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Why Does Rainfall Leave Behind a Scent?

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Rainfall has a particular smell, and now scientists know what causes that familiar odor of fresh precipitation.

Massachusetts Institute of Technology (MIT) researchers believe they can identify the process which creates the earthy aroma, as well as other aerosols, into the air.

Raindrops were studied as they hit the ground, using high-speed cameras. When the droplets hit the ground, images revealed that tiny air bubbles trapped under the raindrop rise up through the water, like bubbles in soda. Aerosols were then releases as these tiny pockets of air burst through the top of the raindrop. The velocity at which raindrops strike the ground, as well as the structure of the surface, were found to play a role in the quantity of aerosols released by the actions.

Around 600 runs of the experiment were conducted, on 28 types of surfaces, including 16 soil samples and 12 man-made materials. The research team measured the permeability of different soil specimens through placing samples in tubes, placing water underneath the soil, and recording the length of time the liquid took to rise to the top.

Aerosols that come up through raindrops can include bacteria and other microbes previously stored in soil.

"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," Youngsoo Joung, a postdoctoral student at MIT, said.

Greater quantities of aerosols were found to be released during light and moderate rain events, compared to heavier downpours, the study revealed. Water droplets from heavy rains tend to hit the ground hard, not allowing enough time for a network of air bubbles to form before the liquid is dispersed.

Petrichor is a term first coined by Australian researchers to describe a mixture of smells, including bacteria and plant oils, that could be responsible for the smell of rain.

"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." Cullen Buie, assistant professor of mechanical engineering at MIT, stated in a university press release.

Researchers already knew that aerosols can be trapped and released when raindrops strike the surface of water, but this study was the first to show that effect on land.

Aerosol Generation by Raindrop Impact on Soil, detailing the experiment detailing the function of aerosols and raindrops in creating the unique odor of rain, was published in the journal Nature Communications.

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