1 FIRST RESULTS
The first physics results from CLOUD  were published in Nature, August 2011 . The measurements
represent the most rigorous laboratory evaluation yet accomplished of binary, ternary and ion-induced
nucleation of sulphuric acid/ammonia aerosol particles under atmospheric conditions. Several new findings
were reported. Firstly, CLOUD has shown that the most likely nucleating vapours, sulphuric acid
and ammonia, cannot account for nucleation in the lower atmosphere. The nucleation observed in the
chamber occurs at only one-tenth to one-thousandth of the rate observed in the lower atmosphere (Fig. 1).
In view of the CLOUD results, the treatment of aerosol formation in climate models will need to be substantially
revised since all models assume that nucleation is caused by these vapours and water alone.
Secondly, CLOUD has found that cosmic ray ionisation can substantially enhance nucleation of sulphuric
acid/ammonia particles—by up to a factor of 10. Ion-enhancement is particularly pronounced in
the cool temperatures of the mid-troposphere and above, where CLOUD has found that sulphuric acid
and water vapour can nucleate without the need for additional vapours.
The publication of the first CLOUD results in Nature  attracted wide interest in the media and scientific
press, including articles in The Economist ,Wall Street Journal, CERN Courier , Nature News, and
Nature Geoscience . The results were also reported at numerous scientific meetings, including an
invited plenary talk at the European Aerosol Conference, EAC2011, Manchester, September 2011.
The CLOUD4 and CLOUD5 runs in 2011 have revealed a number of important new findings
which are currently being analysed. Several high profile (Nature/Science) manuscripts are in preparation.
At least two of these will be submitted for publication before 31 July 2012. This date marks the
submission deadline for papers to be eligible for inclusion in the 5th Assessment Report (AR5) of the
Intergovernmental Panel on Climate Change (IPCC), which is due to be completed by the end of 2013.
For the first time, a chapter in AR5 will be devoted to “Clouds and Aerosols”, and it will contain a section
on the influence of galactic cosmic rays.
In addition to the high profile manuscripts, a further 20 papers are in preparation for submission
to Atmospheric Chemistry and Physics, an open access—and highly popular—Journal of the European
Geosciences Union. Several papers have already been published [6, 7, 8, 9] and about ten more are
expected to be submitted by mid 2012. Fourteen CLOUD abstracts have been submitted to the European
Aerosol Conference, EAC2012, Granada, September 2012.
Several new partners joined CLOUD during 2011, with special expertise as indicated:
Karlsruhe Institute of Technology, Germany: laboratory measurements of aerosols, liquid- and ice
clouds (AIDA facility).
University of Stockholm, Sweden: atmospheric aerosol growth from organic vapours.
Carnegie Mellon University, USA: atmospheric organic chemistry and aerosol growth.
The Memorandum of Understanding for the maintenance and operation of CLOUD has been finalised
and signed by the CERN Director for Research and Computing. It is currently being signed by
the 19 CLOUD partners (U Innsbruck, U Vienna, U Helsinki, Finnish Meteorological Institute, U Eastern
Finland, U Frankfurt, Karslruhe Institute of Technology, Institute for Tropospheric Research – Leipzig,
U Lisbon, Lebedev Physical Institute – Moscow, U Stockholm, CERN, PSI, U Leeds, U Manchester,
Caltech, Carnegie Mellon, Aerodyne Research – Billerica and TOFWERK – Thun).
During 2011, CLOUD Collaboration meetings and data workshops were held at the University of
Vienna, 14–18 February and Goethe University of Frankfurt, 25–30 September.
6 PHYSICS AIMS AND BEAM REQUEST 2012
The beam requests and experimental aims for 2012 are as follows:
CLOUD6: 4 June – 2 July 2012 (4 weeks): Commissioning of the adiabatic expansion system to
operate CLOUD in a classical Wilson expansion chamber mode for generation of liquid and ice clouds.
Instruments will be attached to CLOUD to measure, for the first time, the formation of liquid droplets
and ice particles inside the chamber. The purpose of this run is to prepare for future studies of the so
called “near-cloud” mechanism by which cosmic rays may directly influence cloud microphysics rather
than through the production of cloud condensation nuclei (Fig. 3) . Three spills per supercycle are
CLOUD7: 1 October – 3 December 2012 (9 weeks): Ion-induced and neutral nucleation and growth
of sulphuric acid particles in the presence of oxidised organic vapours from pinanediol and alpha-pinene
(Fig. 4). This represents a follow-up investigation of the new processes discovered during the CLOUD4
run, and a new study of aerosol growth up to the size of cloud condensation nuclei (CCN). Three spills
per supercycle are requested.
