atmosphere Review New Particle Formation: A Review of Ground-Based Observations at Mountain Research Stations Karine Sellegri 1,*, Clemence Rose 1, Angela Marinoni 2, Angelo Lupi 2 , Alfred Wiedensohler 3, Marcos Andrade 4,5 , Paolo Bonasoni 2 and Paolo Laj 6 1 Laboratoire de Météorologie Physique (LaMP), CNRS, Université Clermont Auvergne, F-63000 Clermont-Ferrand, France 2 Institute of Atmospheric Sciences and Climate (ISAC), CNR, 40129 Bologna, Italy 3 TROPOS, Permoserstr. 15, 04318 Leipzig, Germany 4 Laboratory for Atmospheric Physics, Institute for Physics Research, Universidad Mayor de San Andrés, La Paz, Bolivia 5 Department of Atmospheric and Oceanic Sciences, University of Maryland, College Park, MD 20742, USA 6 CNRS, IRD, G-INP, IGE, University of Grenoble Alpes, 38000 Grenoble, France * Correspondence:
[email protected] Received: 4 July 2019; Accepted: 13 August 2019; Published: 26 August 2019 Abstract: New particle formation (NPF) was predicted to contribute to a major fraction of free tropospheric particle number and cloud condensation nuclei (CCN) concentrations by global models. At high altitudes, pre-existing particle concentrations are low, leading to limited condensational sinks for nucleation precursor gases, and temperatures are cooler compared to lower altitudes, whereas radiation is higher. These factors would all be in favor of nucleation to occur with an enhanced frequency at high altitudes. In the present work, long term data from six altitude stations (and four continents) at various altitudes (from 1465 to 5240 m a.s.l) were used to derive statistically relevant NPF features (frequency, formation rates, and growth rates) and seasonal variability. The combined information together with literature data showed that the frequencies of NPF events at the two Southern hemisphere (SH) stations are some of the highest reported thus far (64% and 67%, respectively).