Measurements at the Mauna Loa Observatory (MLO) of particle size distributions between 15 nm and 0.5 μm illustrated many of the mechanisms that influence the concentrations of these aerosols. The time variation of the size distribution suggested that homogeneous nucleation and growth by sulfuric acid vapor condensation dramatically increased the concentrations of particles in the Aitken mode (diameters between ~15 nm and 80 nm). Bimodal distributions, similar in shape to those commonly observed in marine environments, were frequently measured at mid-day to late afternoon, in upslope air when the aerosol was likely to have passed through clouds. When bimodal distributions were recorded, there was always an associated local maximum in the measured aerosol surface area concentration. Removal of the accumulation mode (diameters between ~80 nm and 1 μm) was observed during periods of fog and light rain. In one episode the atmosphere was cleansed of nearly all fine particles. ¿ American Geophysical Union 1996