Particles Found in High Clouds Help Predict Climate Changes

"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-Marie Schmoltner of the National Science Foundation's Division of Atmospheric Sciences, which funded the research.

"By determining the chemical composition of the very cores of individual ice particles, they discovered that both mineral dust and, surprisingly, biological particles play a major role in the formation of clouds," she said.

The effects of tiny airborne particles called aerosols on cloud formation have been some of the most difficult aspects of weather and climate for scientists to understand.

Wave cloud over Wyoming (Photo by Mirasoccer)

Aerosols, ranging from dust, soot, and sea salt to organic materials, some of which travel thousands of miles, form the skeletons of clouds.

Around these nuclei, water and ice in the atmosphere condense and grow, leading to rain and snow. Scientists are trying to understand how the nuclei form, as clouds play a critical role by both cooling the atmosphere and affecting regional precipitation.

The interactions between aerosols and clouds have been what scientists consider the greatest uncertainty in making climate predictions, which are mainly projected from computer simulations of climate phenomena.

The airborne team, led by Kimberly Prather and Kerri Pratt of the University of California at San Diego, Scripps Institution of Oceanography, sampled water droplet and ice crystal residues at high speeds.

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

Scientists have known that microorganisms become airborne and travel long distances, but this study is the first to yield direct data on how they work to influence cloud formation.

Results of the Ice in Clouds Experiment - Layer Clouds (ICE-L) appeared Sunday in the advance online edition of the journal "Nature Geoscience."

The ICE-L team mounted a mass spectrometer onto a C-130 aircraft operated by the National Center for Atmospheric Research in Boulder, Colorado, and made a series of flights through a type of cloud known as a wave cloud.

The researchers performed measurements of cloud ice crystal residues and found that half were mineral dust and about a third were made up of inorganic ions mixed with nitrogen, phosphorus and carbon - the signature elements of biological matter.

The second-by-second speed of the analysis allowed the researchers to make distinctions between water droplets and ice particles. Ice nuclei are rarer than droplet nuclei.

The team demonstrated that both dust and biological material indeed form the nuclei of these ice particles, something that previously could only be simulated in laboratory experiments.

"This has really been kind of a holy grail measurement for us," said Prather.

"Understanding which particles form ice nuclei, and which have extremely low concentrations and are inherently difficult to measure, means you can begin to understand processes that result in precipitation. Any new piece of information you can get is critical," she said.

The findings suggest that the biological particles that get swept up in dust storms help to induce the formation of cloud ice, and that their region of origin makes a difference.

Evidence is increasing that dust transported from Asia could be influencing precipitation in North America, the researchers suggest.

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