7 FUTURE PLANS
CLOUD (Fig. 5) is tackling one of the most challenging problems in atmospheric science—to understand
how new aerosol particles are formed and grow in the atmosphere, and the effect these particles have on
the global atmosphere and climate. CLOUD also aims to answer definitively the question of whether or
not galactic cosmic rays affect clouds and climate, either by affecting aerosols or by directly influencing
the microphysics of liquid or ice clouds. The contribution of aerosols and clouds is recognised by the
Intergovernmental Panel on Climate Change as the most important source of uncertainty in the radiative
forcing of climate change.
The present poor experimental understanding of aerosol nucleation and growth is preventing the
inclusion of physics-based mechanisms in global models, and limiting our understanding of how a major
fraction of atmospheric aerosol will influence future climate. Development of reliable atmospheric models
requires quantifying the fundamental physics and chemistry of nucleation in the laboratory, as well
as a clear connection between the laboratory and the real world through these models.
In the near-term, the 2012 scientific goals for CLOUD are well-defined. During the 2013 shutdown
of the CERN accelerators for the LHC energy upgrade, the CLOUD collaboration will likely devote a
major effort into analysis of the large quantity of high quality data already collected during the last 3
years. Nevertheless, depending on the results obtained later in 2012, it is likely that CLOUD will operate
at some stage during 2013 for special physics measurements that do not require a particle beam. We
expect to begin normal data-taking operations for CLOUD when the PS beams return in 2014. The
precise physics programme cannot be defined at this stage since it will depend on our findings during
For the first time we have with the CLOUD facility at CERN an experimental chamber of the highest
technological performance which has established itself as the world’s leading experiment for these
studies. CLOUD has also brought together a world-class team of atmospheric and modeling experts to
conduct the experiments, analyse and interpret the data, and assess the impact of the results with global
models. Nevertheless, a multi-parameter experimental phase space must be mapped, involving numerous
variables such as temperature, relative humidity, trace gases and their concentrations, ionisation, nucleation
rates, growth rates, droplet and ice particle activation, as well as liquid and ice cloud microphysics.
CLOUD has been in operation for just over two years and we estimate around ten more years will be
required to carry out the experimental programme.
The projected CLOUD experimental programme is therefore likely to extend well beyond the
planned upgrade of the East Hall beamlines. After several year’s experience in the T11 beamline we
would like to request that T11 be retained for CLOUD in the new East Hall beamline layout, ideally with
1–2 m additional space in the experimental zone in the direction towards T10. A dedicated T11 beamline
for CLOUD will maximise the efficiency and output of the experiment, and provide the maximum
availability of the T9 and T10 beamlines for test-beam users.
We would like to thank CERN PH-DT, EN-MME, EN-MEF and TE-VSC for their excellent support of
CLOUD and, in addition, to thank the CERN PS machine team and the PS Coordinator for their strong
support of CLOUD and for efficient operation of the PS.
 CLOUD Collaboration: A study of the link between cosmic rays and clouds with a cloud chamber
at the CERN PS.
CERN-SPSC-2000-021 (2000), <http://cdsweb.cern.ch/record/444592>
CERN-SPSC-2000-030 (2000), <http://cdsweb.cern.ch/record/462623>
CERN-SPSC-2000-041 (2000), <http://cdsweb.cern.ch/record/497173>
CERN-SPSC-2006-004 (2006). <http://cdsweb.cern.ch/record/923140>
 Kirkby, J., et al. Role of sulphuric acid, ammonia and galactic cosmic rays in atmospheric aerosol
nucleation. Nature 476, 429–433 (2011).
 The Economist. Clouds in a jar; a new experiment with old apparatus reveals a flaw in models of
the climate (27 August 2011). <http://www.economist.com/node/21526788>
 Kirkby, J. CLOUD: closing in on the initial steps of cloud formation. CERN Courier 51, 8, 28–31
(October 2011). <http://cerncourier.com/cws/article/cern/47208>
 Pierce, J. Particulars of particle formation. Nature Geoscience 4, 665–666 (2011).
 Kupc, A., et al.. A fibre-optic UV system for H2SO4 production in aerosol chambers causing
minimal thermal effects. J. Aerosol Sci. 42, 8, 532–543 (2011).
 Voigtl¨ander, J., Duplissy, J., & Stratmann, F. Numerical simulation of flow, H2SO4 cycle and new
particle formation in the CERN CLOUD chamber. Atmos. Chem. Phys. Discuss. 11, 20013–20049
 Bianchi, F., Dommen, J., Mathot, S., & Baltensperger, U. On-line determination of ammonia at low
pptv mixing ratios in the CLOUD chamber. Atmos. Chem. Phys. Discuss. 5, 2111–2130 (2012).
 Praplan, A.P., Bianchi, F., Dommen, J., & Baltensperger, U. Dimethylamine and ammonia measurements
with ion chromatography during the CLOUD4 campaign. Atmos. Chem. Phys. Discuss.
5, 2395–2413 (2012).
 Carslaw, K.S., Harrison, R.G., & Kirkby, J. Cosmic rays, clouds, and climate. Science 298, 1732–