Abstract
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.
Highlights
The Intergovernmental Panel on Climate Change (IPCC) claims that the largest source of uncertainty in anthropogenic radiative forcing of the climate is due to increased aerosol since pre-industrial times and its effect on clouds (Myhre et al, 2013)
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
Most of the increased aerosol has resulted from anthropogenic precursor vapours that, after oxidation in the atmosphere, can form particles which may grow into new cloud condensation nuclei (CCN)
Summary
The Intergovernmental Panel on Climate Change (IPCC) claims that the largest source of uncertainty in anthropogenic radiative forcing of the climate is due to increased aerosol since pre-industrial times and its effect on clouds (Myhre et al, 2013). Theoretical considerations and early measurements (Lovejoy, 2004) indicated a strong temperature dependence for the nucleation rates of sulfuric acid particles. This has been confirmed by CLOUD, for example in Kirkby et al (2011) and Kürten et al (2016). Expansion chambers are used to study in-cloud processes such as the homogeneous freezing of super-cooled liquid droplets (Möhler et al, 2003) or cloud droplet chemistry (Jurányi et al, 2009) These experiments require the formation of cloud droplets or ice particles on CCN in the chamber. We present a study of the temperature uniformity and accuracy achieved in the CLOUD chamber under (a) ideal conditions (with no deliberate additional heat) and (b) operational conditions (where additional heat is introduced into the chamber by UV light and warm gases)
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