Articles | Volume 10, issue 12
Research article
22 Dec 2017
Research article |  | 22 Dec 2017

Temperature uniformity in the CERN CLOUD chamber

António Dias, Sebastian Ehrhart, Alexander Vogel, Christina Williamson, João Almeida, Jasper Kirkby, Serge Mathot, Samuel Mumford, and Antti Onnela

Abstract. The CLOUD (Cosmics Leaving OUtdoor Droplets) experiment at CERN (European Council for Nuclear Research) investigates the nucleation and growth of aerosol particles under atmospheric conditions and their activation into cloud droplets. A key feature of the CLOUD experiment is precise control of the experimental parameters. Temperature uniformity and stability in the chamber are important since many of the processes under study are sensitive to temperature and also to contaminants that can be released from the stainless steel walls by upward temperature fluctuations. The air enclosed within the 26 m3 CLOUD chamber is equipped with several arrays (strings) of high precision, fast-response thermometers to measure its temperature. Here we present a study of the air temperature uniformity inside the CLOUD chamber under various experimental conditions. Measurements were performed under calibration conditions and run conditions, which are distinguished by the flow rate of fresh air and trace gases entering the chamber at 20 and up to 210 L min−1, respectively. During steady-state calibration runs between −70 and +20 °C, the air temperature uniformity is better than ±0.06 °C in the radial direction and ±0.1 °C in the vertical direction. Larger non-uniformities are present during experimental runs, depending on the temperature control of the make-up air and trace gases (since some trace gases require elevated temperatures until injection into the chamber). The temperature stability is ±0.04 °C over periods of several hours during either calibration or steady-state run conditions. During rapid adiabatic expansions to activate cloud droplets and ice particles, the chamber walls are up to 10 °C warmer than the enclosed air. This results in temperature differences of ±1.5 °C in the vertical direction and ±1 °C in the horizontal direction, while the air returns to its equilibrium temperature with a time constant of about 200 s.

Short summary
The CERN CLOUD chamber is used to understand different processes of particle formation in the atmosphere. This information can be used by global climate models to update the influence of cloud formation. To provide the most accurate information on these processes, a thorough understanding of the chamber is necessary. Temperature measurements were performed inside the entire volume of the CLOUD chamber to ensure temperature stability and more accurate estimations of particle formation parameters.