Up In The Clouds

LABORATORY EXPERIMENTS have suggested that bacteria, fungal spores, cellular fragments, and other biological particles could nucleate ice crystals such as those found in clouds, but such particles hadn't been seen in actual ice clouds (C&EN, April 14, 2008, page 40). Measurements taken with an airborne mass spectrometer now confirm the presence of biological material in ice crystals in clouds over Wyoming, scientists report (Nat. Geosci., DOI: 10.1038/ngeo521).

"In the absence of having detected and identified biological particles in real ice clouds, the ice-nucleating potential of biological particles was just a plausible hypothesis," says Ulrike Lohmann, an atmospheric physicist at the Swiss Federal Institute of Technology, Zurich, commenting on work by chemistry professor Kimberly A. Prather and grad student Kerri A. Pratt of the University of California, San Diego; atmospheric scientist Paul J. DeMott of Colorado State University, Fort Collins; and coworkers. "These findings are a major advance in our knowledge," Lohmann says.

Prather's team used a newly developed airborne aerosol time-of-flight mass spectrometer aboard a C-130 aircraft to obtain simultaneous positive- and negative-ion mass spectra of the residues of evaporated cloud ice crystals. The negative-ion spectra are key for distinguishing biological particles from mineral dust particles, Prather says. Mineral dust accounts for about 50% of the ice crystal residues, and biological material accounts for about 33%. "I was surprised that dust and bioparticles were as dominant as they were," Prather says.

In the lab, biological particles are among the better ice nucleators, initiating ice formation at higher temperatures than other particles, DeMott says. "We don't know how important they are numberwise in contributing to ice formation in atmospheric clouds," he says. "We want to know whether they get widely dispersed around the globe."

"Understanding clouds is the largest uncertainty in our climate models right now," Prather says. "By understanding some of the key factors involved in forming the clouds, we can start to predict cloud formation better," she says.