A new experiment, known as CLOUD¹, began operation at the CERN PS in November 2009 [CERN Bulletin (2009)]. The CLOUD collaboration comprises 17 institutes from Europe, Russia and the United States, and brings together a diverse interdisciplinary team of atmospheric physicists and chemists, and space, cosmic ray and particle physicists [CLOUD Collaboration].

Over the next few years, CLOUD aims to carry out a quantitative investigation of the potential link between galactic cosmic rays, clouds and climate [CLOUD Collaboration (2000, 2004, 2006),Kirkby (2002)]. Since the cosmic ray flux is modulated by the variable solar wind, this could provide the long-sought mechanism for solar-climate variability; a mystery that has attracted the interest of scientists for more than two centuries since the observation by the Astronomer Royal, William Herschel, of acorrelation between sunspots and the price of wheat in England [Herschel (1801)].

The subject is currently of considerable interest since it addresses the open question of a possible unaccountedsolar-indirect contribution to climate change [Carslaw et al. (2002), Kirkby (2007),Carslaw (2009)]. Recent palaeoclimatic reconstructions show that the climate has frequently varied on 100-year time scales during the Holocene (last 10 kyr) by amounts comparable to the present warming and yet the forcing mechanism or mechanisms are not understood. Some of these reconstructions show clear associations with solar variability, which is recorded in the light radio-isotope archives (such as ¹⁴C in tree rings or ¹⁰Be in ice cores) that measure past variations of cosmic ray intensity [Eichler et al. (2009)]. However, despite the increasing evidence of its importance, solar-climate variability is likely to remain controversial until a physical mechanism is established.

Two different classes of mechanisms have been proposed to connect cosmic rays with clouds: firstly, an influence of cosmic rays on the production of cloud condensation nuclei (CCN)—the seeds of cloud droplets—and, secondly, an influence of cosmic rays on the global electrical circuit in the atmosphere and, in turn, on ice nucleation and other cloud microphysical processes. Considerable progress on understanding ion-aerosol-cloud processes has been made in recent years, and the results are suggestive of a physically-plausible link between cosmic rays, clouds and climate. However, definitive laboratory measurements of the fundamental physical and chemical processes involved have not yet been made, and their climatic significance is poorly known. CLOUD aims to fill this important gap in our scientific understanding of the natural agents of climate change.

The concept of the CLOUD experiment is simple: fill a large chamber with air containing selected trace gases, expose the chamber to an adjustable “cosmic ray” beam from the CERN PS and then analyse how the contents of the chamber change with time. In this way, the effect of the beam can be measured on the formation and growth of CCN from trace condensable vapours found in the atmosphere. In addition, by operating the chamber as a classical cloud chamber, cloud droplets or ice particles can be created and the influence of the beam can be measured on their microphysical properties and interactions.
Although the experimental concept of CLOUD is simple, the practice is not. The construction of the chamber and its associated equipment (Fig. 1) has required state-of-the-art technologies and a wide range of specialised CERN expertise in areas such as cryogenics, gas systems, UHV surface preparation and procedures, ultra-clean welding, ceramic-metal welding for the fibre optic feedthroughs, and the application of partially-conducting ceramics for the field cage standoffs. The participation of experts from PSI and from CERN PH-DT, EN-MME, EN-MEF and TE-VSC has been essential to resolve these challenges.

Fig.1: View of CLOUD in the East Hall T11 zone during the November–December 2009 run. The chamber is surrounded by a thermal housing through which air is circulated via the duct seen to the left. The temperature of the air is adjusted and precisely controlled by a 20 kW thermo-regulator unit seen at the far left. The pion beam, with large transverse dimensions of around 2x2 m², enters the chamber from the right, after passing through a counter hodoscope. The chamber is surrounded by analysing instruments which continuously extract small samples via probes that penetrate 50 cm into the chamber volume (27 m³) in the mid plane. The fibre optic light guides of the UV system can be seen passing up the outside of the thermal housing, slightly to the left of centre. The white box located on the shielding wall is monitoring the galactic cosmic ray intensity in the East Hall. Various other CLOUD equipment, such as the cryogenic ultrapure air supply, gas system and air circulation system are located either outside the building or underneath the platform and are therefore not seen in the image.

