Cloud ice crystals carry biological matter

Ice crystals plucked from clouds and quickly analysed in flight show bits of biological material - bacteria, spores and plants - play a role in the formation of clouds, according to US researchers.

The finding, reported in the journal Nature Geoscience, offers the first direct evidence of airborne bacteria in clouds, they said.

Climate scientists typically rely on computer models to predict climate change, but until now it has been difficult to directly measure the composition of ice crystals in clouds, which are the very seeds that form clouds.

"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," Anne-Marie Schmoltner of the National Science Foundation's Division of Atmospheric sciences, said in a statement.

"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."

For this study, Kim Prather of Scripps Institution of Oceanography and the University of California San Diego loaded up an aircraft with a specially designed lab instrument called a mass spectrometer.

The team made a series of flights over the skies of Wyoming to analyse the chemical composition of ice-forming particles in clouds.

"The key to cloud formation is these little seeds that feed the clouds," said Dr Prather. "We are basically trying to understand what is forming clouds."

The team, lead by Dr Prather's graduate student Kerri Pratt, found that biological matter accounted for 33 per cent of the particles in ice crystals, and mineral dust accounted for 50 per cent.

"The big deal was to be able to measure the chemistry of each particle one at a time," Dr Prather said.

Other teams have captured ice crystals and examined their contents on the ground, she said, but by that time they had melted, and it was impossible to know for sure what they were analysing.

She said the ice pocket they measured contained Asian dust. The findings suggest biological particles that get swept up in dust storms help induce cloud formation.

It was impossible to tell whether bits of plant matter, fungal spores and bacteria they detected had been alive because the instrument they used "smashed them to bits" and analysed their composition, she said.

"They were potentially living. We can't say for sure."

But other teams have found living bacteria in studies of cloud water, she said. And some scientists suspect clouds may be capable of transporting viruses and bacteria and depositing them long distances in the form of rain and snow.

"It's almost like the biological material is hitching a ride along with the dust," she said.

Dr Prather said understanding the exact composition of clouds will help climate scientists better predict climate change, and may even lead to new ways of forming rain clouds to alleviate drought.

The study was funded by the National Science Foundation and the National Centre for Atmospheric Research.