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The formation of new aerosol particles in the atmosphere is a key processinfluencing the aerosol number concentration as well as the climate, inparticular at high altitude, where the newly formed particles directlyinfluence cloud formation. However, free tropospheric new particle formation(NPF) is poorly documented due to logistic limitations and complexatmospheric dynamics around high-altitude stations that make the observationof this day-time process challenging. Recent improvements in measurementtechniques make now possible the detection of neutral clusters down to~ 1 nm sizes, which opens new horizons in our understanding ofthe nucleation process. Indeed, only the charged fraction of clusters hasbeen reported in the upper troposphere up to now. Here we report day-timeconcentrations of charged and neutral clusters (1 to 2.5 nm mobilitydiameter) recorded at the interface between the boundary layer (BL) and theFT as well as in the FT at the altitude site of Puy de Dôme  (1465 m a.s.l.), central France, between 10 and 29 February  2012. Ourfindings demonstrate that in the FT, and especially at the interface betweenthe BL and the FT, the formation of 1.5 nm neutral clusters significantlyexceeds the one of ionic clusters during NPF events, clearly indicating thatthey dominate in the nucleation process. We also observe that the totalcluster concentration significantly increases during NPF events compared tothe other days, which was not clearly observed for the charged clusterpopulation in the past. During the studied period, the nucleation processdoes not seem to be sulfuric acid-limited and could be promoted by thetransport of pollutants to the upper troposphere, coupled with lowtemperatures

Major contribution of neutral clusters to new particle formation at the interface between the boundary layer and the free troposphere