CLOUD chamber

The CLOUD chamber is situated at the European Organization for Nuclear Research (CERN), in the East Hall, at the end of one of the beamlines from the proton synchrotron. The chamber is an electropolished stainless steel cylinder with a volume of 26.1 m3. The main feature of the CLOUD chamber is the extraordinarily precise control over physical and chemical conditions which can be achieved. Experiments can be performed at temperatures between 183 and 300 K, with temperature variation within the chamber being less than 0.1 K. The chamber can be exposed to a 3.5 GeV/c secondary π+ beam from the CERN Proton Synchrotron, spanning the galactic cosmic-ray intensity range from ground level to the stratosphere, for the investigation of the influence of ionization on particle formation. Liquid or ice clouds can be created in the chamber, by controlling the pressure and performing rapid adiabatic expansions.

Chamber air is created by evaporation of cryogenic liquid N2 and O2, at a ratio of 79:21, and humidity is controlled using a Nafion humidifier, with water purified by recirculation through a bank of Millipore Super-Q filters and irradiated with ultraviolet radiation to suppress biological activity. Each trace gas used is added through a dedicated gas line to prevent cross-contamination, and to avoid contamination from plastic materials, all gas piping is made from stainless steel, with metal seals (gold coated to render them chemically inert). UV lighting for photochemistry experiments is introduced via optical fibres and a temperature controlled set of light emitting diodes, so as to ensure minimal transfer of heat to the chamber. The result is that the CLOUD chamber is the cleanest and most stable smog chamber in the world, with any contaminants in the chamber typically present in concentrations below the detection limits of the instrumentation.

The CLOUD chamber is operated by a collaboration of more than 20 research institutions worldwide (including the Paul Scherrer Institute), who meet at CERN once a year for 6-12 week experimental campaigns. The advantage of this operational approach is that each collaboration partner can contribute a suite of state-of-the-art instrumentation for the quantification of gas and particle composition, and little investment in permanent instrumentation is necessary. The huge range of instrumentation which is typically deployed at CLOUD make these the most precisely observed and quantified experiments in the world. Over the last years, this effort has led to fundamental advances in the understanding of the formation and growth of aerosol particles.

This includes the proof that sulfuric acid and water alone are not enough to explain observed new particle formation rates in the boundary layer, and that the addition of ammonia, although it enhances the nucleation rate, is also not sufficient. Furthermore, the effect of ionizing radiation on particle formation was quantified (Kirkby et al. Nature)

In a subsequent work, it was found that adding small amounts of amines to the mixture of sulfuric acid, water and ammonia in the CLOUD chamber, would lead to nucleation rates similar to those observed in the boundary layer (Almeida et al 2013). The addition of oxidation products of biogenic gases (pinanediol) was then also shown to enhance nucleation rates sufficiently to explain those observed in the atmosphere (Riccobono et al 2014).

Most recently, owing to the extremely low contaminant level in the CLOUD chamber, it was possible for the first time to prove that new particle formation in the atmosphere was possible entirely without the contribution of sulfuric acid (Kirkby et al 2016). These laboratory observations were simultaneously confirmed with atmospheric measurements (Bianchi et al 2016). This finding has significant implications for our understanding of atmospheric particulate concentrations in the pre-industrial period, before anthropogenic enhancements of sulfuric acid occurred (Gordon et al 2016). The role of highly oxidised organic molecules in the growth of newly formed particles was also quantified (Tröstl et al 2016).

The extraordinary nature of these rapid advances in our understanding of atmospheric aerosols make CLOUD the most successful aerosol experiment ever run.

References

Molecular understanding of sulphuric acid-amine particle nucleation in the atmosphere
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New particle formation in the free troposphere: A question of chemistry and timing
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Ion-induced nucleation of pure biogenic particles
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Oxidation Products of Biogenic Emissions Contribute to Nucleation of Atmospheric Particles
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