Collection of microparticles at high balloon altitudes in the stratosphere

Abstract

We report the collection of stratospheric particles at 34–36 km using balloon-borne collectors. The concentration of particles on the collection surfaces and element concentrations were measured on the majority of the particles using scanning electron microscope (SEM) and proton-induced X-ray emission (PIXE) analysis respectively. Particle morphologies, elemental composition, and electron diffraction data were obtained on a small number of the collected particles using transmission electron microscope (TEM) techniques. The concentration of particles between 0.045 (lower imaging limit) and 1 μm in radius is several orders of magnitude above the blank levels on the collection surfaces while the concentration of particles above 1 μm is near blank levels. More than 10 6 submicron particles were collected. The concentration of submicron particles is between 10 and 50 times the concentration expected at the sampling altitude based on models using the measured flux of extraterrestrial particles to the atmosphere. The higher concentration of submicron particles may be due to contributions from volcanic particles, inaccuracies in the influx of particles of this size to the earth, or breakup of larger particles. Analysis of the elemental composition using PIXE showed Cl, S, Ti, Fe, Br, Ni, Zr, Zn, Sr, and Cu in decreasing order of concentration. Elements below S could not be detected in the analysis. Cl, S, and Br are believed to be present due to reaction of the collection surfaces and particles with atmospheric gases while the other elements are present in the particles. The elemental composition of the particles does not allow an unequivocal origin to be assigned because particles from several of the possible sources listed above contain the measured elements. TEM analysis of 23 of the submicron particles showed 16 to be non-graphitic. The particles ranged from Al rich silicates to almost pure Fe to one containing almost exclusively Ba and S. None were definitely chondritic in composition. This collection of particles complements existing collections obtained by aircraft. They were colleted at much higher altitude and are predominantly submicron in size. Collection and analysis of particles at high altitudes using balloons presents unique challenges and opportunities in understanding the particle population in the stratosphere.