Experimental runs typically last up to ten hours or more before the contents of the chamber need to be flushed out in preparation for a new run. In order to reduce the wall losses of trace gases and aerosol particles to low enough rates for these long-duration experiments, a large aerosol chamber is needed. The CLOUD chamber dimensions are 3 m diameter and 4 m height. It is constructed from 316L stainless steel—electropolished on the inside surface—and equipped with various ports for instruments or personnel access (Fig. 2). The chamber can eventually be operated at any temperature between the warmest regions of the troposphere (40 ^οC) and the coldest regions of the polar stratosphere (-90 ^οC).

The trace gas concentrations are typically in the range of a few parts per billion—and in some cases less than one part per trillion. Condensable vapour backgrounds must be kept well below concentrations of one part per trillion which is up to a factor one million times more stringent than background requirements for gaseous drift chamber detectors. In order to meet this performance, extremely carefull attention has been paid to the preparation of the inner surfaces and welds of the CLOUD chamber and in the design of the gas and water supply systems.

Fig. 2:Fisheye image looking up into the CLOUD chamber in November 2009, before installing the lower manhole cover and final sections of the thermal housing. The image shows the fibre optic UV illumination from 240 individual optical feedthroughs, with reflections off the electropolished interior of the chamber, the field cage electrodes and standoffs, and four of the 50 external Pt100 thermometers. The chamber is maintained at a temperature stability of around 0.01 ^oC by a surrounding thermal housing through which precisely thermally-controlled air is circulated (Fig. 1); the duct on the right hand side of the image marks the entry point of the air at the bottom of the chamber. The chamber was operated between room temperature and 100 ^oC in 2009, but the eventual operation range will be -90 ^oC -> +100 ^oC, from Spring 2011 onwards

The chamber is equipped with fibre-optic illumination of UV light (for photolytic reactions) and an electric field cage to control the concentrations of small ions and charged aerosols when the beam is off (Fig. 2). During beam exposure, the contents of the chamber are continuously analysed by a suite of sensitive instruments connected to the chamber via sampling probes and (in future) optical ports (Fig. 1). For the 2009 run, three of the instruments were state-of-the-art mass spectrometers: a CIMS (measuring sulphuric acid vapour at 0.001 ppt sensitivity), a PTR-MS (organic vapours at 10–100 ppt sensitivity) and an API-ToF (ion masses in the range up to 2000 amu, at 0.001 amu resolution).

The first CLOUD measurements were carried out in 2006 with a pilot experiment, shortly after the approval of the project. The pilot experiment gave a first glimpse of ion-induced nucleation and, most importantly, provided invaluable technical input for the CLOUD design [Duplissy et al.(2009)].

After a three-year period for design and construction, CLOUD was installed in the T11 zone of the East Hall during Fall 2009. With the efforts of a strong and enthusiastic team of young physicists and chemists from the CLOUD institutes, and with superb support from CERN PH, EN, TE, GS and RP staff and contractors, the installation was completed by 14 November and the experiment began operation. A special extension of PS operations for CLOUD was granted by the CERN directorate, and CLOUD completed a very successful first run on 7 December, 2009. Within a few days of starting, the detector  showed excellent technical performance and had the feel of a precision instrument. Temperature stabilities of around 0.01 ^oC were achieved—a factor 10 better than the design requirement and background contaminations were measured around a factor 100 below other aerosol chambers. A large amount of high quality data were recorded (an example is shown in Fig. 3) and are currently being analysed in preparation for publication.

