Do you always know before and after it rains simply because of the smell? Now, researchers have discovered the origin of the earthy, sweet smell that lingers in the air. And though scientists have been baffled by the source of this aroma, known as Petrichor, for many years now researchers from MIT have found its origin with the help of high-speed photography.
The study, published this week in the journal Nature Communications, used super-slow-motion footage created by researchers to demonstrate the way the smell of rain 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" professor of mechanical engineering and lead researcher of the study, Cullen R. Buie says. "Rain happens every day - it's raining now, somewhere in the world."
The researchers observed that when a raindrop hits a porous surface, tiny bubbles of air are trapped into it, which then shoot upwards like the bubbles in glass of champagne. These bubbles eventually burst from the raindrop, creating a sort of fizz of aerosols. These aerosols are then picked up and spread by the winds, bringing the smell to noses everywhere.
According to the study, light and moderate rainfall produces more aerosols than heavy rain, explaining the earthy smell one can pickup before and after a light shower. Postdoctoral researcher involved in the study, Youngsoo Joung says that "heavy rain has a high impact speed, which means there's not enough time to make bubbles inside the droplet."
Researchers filmed raindrops falling on a variety of very different surfaces include sixteen types of soil, as well as 12 manmade materials. Joung believes that understanding how the aerosols generated by rain are generated and spread could help with the understanding of the spread of disease. "Until now, people didn't know that aerosols could be generated from raindrops on soil. 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."
MIT researchers plan to carry out further studies with surfaces containing soil bacteria and other pathogens to see if or how this mechanism of spreading aerosols affects the spread of bacteria and pathogens, as well.
"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."
