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 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.


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.


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."


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