Approximately monthly balloon soundings, at mid-latitude using an aerosol particle counter, are utilized in a study of the time development of an order of magnitude increase in the stratospheric aerosol level which began in late 1974. A substantial reduction, by a factor of 5, was observed in the total number of stratospheric particles with r?0.15 μm from the peak observed in February 1975 to the minimum observed in October 1975. The characteristic decay time for the peak particle mixing ratio was about 1 year. Aerosol vaporization measurements show that all new aerosol having r?0.15 μm was volatile 3--4 months after the increase began, a result suggesting a relatively rapid gas-to-particle conversion process. If the gas involved was volcanic SO2, then an injection of about 0.7 Mt would be required to explain the aerosol increase.