The CLOUD facility
KL Katrianne Lehtipalo  CY Chao Yan LD Lubna Dada FB Federico Bianchi MX Mao Xiao RW Robert Wagner DS Dominik Stolzenburg LA Lauri R. Ahonen AA Antonio Amorim AB Andrea Baccarini PB Paulus S. Bauer BB Bernhard Baumgartner AB Anton Bergen AB Anne-Kathrin Bernhammer MB Martin Breitenlechner SB Sophia Brilke AB Angela Buchholz SM Stephany Buenrostro Mazon DC Dexian Chen XC Xuemeng Chen AD Antonio Dias JD Josef Dommen DD Danielle C. Draper JD Jonathan Duplissy ME Mikael Ehn HF Henning Finkenzeller LF Lukas Fischer CF Carla Frege CF Claudia Fuchs OG Olga Garmash HG Hamish Gordon JH Jani Hakala XH Xucheng He LH Liine Heikkinen MH Martin Heinritzi JH Johanna C. Helm VH Victoria Hofbauer CH Christopher R. Hoyle TJ Tuija Jokinen JK Juha Kangasluoma VK Veli-Matti Kerminen CK Changhyuk Kim JK Jasper Kirkby JK Jenni Kontkanen AK Andreas Kürten ML Michael J. Lawler HM Huajun Mai SM Serge Mathot RI Roy L. Mauldin III UM Ugo Molteni LN Leonid Nichman WN Wei Nie TN Tuomo Nieminen AO Andrea Ojdanic AO Antti Onnela MP Monica Passananti TP Tuukka Petäjä FP Felix Piel VP Veronika Pospisilova LQ Lauriane L. J. Quéléver MR Matti P. Rissanen CR Clémence Rose NS Nina Sarnela SS Simon Schallhart SS Simone Schuchmann KS Kamalika Sengupta MS Mario Simon MS Mikko Sipilä CT Christian Tauber AT António Tomé JT Jasmin Tröstl OV Olli Väisänen AV Alexander L. Vogel RV Rainer Volkamer AW Andrea C. Wagner MW Mingyi Wang LW Lena Weitz DW Daniela Wimmer PY Penglin Ye AY Arttu Ylisirniö QZ Qiaozhi Zha KC Kenneth S. Carslaw JC Joachim Curtius ND Neil M. Donahue RF Richard C. Flagan AH Armin Hansel IR Ilona Riipinen AV Annele Virtanen PW Paul M. Winkler UB Urs Baltensperger MK Markku Kulmala  DW Douglas R. Worsnop

The CLOUD chamber (16, 17) is a temperature-controlled stainless steel cylinder with a volume of 26.1 m3 located at CERN, Geneva, Switzerland. To ensure cleanliness, all inner surfaces of the chamber are electropolished. Before each campaign, the chamber was rinsed with ultrapure water and subsequently heated to 373 K. While cooling down to operating temperature, the chamber was flushed with humidified synthetic air containing several ppmv (parts per million by volume) of ozone. Thus, the background total VOC concentration is in the sub-ppbv level (39) and the contamination from condensable vapors is mostly below the detection limit of our instruments [sub-pptv (15)]. A sophisticated gas supply system was used to carefully control the amounts of trace gases added to the chamber.

A high voltage field cage (±30 kV) inside the chamber can be switched on to remove all ions from the chamber (referred to as “neutral conditions,” N). When the electric field is off, natural GCRs are creating ions in the chamber, as is the situation in the atmosphere. This is referred to as “GCR conditions.” Ion concentrations in the chamber can be artificially increased by using the pion beam from the CERN Proton Synchrotron (3.5 GeV/c). This is called “π conditions” (not used in this study).

The chamber was equipped with several UV light systems. In all the experiments described in this study, so-called UVH light (4 × 200 W Hamamatsu Hg-Xe lamps producing light in the wavelength range of 250 to 450 nm) was used to produce OH. In CLOUD10, additionally, a UV laser (4-W excimer laser; KrF, 248 nm) was used in some of the experiments to achieve higher H2SO4 concentrations. Between the CLOUD10 and CLOUD11 campaigns, the intensity of the UVH light was increased by renewing and shortening the optical fibers, which deliver the light into the chamber. Therefore, the use of the UV laser was not necessary, as the UVH system could supply the same wavelengths. In CLOUD11, also a UV-sabre (400-W UVS3, centered on 385 nm) was available, with the main purpose to form NO from NO2. Thus, the NO/NO2 ratio could be controlled by changing the UV-sabre light intensity. The NO2 photolysis frequency, jNO2, was characterized using NO2 actinometry and varying the UV-sabre intensity. In CLOUD10, we injected NO directly into the chamber (leading to a constant NO/NO2). More details of the facility can be found elsewhere (16, 17).

The instruments used to record chamber conditions, gas and particle concentration, as well as methods to calculate particle formation and GRs were similar to previous CLOUD publications, and they are described in Supplementary Materials and Methods.

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