Abstract
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.
Highlights
In polluted areas, atmospheric aerosol particles often affect visibility and have undesirable effects on human health (Seaton et al, 1995)
The purpose of this section is only to discuss new particle formation (NPF) event frequency and type at the Puy de Dôme (PDD) station in order to further examine the behaviour of cluster ions on event and non-event days
The monthly mean NPF event frequency at the Puy de Dôme is presented in Fig. 1, initially considering the whole data set
Summary
Atmospheric aerosol particles often affect visibility and have undesirable effects on human health (Seaton et al, 1995). On a more global scale, aerosol particles influence the Earth’s climate system by scattering and absorbing incoming solar radiation (direct effect) and by affecting several cloud properties (indirect effect). A better understanding of the indirect effect requires, in particular, more accurate information on secondary aerosol particle sources and on the nucleation process. Measurements, as well as recent model investigations, suggest that atmospheric nucleation is an important source of aerosol particles and cloud condensation nuclei (CCN) (Kerminen et al, 2012; Makkonen et al, 2012) but the very first steps of the nucleation process remain uncertain. The formation and growth of ultrafine aerosol particles in the atmosphere has been studied during recent decades in various locations (see Kulmala et al, 2004 for a review) but the mechanisms involved in the particle formation are still unclear, mostly because of instrumental limitations
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