Researchers say they've used high-speed photography to show the origin of the familiar earthy, sweet smell that lingers in the air following a rainstorm.
Scientists call that aroma petrichor, and have long ascribed it to chemicals and oils in soil released as aerosols when raindrops hit the ground.
Now researchers at MIT say they've created super-slow-motion footage to demonstrate how that "rain smell" moves from the ground into the air.
"It's a very common phenomenon, and it was intriguing to us that no one had observed this mechanism before," says professor of mechanical engineering Cullen R. Buie.
"Rain happens every day -- it's raining now, somewhere in the world," he says.
When a raindrop impacts a porous surface, the researchers found, it traps tiny bubbles of air that then shoot upwards like the bubbles in a glass of champagne, ultimately bursting out of the raindrop in a fizz of aerosols.
Those aerosols contain aromatic elements that can be released by light rainfall and spread by winds, they say.
More aerosols are produced by light or moderate rainfall than during heavy rainfall, which is why the familiar petrichor odor is more commonly apparent after a light shower, they add.
"Heavy rain [has a high] impact speed, which means there's not enough time to make bubbles inside the droplet," says postdoctoral researcher Youngsoo Joung.
The scientists filmed raindrops falling on a variety of surfaces, including 16 kinds of soil and 12 engineered materials.
Identifying a mechanism for raindrop-induced generation of aerosols may help to explain how some kinds of soil-based diseases are spread, Joung says.
"Until now, people didn't know that aerosols could be generated from raindrops on soil," he 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."
The MIT scientists are conducting further experiments with surfaces containing soil bacteria and pathogens, including E. coli, to see if rainfall and its aerosol-generating mechanism can spread them.
"Aerosols in the air certainly could be resulting from this phenomenon," Buie says. "Maybe it's not rain, but just a sprinkler system that could lead to dispersal of contaminants in the soil, for perhaps a wider area than you'd normally expect."
The results of the MIT study have been published in the journal Nature Communications.
