Volcanic dust in the stratosphere between 34 and 36 km altitude during May 1985

Abstract

Abstract The morphology, mineralogy, chemistry and size distribution of nonspherical particles collected in the stratosphere between 34 and 36 km altitude during May, 1985 were determined by analytical electron microscope analysis. The single-crystal grains are between a few nanometers in diameter and ∼ 14 μ m in length and occur as single particles and in mono- and polymict clusters. About ∼ 70% of all particles are tridymite, cristobalite, keatite and anorthite that delineate cotectic plagioclase-mullite and tridymite-mullite crystallisation. In order of decreasing abundance, the particles further include orthopyroxene, olivine, barite and metal oxides. All particle properties, including salt microcrystals and sulfuric acid droplets on their surface, uniquely support a volcanic origin. The abundance of Ca-Al-silicate and silica particles could indicate fractionation in ascending volcanic plumes or be an artifact of lacking data for ash-fall deposits at comparable size fractions. The observed upper size limit of nonspherical particles is accurately predicted by the Wilson-Huang settling law when particle diameters are equal to, or larger than, the gas mean free path of the atmosphere. The abundance of particles μ m in size at collection altitude suggests that nonspherical particles smaller than the gas mean free path behave as spherical particles with faster settling rates. Settling rate modelling, the mineralogical and chemical properties of the nonspherical particles, support that they could be from the top of the 1982 El Chichon eruption plume that reached an altitude similar to the 1985 collection altitude.