Isotopic studies of the sulfur component of the stratospheric aerosol layer1, 2"

A major fraction of the large-size population of the Junge aerosol layer is comprised of sulfates thought to be of terrestrial origin. In order to obtain direct evidence concerning the importance of various contributing sulfur sources, as well as the significance of in situ gas-to-particle reactions, we undertook an extensive study of the sulfur concentration and isotopic ratio of particles, collected from 1962 to 1971 in the lower stratosphere over a wide range of latitudes. The data show that the major perturbations to the low-level persistent aerosol layer result from a few explosive volcanic eruptions. The extent of the initial tephra contribution to the layer gradually subsides, but particles are continuously replenished by aerosols formed from the gaseous portion of the eruptions. Trends in δS³⁴ enrichment following an event are consistent with a simple one-dimensional model accounting for both transport and chemical reaction. During periods of comparative volcanic quiescence, the isotopic ratios of stratospheric sulfates in both Hemispheres exhibit a similar value, suggesting continual input from a common equatorial source.