25 August 2011

Cosmic rays may influence cloud formation

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The very first steps of cloud formation have been revealed in unprecedented detail, exposing gaps in current global climate models.
The CLOUD experiment

The CLOUD experiment at CERN. Credit: CERN

GENEVA: The very first steps of cloud formation have been revealed in unprecedented detail, exposing gaps in current global climate models.

The CLOUD experiment at the European Organisation for Nuclear Research (CERN) was designed to study the effect of cosmic rays on the formation of atmospheric aerosols.

The first results have revealed something unexpected: that the suite of trace vapours currently used by climate models to predict aerosol formation in the lower atmosphere is incomplete, and can only explain a tiny fraction of the observed atmospheric aerosol production.

“Half of all seeds for cloud droplet formation come from nucleation [clustering] of trace atmospheric vapours, but these processes are poorly understood, to the extent that – as this paper shows – we’re not even sure what all of the gases are,” said lead author Jasper Kirkby from CERN in Geneva, Switzerland. “We’ve found that [in addition to sulphuric acid and ammonia] there has to be another vapour (or vapours) involved, which has a controlling influence.”

What’s in a CLOUD?

Atmospheric aerosols, which are the tiny solid or liquid particles suspended in the atmosphere, play a crucial role in cloud formation. Trace ammonia and sulphuric acid vapours form clusters with water molecules, and can eventually grow large enough to seed cloud droplets.

Higher atmospheric concentrations of aerosols are thought to promote brighter, longer-lived clouds, but estimations of the net cooling effect of aerosols on the Earth’s climate are subject to larger uncertainties than any other contributing factor.

The CLOUD study is the first in the experiment’s quest to quantitatively answer a question that has been bugging atmospheric scientists since the 1970s: How do cosmic rays influence cloud droplet formation and, consequently, climate?

The measurements were performed within the unique CLOUD facility – a three-metre stainless steel chamber filled with ultrapure humidified air and exact concentrations of trace gases to replicate different parts of the atmosphere. Beams of particles from CERN’s Proton Synchrotron accelerator were used to perfectly mimic cosmic rays passing through the gas, which was continuously sampled through various ports and probes.

Cosmic enhancing and organic additives

The researchers found ionisation by cosmic rays to enhance aerosol production rates by up to 10 times. But they were surprised to learn that ammonia and sulphuric acid vapours alone could nowhere near account for the concentrations of atmospheric aerosols seen in the lower atmosphere, even with the newly-quantified cosmic ray enhancement effect factored in.

“What we’ve found in this first experiment … is that organic vapours have to be involved in the nucleation process [in the low atmosphere]. It’s the first time that’s really been established unambiguously, and now we need to urgently identify what those vapours are,” said Kirkby. This next step is already underway, he said, in collaboration with a swathe of field researchers who provide the CLOUD team with suggested identities and concentrations of molecules to investigate.

Just the beginning

Atmospheric scientist Renyi Zhang of Texas A&M University in the U.S., who was not involved in the research, pointed out that in previous studies of how aerosols form in the atmosphere, “a lot of times the data have been extremely conflicting.”

He said the state-of-the-art CLOUD set-up is able to shed light on the controversial issue of which species play a role in these processes under atmospheric conditions, and added: “It’s extremely important for atmospheric modellers to put that into climate models. Right now, people just don’t have a good understanding of how aerosols form in the atmosphere.”

Ultimately, CLOUD will try to answer the question of whether cosmic rays have a climatically significant effect on clouds and, in turn, whether an element of climate variability might then be attributed to the Sun – the source of solar wind and galactic cosmic rays. “I’m absolutely sure that we’ll settle this question, but it’s going to take five or ten years to do so,” said Kirkby. “This is just the beginning.”

Ceri Perkins is a part-time employee at the CERN ATLAS experiment.
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