1Laboratoire de Météorologie Physique CNRS UMR6016, Observatoire de Physique du Globe de Clermont-Ferrand, Université Blaise Pascal, France
2Finnish Meteorological Institute, P.O. Box 503, 00101 Helsinki, Finland
3Laboratoire des Sciences du Climat et de l'Environnement, UMR Commissariat à l'Energie Atomique/CNRS 1592, Gif-sur-Yvette, France
4Laboratoire de Glaciologie et Géophysique de l'Environnement, CNRS UMR5183, Université Joseph Fourier Grenoble 1, Saint Martin d'Héres, France
Abstract. Cluster particles (0.8–1.9 nm) are key entities involved in nucleation and new particle formation processes in the atmosphere. Cluster ions were characterized in clear sky conditions at the Puy de Dôme station (1465 m a.s.l.). The studied data set spread over five years (February 2007–February 2012), which provided a unique chance to observe seasonal variations of cluster ion properties at high altitude. Statistical values of the cluster ion concentrations and diameters are reported for both positive and negative polarities. Cluster ions were found to be ubiquitous at the Puy de Dôme and displayed an annual variation with lower concentrations in spring. Positive cluster ions were less numerous than negative, but were larger in diameter. Negative cluster ion properties were not sensitive to the occurrence of a new particle formation (NPF) event, while positive cluster ions appeared to be significantly more numerous and larger on event days. The parameters of the balance equation for the positive cluster concentration are reported separately for the different seasons and for the NPF event days and non-event days. The steady-state assumption suggests that the ionization rate is balanced with two sinks: the ion recombination and the attachment onto background aerosol particles, referred to as "aerosol ion sink". The aerosol ion sink was predominant compared to the recombination sink. The positive ionization rates derived from the balance equation (Qcalc) were well correlated with the ionization rates obtained from radon measurement (Qmeas). When ignoring the gamma radiation contribution to the ion production, Qcalc is on average higher than Qmeas during the warm season. In contrast, when a seasonal gamma contribution is taken into account, Qmeas always exceeds Qcalc. We found that neither the aerosol ion sink nor the ionization rate (calculated or measured, with or without the gamma contribution) were significantly different on event days compared to non-event days, and thus, they were not able to explain the different positive cluster concentrations between event and non-event days. Hence, the excess of positive small ions on event days may derive from an additional constant source of ions leading to a non-steady state.