May 19, 2009 The climate change debate has focused our collective attention on the importance of understanding the complex workings of our planet's weather system, but there is still much we don't know. In this latest breakthrough, a UC San Diego-led team of atmospheric chemistry researchers has made the first-ever direct detection of biological particles within ice clouds. By providing insights into, for example, how particles from Asia effect rainfall in North America, the research aims to shed light on one of the most uncertain factors of climate modeling and enhance our understanding of atmospheric cooling and regional precipitation.

Biological Materials and cloud formation

In the first aircraft-based deployment of the aircraft aerosol time-of-flight mass spectrometer (A-ATOFMS), the team led by Kerri Pratt, a Ph.D. and Professor Kim Prather, identified biological material in sampled water droplet and ice crystal residues taken at high speeds from a specially outfitted C-130 aircraft flying over Wyoming.

Clouds form with the aid of aerosols (airborne particles) ranging from dust, soot, sea salt to organic materials. Water and ice in the atmosphere condense around these and grow, leading to precipitation. It is a clearer understanding of this process that the research seeks to discover.

"If we understand the sources of particles that nucleate clouds and their relative abundance, then we can determine the impact of these different sources on climate," said Pratt.

The findings have led to the discovery that not only mineral dust but also biological particles play a major role in the formation of clouds. Of particular interest is that the region of origin of the biological particles appears to play an important part in cloud formation.

It has been known for some time that microorganisms or parts of them get airborne and travel great distances; however this study is the first to yield in-situ data on their participation in cloud ice processes.

"By sampling clouds in real time from an aircraft, these investigators were able to get information about ice particles in clouds at an unprecedented level of detail," said Anne-Marine Schmoltner of the NSF's Division of Atmospheric Sciences.

Using modeling techniques and the chemical composition of measured dust, the ICE-L team identified the source of the dust as Asia or Africa. "This has really been kind of a holy grail measurement for us," said Prather. "Understanding which particles form ice nuclei, which occur at extremely low concentrations and are inherently difficult to measure, means you can further understand processes that result in precipitation. Any new piece of information you can get is critical."

The findings not only suggest that the biological particles that get swept up in dust storms help to induce the formation of cloud ice, but their region of origin makes a difference. The evidence suggests that the dust transported from Asia could be influencing precipitation in North America, for example, says Prather.

Results of these studies appear in the May 17 in the advance online edition of the journal Nature Geoscience .

The Ice in Clouds Experiment - Layer Clouds (ICE-L) was funded by the National Science Foundation(NSF) and the National Center for Atmospheric Research (NCAR).

David Greig

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