New particle formation and ultrafine charged aerosol climatology at a high altitude site in the Alps (Jungfraujoch, 3580 m a.s.l., Switzerland)

摘要

We investigate the formation and growth of charged aerosols clusters atJungfraujoch, in the Swiss Alps (3580 m a.s.l.), the highest altitudesite of the European EUCAARI project intensive campaign. Charged particlesand clusters (0.5–1.8 nm) were measured from April 2008 to April 2009 and allowed the detection of nucleation events in this very specific environment (presence of free tropospheric air and clouds). We found that the naturally charged aerosol concentrations, which are dominated by the cluster size class, shows a strong diurnal pattern likely linked to valley breezes transporting surface layer ion precursors, presumably radon. Cosmic rays were found not to be the major ion source at the measurement site. However, at night, when air masses are more representative of free tropospheric conditions, we found that the cluster concentrations are still high. The charged aerosol size distribution and concentration are strongly influenced by the presence of clouds at the station. Clouds should be taken into account when deriving high altitude nucleation statistics. New particle formation occurs on average 17.5% of the measurement period and shows a weakseasonality with a minimum of frequency during winter, but this seasonalityis enhanced when the data set is screened for periods when the atmosphericstation is out of clouds. The role of ions in the nucleation process wasinvestigated and we found that the ion-mediated nucleation explains 22.3%of the particle formation. The NPF events frequency is correlated with UVradiation but not with calculated H₂SO₄ concentrations,suggesting that other compounds such as organic vapors are involved in thenucleation and subsequently growth process. In fact, NPF events frequencyalso surprisingly increases with the condensational sink (CS), suggestingthat at Jungfraujoch, the presence of condensing vapours probably coupledwith high CS are driving the occurrence of NPF events. A strong link to theair mass path was also pointed out and events were observed to be frequentlyoccurring in Eastern European air masses, which present the highestcondensational sink. In these air masses, pre-existing cluster concentrationsare more than three time larger than in other air masses during event days,and no new clusters formation is observed, contrarily to what is happening inother air mass types.