MIT researchers believe they’ve identified the mechanism that gives us the elusive aroma of new rain.
To many of us it ranks high in the order of favorite smells, though we may not know the name. Petrichor: that deeply pleasant, earthy fragrance that wafts through the air as rain first hits the ground.
Named by Australian researchers in 1964 – from the Greek ichor, the ethereal fluid that flowed in the veins of Greek gods, and petros, for stone – petrichor is described as a combination of plant oils and the chemical compound geosmin (what we know as the smell of loamy dirt), which are released from the soil when it drizzles.
Now researchers at MIT believe they may have identified the mechanism that sends this magical mix of molecules, as well as other aerosols, into the environment.“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.”
Buie and researcher Youngsoo Joung set up a system of high-speed cameras to see what happens to raindrops upon impact, and discovered something that had not been previously detected. When a drop hits a surface, it begins to flatten out and traps tiny air bubbles at the point of contact. The bubbles then shoot up through the drop and burst into the air with a fizz. Think of a glass of champagne. And depending on how quickly the drop hits the surface – and the properties of the surface itself – a cloud of frenzied aerosols are dispersed. And no, “frenzied aerosols” is not a case of this writer being poetic. That’s what the researchers call it.
“Frenzied means you can generate hundreds of aerosol droplets in a short time — a few microseconds,” Joung explains. “And we found you can control the speed of aerosol generation with different porous media and impact conditions.”
The scientists believe that in natural environments, these aerosols may carry the aromatic elements that we humans find so beguiling.
Referring to the Australian researchers who first characterized the aroma, Buie says, “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,” Buie says. “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.”
So it looks like we like can thank the fizz and frenzy of raindrops for liberating the ground's unique fragrance into the air for all to smell. You can see the mechanism in action in the MIT video below.