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High-speed video shows how everyone’s favorite after-rain smell is created

A study recently published in the journal Nature Communications reveals that water droplets release clouds of tiny particles called aerosols when they make contact with porous surfaces, explaining why many people can detect the distinctive earthy aroma in the air after a light rain.

“Rain happens every day — it’s raining now, somewhere in the world,” said co-author Cullen Buie, an assistant professor of mechanical engineering at the Massachusetts Institute of Technology (MIT), in a recent statement. “It’s a very common phenomenon, and it was intriguing to us that no one had observed this mechanism before.”

The research team of Buie and Youngsoo Joung, a postdoctoral researcher in Buie’s laboratory, utilized high-speed cameras to observe that when a raindrop hits a porous surface such as soil, the droplet traps tiny air bubbles at the point of contact, which then shoot upward and burst to release aerosols into the environment.

“Until now, people didn’t know that aerosols could be generated from raindrops on soil,” said Joung. “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.”

According to MIT, Buie and Joung conducted approximately 600 experiments on 28 types of surfaces, which were comprised of 16 soil samples and 12 engineered materials. In separate experiments, they dropped single drops of water on each surface from varying heights to simulate different rainfall intensities, with larger heights meaning faster droplet speeds. The team found that more aerosols were produced in light and moderate rain compared to during heavy rain.

The researchers believe that this mechanism may explain petrichor, a phenomenon that describes the smell released after a light rain. “They talked about oils emitted by plants, and certain chemicals from bacteria, that lead to this smell you get after a rain following a long dry spell,” said Buie. “Interestingly, they don’t discuss the mechanism for how that smell gets into the air. One hypothesis we have is that that smell comes from this mechanism we’ve discovered.”