The size distribution and interaction of radioactive and natural aerosols in the stratosphere

Abstract

Artificial radioactivity which persists in the stratosphere on a time scale of years is shown to be associated with particles below 0.02 μ radius above 27 km and with particles very nearly 0.1 μ radius between 21 km and the tropopause. Assuming the artificial radioactivity to be associated with natural aerosols at each level, the radioactive particle size data provide insight on the size distribution of micrometeorites and other particulates in the upper stratosphere and their interaction with sulfate particles in the lower stratosphere. The decrease in specific radioactivity with increased sulfate particle radius near 20 km suggests that the photochemical oxidation of SO2 may not be the important mechanism for large particle formation at this level. Chemical factors and residence time considerations support the view that Aitken nuclei in the upper troposphere are sulfate particles of sufficient size and population to account for the production of large sulfate particles near and above the tropopause by Aitken nuclei coagulation. Approximate estimation of the total sulfate mixing ratio with altitude above 5 km indicates a broad maximum in the upper troposphere. Photochemical and radiochemical evidence for the rapid oxidation of SO2 in the stratosphere reinforces these views. It is tentatively concluded that Aitken particles in the troposphere account for most of the sulfate in the atmosphere and that there is no stratospheric sulfate layer but only a stratospheric “large particle” layer.DOI: 10.1111/j.2153-3490.1966.tb00261.x