That smell has a name. It's called petrichor. For decades, scientists have speculated on where this smell comes from, but the mechanism behind the phenomenon has remained elusive. Now, researchers at MIT studying high-speed video of raindrops think they've found the answer: The smell is released in tiny aerosol clouds that raindrops emit upon impact.
Researchers, led by MIT mechanical engineer Cullen R. Buie, used high-speed cameras to show that, when a raindrop hits a porous surface, tiny bubbles of air are trapped at the point of contact. A fraction of a second later, the bubbles issue forth, rising up and out of the drop in the form of a fine spray:
Using high-speed cameras, the researchers observed that when a raindrop hits a porous surface, it traps tiny air bubbles at the point of contact. As in a glass of champagne, the bubbles then shoot upward, ultimately bursting from the drop in a fizz of aerosols.
The researchers suspect that in natural environments, aerosols may carry aromatic elements, along with bacteria and viruses stored in soil. These aerosols may be released during light or moderate rainfall, and then spread via gusts of wind.
"Rain happens every day — it's raining now, somewhere in the world," says Cullen R. Buie, an assistant professor of mechanical engineering at MIT. "It's a very common phenomenon, and it was intriguing to us that no one had observed this mechanism before."
Youngsoo Joung, a postdoc in Buie's lab, adds that now that the group has identified a mechanism for raindrop-induced aerosol generation, the results may help to explain how certain soil-based diseases spread.
"Until now, people didn't know that aerosols could be generated from raindrops on soil," Joung says. "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."