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Slow-mo video of raindrops reveals how rain gets its distinctive smell

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Petrichor: It's a great word and an even better smell — the one that hangs in the air after a rain storm. By filming raindrops in super-slow-motion, MIT researchers think they've figured out how this smell works.

It turns out that raindrops release little bursts of aerosols, or fine particles of liquid suspended in gas.Just as the aerosols released by champagne bubbles make the wine smell and taste distinctive, the aerosols released when rain hits the ground may be sending out signature smells that get carried away on the wind. The study was published this week in Nature Communications.

“Until now, people didn’t know that aerosols could be generated from raindrops on soil,” Youngsoo Joung, a postdoctoral student who worked on the research, said in a statement. “This finding should be a good reference for future work, illuminating microbes and chemicals existing inside soil and other natural materials, and how they can be delivered in the environment, and possibly to humans.”

But heavy rain actually produces fewer aerosols than a light or medium rainfall, according to their findings. So that might be why petrichor is associated with the moments after a drizzle. When the physics of a raindrop hitting a porous surface — like wet soil — are just right, it might release a frenzy of aromatic plant oils and bacteria that sit in the dirt.

This could also explain how soil microbes spread from place to place, the researchers said. James Bird, an assistant professor of mechanical engineering at Boston University who wasn't involved in the study, told MIT that the findings could solve some long-standing mysteries.

"Microbes from soil have been observed high in the atmosphere; this paper provides an elegant mechanism by which these microbes can be propelled past the stagnant layer of air around them to a place where the breeze can take them elsewhere," Bird said.

Further study will help the researchers determine whether or not petrichor really is a sign that soil microbes are on the move.

With the help of high-speed cameras, researchers at the Massachusetts Institute of Technology were able to figure out how rainfall produces a distinctive, earthy smell. (Video: Reuters)