Aerosol measurements were made in the marine boundary layer along the coast of Washington State during the Pacific Stratus Sulfur Investigation. On April 22 the particle concentration increased to levels much higher than usual for the clean marine boundary layer. The total particulate number concentration greater than 3 nm diameter increased rapidly from about 250 cm-3 to 3200 cm-3, remained near that level for 7 hours, and then decreased over the next 2 hours to less than 400 cm-3. The change could not be attributed to either local or distant contamination. Immediately before the increase particulate surface area concentration dropped from 25 μm2 cm-3 to less than 5 μm2 cm-3. The SO2 concentration increased from about 20 pptv to 40--60 pptv just before the increase in particle concentration. While these measurements cannot distinguish between changes in number concentration caused by particle nucleation versus advection or vertical mixing, clearly there was recent or continuing particle production on a mesoscale in the air mass. Related aircraft measurements and model results support the hypothesis of new particle formation. These data provide evidence that at times high concentrations of new, ultrafine particles are formed at low SO2 concentrations under marine conditions. This homogeneous nucleation, as opposed to heterogeneous condensation on existing particles, is strongly and inversely dependent on the concentration of existing particles. ¿ American Geophysical Union 1992