by David Danelski,University of California - Riverside
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Tiny, invisible gases long thought to be irrelevant in cloud formation may actually play a major role in determining whether clouds form—and possibly whether it rains.
That's the surprising finding from a new UC Riverside-led study, which runs counter to more than a century of assumptions about the physics behind cloud droplet formation. Published in the journalScience Advances,the researchreveals that removing common trace gases from air samples significantly alters how water vapor condenses into cloud droplets.
"This was not something we expected to see," said Markus Petters, a co-author of the study and an atmospheric chemist in UCR's Bourns College of Engineering. "It turns out that these volatile organic compounds—trace gases—can either help or hinder the ability of tiny particles in the atmosphere to become cloud droplets."
Clouds begin to form when water vapor rising into cooler air condenses around aerosol particles—tiny bits of salt, dust, or pollution suspended in the air.
These particles act as "cloud condensation nuclei," or CCN. For a droplet to form around a CCN, the air must be supersaturated, meaning it is just over 100% humidity, such as 100.1% or 100.5%, and the threshold of supersaturation depends on the size and chemical makeup of the CCN.
But Petters and UCR doctoral student Elavarasi Ravichandran, who is the first author of the study, investigated a third factor—trace gases like organic acids, which exist in the atmosphere at low concentrations. When these gases were removed from test samples using a charcoal-based scrubber, the ability of particles to form droplets changed dramatically.
Source: Phys.org
