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Mystery factor clouds climate science

Missing piece Climate scientists have discovered a mystery factor in climate change models, following new research at the CERN particle accelerator, near Geneva.

First results from the CLOUD (Cosmics Leaving Outdoor Droplets) experiment show that trace vapours in the atmosphere, which until now had been thought to account for all aerosol formation, actually only explain a minute proportion of atmospheric aerosol production.

The research, published in the journal Nature, also shows that ionisation from cosmic rays may play a significant role in the process.

Understanding how new aerosol particles form in the atmosphere, and the effect these particles have on climate, is one of the big challenges of atmospheric science.

The CLOUD team studied the nucleation of new particles in a special chamber with tightly controlled vapour concentrations under regulated temperature and humidity. The experiment was also the first of its kind to measure nucleation due to ionising cosmic rays, which reach Earth from outside the solar system. The amount that arrives at Earth is regulated by the Sun - greater solar output equals less cosmic radiation. To simulate the cosmic rays, the researchers used CERN's proton synchrotron to send a beam of particles into the reaction chamber.

Forming nucleation points

"We've made a number of first observations of some very important atmospheric processes", says the paper's lead author and CLOUD team spokesman, Jasper Kirkby.

Aerosols, tiny liquid or solid particles present in the atmosphere (such as dust, smoke particles and water vapour), act as nucleation points for water droplets and are thus important in cloud formation.

An increased concentration of aerosols in the atmosphere due to human activities is thought to offset some of the warming caused by the recent increase in greenhouse gases.

Additional aerosol particles would cool the climate by reflecting more sunlight and by forming additional cloud droplets, thus making clouds brighter and extending their lifetime.

Current estimates indicate that around half of all cloud droplets begin with the clustering of molecules present in low concentrations in the atmosphere. Some of these embryonic clusters eventually become large enough to seed cloud droplets. Trace amounts of sulphuric acid and ammonia are generally regarded as important in the process.

Cosmic ray effect

The CLOUD results show that a few kilometres up in the atmosphere sulphuric acid and water vapour can rapidly form clusters. The research also showed that cosmic ray ionisation can accelerate the formation ten-fold or more.

"We've found that cosmic rays significantly enhance the formation of aerosol particles in the mid troposphere and above. These aerosols can eventually grow into seeds for clouds," he says.

But the authors say that within about one kilometre of the Earth's surface additional vapours such as ammonia are needed.

The CLOUD results also show that even with the action of cosmic rays, sulphuric acid, ammonia and water vapour are not enough to explain actual observations of aerosol formation. This, the team says, means as yet unknown additional vapours must be involved in the process.

"It was a big surprise to find that aerosol formation in the lower atmosphere isn't due to sulphuric acid, water and ammonia alone," Kirkby says.

"Now it's vitally important to discover which additional vapours are involved, whether they are natural or of human origin, and how they influence clouds," he says. "This will be our next job."

The CLOUD group also aims to investigate the extent to which cosmic rays affect cloud cover; and to understand how solar activity may affect climate change.