Measurement report: Molecular-level investigation of atmospheric cluster ions at the tropical high-altitude research station Chacaltaya (5240 m a.s.l.) in the Bolivian Andes

30 Air ions are the key components for a series of atmospheric physicochemical 31 interactions, such as ion-catalyzed reactions, ion-molecule reactions, and ion-induced 32 new particle formation. They also control atmospheric electrical properties with effects 33 on global climate. We performed molecular-level measurements of cluster ions at the 34 high-altitude research station Chacaltaya (CHC; 5240 m a.s.l.), located in the Bolivian 35 Andes, from January to May 2018 using an atmospheric pressure interface time-of- 36 flight mass spectrometer. The negative ions mainly consisted of (H 2 SO 4 ) 0-3  HSO 4- , 37 (HNO 3 ) 0-2  NO 3- , SO 5- , (NH 3 ) 1-6  (H 2 SO 4 ) 3-7  HSO 4- , malonic acid-derived, and 38 CHO/CHON  (HSO 4- /NO 3- ) cluster ions. Their temporal variability exhibited distinct 39 diurnal and seasonal patterns due to the changes in the corresponding neutral species’ 40 molecular properties (such as electron affinity and proton affinity) and concentrations 41 resulting from the air masses arriving at CHC from different source regions. The 42 positive ions were mainly composed of protonated amines and organic cluster ions, but 43 exhibited no clear diurnal variation. H 2 SO 4 -NH 3 cluster ions likely contributed to the 44 new particle formation process, particularly during wet-to-dry transition period and dry 45 season when CHC was more impacted by air masses originating from source regions 46 with elevated SO 2 emissions. Our study provides new insights into the chemical 47 composition of atmospheric cluster ions and their role in new particle formation in the 48 high-altitude mountain environment of the Bolivian Andes.