectral observations the best in nearly two thirds of the observations. Additionally, the amount of gypsum was dependent on the back-trajectory direction, with a larger fraction of the total AOD due to gypsum when the flow was from east of the Niamey site.