Particle Measuring Systems laser particle spectrometer (ASAS-X and FSSP) probes were used to measure aerosol particle concentrations and size distributions during 11 out of 12 ER-2 flights between Punta Arenas (53 ¿S) and Antarctica (up to 72 ¿S) from August 17 to September 22, 1987. The time resolution was 10 s, corresponding to a spatial resolution of 2 km. The data were divided into two size classes (0.05--0.25 and 0.53--5.5 μm radius) to separate the small particle from the coarse particle populations. Results show that the small particle concentrations are typical for a background aerosol during volcanic quiescence. This concentration is generally constant along a flight track; in only one instance did we measure a depletion of small particles during a polar stratospheric cloud (PSC) encounter, suggesting a nucleation of type I PSC particles on background aerosols. Temporary increases of the coarse particle concentrations indicated the presence of tenuous polar stratospheric clouds that were encountered most frequently at the southernmost portion of a flight track and when the aircraft descended to lower altutides. During ''particle events,'' particle modes were found at 0.6 μm radius, corresponding to type I PSCs, and occasionally, a second mode at 2.0 μm radius, corresponding to type II PSCs. ¿ American Geophysical Union 1989