Eleven AGASP research flights during March-April 1983 collected particle size fractions by cascade impactors. Concentrations of elemental constituents of soil dust—Al, Si, K, Ca, Ti, and Fe—primarily in the coarsest particles, >16 µm aerodynamic diameter (µmad), were much greater in samples from six of the flights than in the samples of the remaining five flights. The relative concentrations of these elements were approximately those in geochemical average shale. They differed from concentrations in the average earth's crust by a several-fold depletion of Si. The composition of stratospheric samples of El Chichon volcanic dust had relatively higher K and Ti than did the high dust aerosols. The results suggest two possible sources for high dust content particles: (1) finely divided clay minerals generated at lower latitudes, e.g., in Asia during late winter, which are transported northward, and under suitable atmospheric conditions before sampling, may grow or coalesce as ice crystals into much larger sizes; and (2) a local Arctic source of soil dust particles.