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Cluster particles (0.8–1.9 nm) are key entities involved in nucleation andnew particle formation processes in the atmosphere. Cluster ions werecharacterized in clear sky conditions at the Puy de Dôme station(1465 m a.s.l.). The studied data set spread over five years (February2007–February 2012), which provided a unique chance to observe seasonalvariations of cluster ion properties at high altitude. Statistical values ofthe cluster ion concentrations and diameters are reported for both positiveand negative polarities. Cluster ions were found to be ubiquitous at the Puyde Dôme and displayed an annual variation with lower concentrations inspring. Positive cluster ions were less numerous than negative, but werelarger in diameter. Negative cluster ion properties were not sensitive to theoccurrence of a new particle formation (NPF) event, while positive clusterions appeared to be significantly more numerous and larger on event days. Theparameters of the balance equation for the positive cluster concentration arereported separately for the different seasons and for the NPF event days andnon-event days. The steady-state assumption suggests that the ionization rateis balanced with two sinks: the ion recombination and the attachment ontobackground aerosol particles, referred to as "aerosol ion sink". Theaerosol ion sink was predominant compared to the recombination sink. Thepositive ionization rates derived from the balance equation(Qcalc) were well correlated with the ionization rates obtainedfrom radon measurement (Qmeas). When ignoring the gamma radiationcontribution to the ion production, Qcalc is on average higherthan Qmeas during the warm season. In contrast, when a seasonalgamma contribution is taken into account, Qmeas always exceedsQcalc. We found that neither the aerosol ion sink nor theionization rate (calculated or measured, with or without the gammacontribution) were significantly different on event days compared tonon-event days, and thus, they were not able to explain the differentpositive cluster concentrations between event and non-event days. Hence, theexcess of positive small ions on event days may derive from an additionalconstant source of ions leading to a non-steady state

Long-term observations of cluster ion concentration, sources and sinks in clear sky conditions at the high-altitude site of the Puy de Dôme, France