During an extended field campaign at a high-alpine site (Jungfraujoch; 3580 m asl, Switzerland) from March 1997 to May 1998, the particle number size distribution (diameter D=18--750 nm) and number concentration N(D>10 nm) were measured on a continuous basis. The number size distribution was dominated by particles with D<100 nm, while most of the surface area was in the accumulation mode size range (0.1--1 μm). Average size distributions exhibited a distinct bimodal shape which is generated and maintained by cloud processes and is less distinct in the free troposphere. Fitted modal diameters and standard deviations &sgr; of the Aitken (20--100 nm) and accumulation modes were surprisingly constant throughout the year (DAit=43¿3 nm; DAcc=140¿6 nm; &sgr;Ait=2.13¿0.11; &sgr;Acc=1.61¿0.03). The relative number concentrations in both modes are responsible for the seasonality observed in the shape of the size distribution. The high seasonality of NAcc with summer and winter values of ~260 and ~40 cm-3, respectively, is mainly due to transport of planetary boundary layer air to the station. In contrast, new particle formation is responsible for the high concentrations of nucleation mode particles (D<20 nm) which exhibited a maximum during the winter months. The relatively low seasonality found for NAit (summer, winter values: ~530, ~310 cm-3) is due to gas-to-particle conversion as well as transport processes. An analysis showed that a significant fraction of nucleation mode particles were formed by photochemical reactions. ¿ 1999 American Geophysical Union