Nucleation events detected at the high altitude site of the Puy de Dôme Research Station, France

Aerosol and ion number size distributions were measured at the top of the Puy de Dome (1465 m above the sea level) for a three-month period. The goals were to investigate the vertical extent of nucleation in the atmosphere and the effect of clouds on nucleation. Nucleation and new-particle formation events were classified into four classes: (1) burst of cluster ions, (2) large ion formation starting from 10 nm, (3) burst of cluster ions followed by large ion formation with a gap of intermediate ions, and (4) burst of ions with continuous growth to the sizes > 10 nm. All together these events were observed during nearly half of the analyzed days. Concentrations of cluster ions (< 1.4 nm) varied typically between 100 and 1000 cm -3 . Intermediate ion (1.4-6 nm) concentrations were usually lower than 500 cm -3 but could exceed 3000 ions cm -3 during nucleation events. Large concentrations of intermediate ions seem to be appropriate to detect the occurrence of most of nucleation events. In clouds, the aerosol condensation sink and cluster ions concentrations were lower, presumably because of scavenging by cloud droplets, but the intermediate ion concentrations remained unchanged. We observed that large ion formation starting at 10 nm (class 2 events) and continuous growth of ions (class 4 events) occurred preferably under clear-sky conditions, and that all except class 2 events could be observed under cloudy conditions.