Fig. 3: An example of one of the measurements recorded by CLOUD in the first run during November–December 2009. The upper panel shows the time development of the aerosol particle concentration in the CLOUD chamber measured by a battery of Condensation Particle Counters (CPCs), each set to a different detection threshold of aerosol diameter in the range 2.5–7 nm (the modulation is due to an alternating HV on the electrostatic precipitator in the sampling line). The lower panel shows the time-development of the aerosol size spectrum measured in a Scanning Mobility Particle Sizer (SMPS), with a detection threshold of around 10 nm. The pion beam was turned on at 16h45 and produced a sharp increase in the aerosol nucleation rate, detected shortly afterwards. This is a clear demonstration of ion-induced nucleation although the beam intensity for this run was high and not representative of the atmosphere. The aerosol particles rapidly grew to diameters of around 50 nm, which is close to the threshold size to seed cloud droplets (lower panel). The run was ended at 17h50 and the chamber cleaned progressively in steps by alternately charging the aerosols with the beam and then sweeping them out with the electric field cage.

References

[CLOUD Collaboration]

University of Innsbruck, Institute of Ion Physics and Applied Physics, Austria

University of Vienna, Institute for Experimental Physics, Austria

Institute for Nuclear Research and Nuclear Energy, Sofia, Bulgaria

Helsinki Institute of Physics and University of Helsinki, Department of Physics, Finland

Finnish Meteorological Institute, Helsinki, Finland

University of Kuopio, Department of Applied Physics, Finland

Tampere University of Technology, Department of Physics, Finland

Goethe-University of Frankfurt, Institute for Atmospheric and Environmental Sciences, Frankfurt am Main, Germany

Leibniz Institute for Tropospheric Research, Leipzig, Germany

University of Lisbon, Department of Physics, Portugal

Lebedev Physical Institute, Solar and Cosmic Ray Research Laboratory, Moscow, Russia

CERN, Physics Department, Switzerland

Paul Scherrer Institut, Laboratory of Atmospheric Chemistry, Switzerland

University of Leeds, School of Earth and Environment, United Kingdom

University of Reading, Department of Meteorology, United Kingdom

Rutherford Appleton Laboratory, Space Science & Particle Physics Dept., United Kingdom

California Institute of Technology, Division of Chemistry and Chemical Engineering, USA.

[Carslaw et al. (2002)] Carslaw, K.S., Harrison, R.G., and Kirkby, J., Cosmic rays, clouds, and climate,Science, 298, 1732–1737, 2002.

[Carslaw (2009)] Carslaw, K.S., Cosmic rays, clouds, and climate, Nature, 260, 332–333, 2009.

[CERN Bulletin (2009)] CERN Bulletin,

On CLOUD nine http://cdsweb.cern.ch/record/1180849, 8 June 2009;

Happily CLOUDy http://cdsweb.cern.ch/record/1221077, 16 November 2009.

[CLOUD Collaboration (2000, 2004, 2006)] CLOUD Collaboration: A study of the link between cosmicrays and clouds with a cloud chamber at the CERN PS,

CERN-SPSC-2000-021  http://cdsweb.cern.ch/record/444592, 2000;

CERN-SPSC-2000-030 http://cdsweb.cern.ch/record/462623, 2000;

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[Duplissy et al. (2009)] Duplissy, J. et al., Results from the CERN pilot CLOUD experiment, Atmos.

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[Eichler et al. (2009)] Eichler, A., S. Olivier, K. Henderson, A. Laube, J. Beer, T. Papina, H.W.G¨aggeler

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may possibly be drawn from solar observations, J. Phil. Trans. Roy. Soc., 91, 265–283, 1801.

[Kirkby (2002)] Kirkby, J., CLOUD: a particle beam facility to investigate the influence of cosmic rays

on clouds, CERN-EP-2002-019 http://cdsweb.cern.ch/record/540780, 2002.

[Kirkby (2007)] Kirkby, J., Cosmic rays and climate, Surv. Geophys., 28, 333–375, 2007; CERN-PHEP-

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