Seasonal and diurnal variation of aerosol size distributions (10 < D < 750nm) at a high-alpine site (Jungfraujoch 3580 m asl)

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 dom inated by particles with D < 100 nm, while most of the surface area was in the accumulation mode size range (0.1-1 mu m). Average size distributions e xhibited a distinct bimodal shape which is generated and maintained by clou d processes and is less distinct in the free troposphere. Fitted modal diam eters and standard deviations most of the Aitken (20-100 nm) and accumulati on modes were surprisingly constant throughout the year (D-Ait = 43 +/- 3 n m; D-Acc = 140 +/- 6 nm; sigma(Air) = 2.13 +/- 0.11; sigma(Acc) = 161 +/- 0 .03). The relative number concentrations in both modes are responsible for the seasonality observed in the shape of the size distribution. The high se asonality of N-Acc with summer and winter values of similar to 260 and simi lar to 40 cm(-3), respectively, is mainly due to transport of planetary bou ndary layer air to the station. In contrast, new particle formation is resp onsible for the high concentrations of nucleation mode particles (D < 20 nm ) which exhibited a maximum during the winter months. The relatively low se asonality found for N-Ait (summer, winter values: similar to 530, similar t o 310 cm(-3)) is due to gas-to-particle conversion as well as transport pro cesses. An analysis showed that a significant fraction of nucleation mode p articles were formed by photochemical reactions.