Cloud Droplet Formation Rate Not Affected By Pollution
February 19, 2013

Pollution Does Not Change The Rate Of Droplet Formation In Clouds

April Flowers for - Your Universe Online

A little bit of oily and viscous organic material doesn't seem to matter much when it comes to forming the droplets that make up clouds. This is good news for reducing the uncertainty of climate model predictions.

For accurate climate modeling, understanding cloud formation is essential. This understanding has to start with droplet formation, which occurs when water vapor is attracted to particles floating in the atmosphere. These particles include dust, sea salt from the ocean, microorganisms, soot, sulfur — and organic material that can be both viscous and oily.

Scientists have believed for years particles coated with this organic "goop" — produced by combusted petroleum and biomass — could form droplets more slowly than other particles. This could have a significant impact on cloud formation.

A new study, led by the Georgia Institute of Technology, suggests the long-held theory isn't true. Based on aerial and ground-based measurements of droplet formation from ten different areas of the northern hemisphere, the research team revealed organic coatings on particles don't seem to significantly affect the rate at which droplets form. To gain a thorough understanding, the team studied a wide range of particles, including organic, hydrocarbon-rich particles from the 2010 Deepwater Horizon oil spill in the Gulf of Mexico.

"It turns out that it doesn't matter how much goop you have — or don't have — the droplets take the same time to form," said Athanasios Nenes, professor in the School of Earth and Atmospheric Sciences and the School of Chemical and Biomolecular Engineering at Georgia Tech. "Even in extreme environments like Deepwater Horizon, the rate of droplet formation on particles found over the spill doesn't differ from that of typical sea salt particles."

Contributing to climate warming, clouds can hold in heat emitted from the Earth's surface. They can also reflect sunlight, producing a cooling effect. Predicting changes in how cloud cover will change in the future is therefore essential to good climate modeling.

The reason we care about droplet formation rates is because the more slowly the droplets form, the more droplets you end up having in clouds," Nenes said. "This, in turn, affects cloud properties and their climate impacts. For many years, there was the perception that having a lot of oily organic compounds from pollution would make water uptake a lot slower and might make droplets take longer to form. If that were true, it would mean that the impact pollution could have on clouds and climate would be much larger than we thought."

The scientists began a series of studies using a mini cloud formation chamber small enough to be operated aboard an aircraft. This formation chamber consists of a long metal tube that is heated at one end, cooled at the other, and the walls are kept moist. Air, containing particles from outside the plane, is allowed to flow through. When air in the chamber becomes cool enough that it can no longer retain the moisture, droplets will form and then exit the chamber so they can be studied.

"With the chamber, we essentially create a cloud in a tube," Nenes said. "The difference between the cloud in the tube and the cloud outside is that the tube allows us to precisely control the temperature and the amount of water vapor available. We know exactly what is going on with that cloud, and this allows for very accurate measurements of cloud formation."

Nenes and his team took the chamber on ten missions operated by NASA, NSF, NOAA, and ONR, flying through the pristine air of the Arctic, smoke from forest fires in Canada and polluted air masses over the United States.

"We have literally hundreds of hours of data studying cloud formation from areas all over the globe," Nenes said. "We didn't see any changes in the droplet nucleation time scale."

Going forward, Nenes and his team would like to study particles from other areas of the world, especially China and Africa. Another step in his research would be to see what happens when the temperature of the air flowing through the cloud chamber is cold enough to form ice. Some evidence suggests the kinetics of ice formation may be different in particles that are rich in "goop."

"This is good for atmospheric and climate scientists, because some of the uncertainty of droplet formation and aerosol impacts goes away," Nenes added. "With careful measurements and global deployment of measuring instruments, you can actually resolve outstanding questions in cloud physics and help simplify the descriptions of clouds in climate models."

The findings of this study were published this week in Proceedings of the National Academy of Science.