The role of ions in new particle formation in the CLOUD chamber
Authors: Wagner Robert , Yan Chao , Lehtipalo Katrianne , Duplissy Jonathan , Nieminen Tuomo , Kangasluoma Juha , Ahonen Lauri R. , Dada Lubna , Kontkanen Jenni , Manninen Hanna E. , Dias Antonio , Amorim Antonio , Bauer Paulus S. , Bergen Anton , Bernhammer Anne-Kathrin , Bianchi Federico , Brilke Sophia , Mazon Stephany Buenrostro , Chen Xuemeng , Draper Danielle C. , Fischer Lukas , Frege Carla , Fuchs Claudia , Garmash Olga , Gordon Hamish , Hakala Jani , Heikkinen Liine , Heinritzi Martin , Hofbauer Victoria , Hoyle Christopher R. , Kirkby Jasper , Kürten Andreas , Kvashnin Alexander N. , Laurila Tiia , Lawler Michael J. , Mai Huajun , Makhmutov Vladimir , Mauldin III Roy L. , Molteni Ugo , Nichman Leonid , Nie Wei , Ojdanic Andrea , Onnela Antti , Piel Felix , Quéléver Lauriane L. J. , Rissanen Matti P. , Sarnela Nina , Schallhart Simon , Sengupta Kamalika , Simon Mario , Stolzenburg Dominik , Stozhkov Yuri , Tröstl Jasmin , Viisanen Yrjö , Vogel Alexander L. , Wagner Andrea C. , Xiao Mao , Ye Penglin , Baltensperger Urs , Curtius Joachim , Donahue Neil M. , Flagan Richard C. , Gallagher Martin , Hansel Armin , Smith James N. , Tomé António , Winkler Paul M. , Worsnop Douglas , Ehn Mikael , Sipilä Mikko , Kerminen Veli-Matti , Petäjä Tuukka , Kulmala Markku 
Published: December 2017

Abstract. The formation of secondary particles in the atmosphere accounts for more than half of global cloud condensation nuclei. Experiments at the CERN CLOUD (Cosmics Leaving OUtdoor Droplets) chamber have underlined the importance of ions for new particle formation, but quantifying their effect in the atmosphere remains challenging. By using a novel instrument setup consisting of two nanoparticle counters, one of them equipped with an ion filter, we were able to further investigate the ion-related mechanisms of new particle formation. In autumn 2015, we carried out experiments at CLOUD on four systems of different chemical compositions involving monoterpenes, sulfuric acid, nitrogen oxides, and ammonia. We measured the influence of ions on the nucleation rates under precisely controlled and atmospherically relevant conditions. Our results indicate that ions enhance the nucleation process when the charge is necessary to stabilize newly formed clusters, i.e., in conditions in which neutral clusters are unstable. For charged clusters that were formed by ion-induced nucleation, we were able to measure, for the first time, their progressive neutralization due to recombination with oppositely charged ions. A large fraction of the clusters carried a charge at 1.5 nm diameter. However, depending on particle growth rates and ion concentrations, charged clusters were largely neutralized by ion–ion recombination before they grew to 2.5 nm. At this size, more than 90 % of particles were neutral. In other words, particles may originate from ion-induced nucleation, although they are neutral upon detection at diameters larger than 2.5 nm. Observations at Hyytiälä, Finland, showed lower ion concentrations and a lower contribution of ion-induced nucleation than measured at CLOUD under similar conditions. Although this can be partly explained by the observation that ion-induced fractions decrease towards lower ion concentrations, further investigations are needed to resolve the origin of the discrepancy.