In Situ Observations of Volatile and Nonvolatile Particle Size Distributions From Balloon-Borne Platforms

Abstract Profiles of volcanogenic aerosol size spectra, primarily sulfate and ash, have been measured in situ using optical particle counters (OPCs), with heated and unheated inlets, deployed on scientific balloons, after the explosive eruptions of Mt. Pinabuto, 1991, and Kelud, 2014. In both cases, the longevity of small (less than 0.5 µm radius) particles in the stratosphere is demonstrated for the dominant sulfate particles. The ash particles following Pinatubo appear to be carried within sulfate particles of this size, but only comprising about 20% of the particle mass and persisting less than a year. In contrast sulfate particles of 0.5 µm persist much longer in the stratosphere. After both volcanoes there is clear evidence of fairly rapid gravitational sorting of the particles. The particles with nonvolatile cores, which are therefore denser, sediment more quickly, and the volcanic layers appear to be sorted based on the fraction of non-volatile material present. Such sorting is initially independent of aerosol size suggesting similar non-volatile fractions across a range of sizes.