Wild is the Wind

Aircraft-mounted instrument makes first-ever direct measurements of biological particles in ice clouds

A UC San Diego-led science team and a particle analyzer named "Shirley" are behind the first detection ever of biological particles -- pieces of bacteria, fungi, and plants --— in cold ice clouds.

Atmospheric chemistry Professor Kim Prather, who holds a joint appointment at Scripps Institution of Oceanography and the Department of Chemistry and Biochemistry at UCSD, described the find as a key step forward toward explaining how airborne particles, or aerosols, affect cloud formation. That dynamic has been considered one of the top challenges of scientists who try to simulate future climate through the use of computer models, she said.

"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," said Prather. "Any new piece of information you can get is critical."

Prather and her graduate student Kerri Pratt installed Shirley, an aircraft aerosol time-of-flight mass spectrometer (A-ATOFMS), onto a C-130 operated by the National Center for Atmospheric Research. (Shirley's partner "Laverne" was back at the UCSD campus with other ATOFMS units "Jake" and "Elwood" and newer units.) The aircraft flew through ice and liquid clouds over Wyoming during a series of flights in fall 2007, channeling the residues of ice crystals and cloud droplets to Shirley's intake valves. The mass spectrometer collected second-by-second data about the chemical composition of these cloud residues, providing information about cloud condensation nuclei and ice nuclei, the aerosols around which moisture condenses. This was the first aircraft deployment of the A-ATOFMS co-designed by Prather.

Analysis of the ice crystal cores revealed that they were made up almost entirely of either dust or biological particles such as bacteria, fungal spores and plant material. While it has long been known that microorganisms or parts of them get airborne and travel great distances, this study is the first to yield in-situ data on their participation in cloud ice processes.

Unclear from this experiment is whether any of the biological matter included living organisms, a matter of key interest to scientists trying to understand how bacteria can propagate around the world. Also to be seen is how much the microbial particles and dust affect rainfall, sometimes in places thousands of miles away from where the aerosols originated.

Prather said initial evidence is increasingly suggesting that dust transported from Asia could be influencing precipitation in North America, for example. Researchers hope to use the ICE-L data to design future studies timed to events when such particles may be playing a bigger role in triggering rain or snowfall.

—Robert Monroe

June 2009