The fresh aroma after a summer downpour is one of life's pleasing scents.

The earthy fragrance was named petrichor by Australian researchers in 1964. They described it as a combination of plant oils and the chemical compound geosmin which are released from the soil when it rains.

Now researchers from Massachusetts Institute of Technology (MIT), USA, believe they have captured this event happening and shown how the smell gets into the air.

No one had observed this mechanism before

Using high-speed cameras the scientists observed how raindrops trap tiny air bubbles as they hit the ground.

They say the bubbles then shoot upwards through the raindrop and erupt into a fizz, producing extremely fine liquid droplets or solid particles that remain suspended in the air as fog or smoke, known as aerosols.

The results are published in the journal Nature Communications

The authors suspect that the tiny particles that are released into the environment not only spread noticeable aromatic elements from the soil but also bacteria and viruses stored there.


They found that more particles were produced by light or moderate rainfall compared with heavy downpours, which are then spread by the wind.

“Rain happens every day — it’s raining now, somewhere in the world,” says Cullen Buie, 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.”

Scientists have already shown that raindrops can trap and release aerosols when falling on water, but this is said to be the first time the process has been seen happening on soil.

The results may help to explain how some soil-based diseases are spread and the authors are now studying whether contaminants such as E. coli can be spread significantly via rainfall.

Video produced and edited by Melanie Gonick/MIT, high-speed droplet footage by Youngsoo Joun, music sampled from "Running Waters" by Jason Shaw.

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