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Research / Discovery

Amount of dust, pollen matters for precipitation in clouds, climate change

June 7, 2010

Large numbers of dust and pollen particles in the atmosphere may make your nose twitch, but when lifted to the heights where clouds form they can lead directly to greater precipitation in some clouds, Colorado State University atmospheric scientists have discovered.

Ice crystals linked to aerosol particles

Paul DeMott in the NCAR plane used in his research.

Formation of ice crystals is necessary for precipitation formation in many clouds, and the numbers of crystals formed is linked to the abundance of larger aerosol particles in the atmosphere, according to a study led by Paul DeMott and Anthony Prenni, research scientists in the Atmospheric Science department at Colorado State, appearing in this week’s issue of the Proceedings of the National Academy of Sciences.

Precipitation and cloud dynamics

Special particles called ice nuclei – with both natural and human sources - serve as catalysts to form ice in clouds. Precipitation falling as rain has its source in these ice crystals or agglomerates of these in some clouds. Variation in the numbers of ice nuclei due to changes in processes affecting emissions at the Earth’s surface or human influences, such as industrial pollution, can thus affect precipitation and the properties of clouds that determine their warming or cooling impacts on climate.

“The sources of most ice nuclei are primary emissions – from windblown desert and soil dust, certain bacteria or other organisms released from plant matter or in sea spray, dust, forest fire smoke, soot or metallic pollution particles, and even volcanic ash,” DeMott said. “Only a small proportion of these particles can act as ice nuclei, but the bigger the particles, the better it is for ice nuclei.”

Clouds have a stronger effect on cooling than previously thought

At the present time, many climate models assume that ice forms at a particular temperature in the atmosphere, regardless of whether ice nuclei are present. This assumption is too simple for climate models to accurately represent what’s occurring in nature, DeMott said.

Using DeMott’s new findings, global climate model simulations conducted by collaborator Xiaohong Liu of the U.S. Department of Energy’s Pacific Northwest National Laboratory predicted that clouds have a stronger cooling effect on the globe than previously estimated. However, future increases in these ice nuclei for cold clouds would reduce the cooling impact on climate and vice versa, the scientists found.

Scientists have spent decades trying to understand the processes. The National Science Foundation, the National Oceanic and Atmospheric Administration, the U.S. Department of Energy and NASA have funded Colorado State’s research in this area.

Collected air samples across the globe

Tony Prenni is part of the team that analyzed 14 years of air samples from in and around clouds.

DeMott and Prenni analyzed data from 14 years of trips across the globe - from the Amazon Rainforest in Brazil to the Arctic to Broomfield, Colo. - to collect air samples, sometimes in specially equipped National Center for Atmospheric Research planes.

The Colorado State scientists also developed the first instrument –that could be used inside a plane to take continuous air samples from in and around clouds and measure in real time the ice-forming ability of particles. The instrument allows the researchers to sample air and detect the total number concentrations of ice nuclei - without first putting them on a filter or other processing.

Capturing air from a plane

CSU scientists take air samples into a small chamber through a special port on the side of a C-130 plane. A diffusion chamber cools and humidifies the air and particles between two plates of ice toward conditions where ice forms, essentially "growing" clouds by simulating the conditions in the atmosphere.

Researchers then evaluate how many particles will form ice crystals for specific cloud conditions. The plane then passes through the wave clouds to measure, with other instruments, how much ice really forms.

Chemical makeup of particles forming ice

Scientists also used specialized instruments to determine the chemical makeup of the particulates forming ice.

“Ice nuclei are hard to measure – they’re microns in size like the size of a bacteria,” Prenni said. “They don’t make haze – there aren’t enough of them. Of all the particles in the atmosphere, one in a million particles in the atmosphere can cause ice to form.” [More]

Contact: Emily Wilmsen
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