The results obtained with an improved balloon-borne condensation nuclei (cn) counter that is capable of operating to altitudes of at least 30 km are presented. Of major interest is the appearance of a quasiannual variation near 30 km which could be described by a sudden concentration increase of unusually small particles occurring in the winter or spring followed by a 1- to 3-month decay period to background levels. The magnitude of the variation has increased dramatically following the recent generally higher levels of volcanic activity affecting the lower stratosphere. Several potential explanations for the event are considered, but none appear entirely satisfactory or complete at the present time. The explanation with the fewest drawbacks would attribute the production of growth of the new cn to a highly supersaturated H2SO4 vapor layer generated by any one of several proposed processes in the upper high latitude stratosphere.