Scientists find why rain gives off fresh, earthy smell

MIT scientists have identified the mechanism that releases an earthy smell in the air after a light rain.
Using high-speed cameras, the researchers at the Massachusetts Institute of Technology observed that when a raindrop hits a porous surface, it traps tiny air bubbles at the point of contact.
As in a glass of champagne, the bubbles then shoot upward, ultimately bursting from the drop in a fizz of aerosols.
The team was also able to predict the amount of aerosols released, based on the velocity of the raindrop and the permeability of the contact surface.
The researchers suspect that in natural environments, aerosols may carry aromatic elements, along with bacteria and viruses stored in soil. These aerosols may be released during light or moderate rainfall, and then spread via gusts of wind.

Youngsoo Joung, a postdoc in the lab of Cullen R Buie, an assistant professor of mechanical engineering at MIT, said that now that a mechanism for raindrop-induced aerosol generation has been identified, the results may help to explain how certain soil-based diseases spread.

"Until now, people didn't know that aerosols could be generated from raindrops on soil," Joung said.
Buie and Joung conducted 600 experiments on 28 types of surfaces: 12 engineered materials and 16 soil samples.
In the lab, the researchers measured each soil sample's permeability by first pouring the material into long tubes, then adding water to the bottom of each tube and measuring how fast the water rose through the soil. The faster this capillary rise, the more permeable the soil.

In separate experiments, the team deposited single drops of water on each surface, simulating various intensities of rainfall by adjusting the height from which the drops were released. The higher the droplet's release, the faster its ultimate speed.
Joung and Buie set up a system of high-speed cameras to capture raindrops on impact. The images they produced revealed a mechanism that had not previously been detected: As a raindrop hits a surface, it starts to flatten; simultaneously, tiny bubbles rise up from the surface, and through the droplet, before bursting out into the air.
Depending on the speed of the droplet, and the properties of the surface, a cloud of "frenzied aerosols" may be dispersed.

"Frenzied means you can generate hundreds of aerosol droplets in a short time - a few microseconds," Joung said.
"And we found you can control the speed of aerosol generation with different porous media and impact conditions," Joung added.
From their experiments, the team observed that more aerosols were produced in light and moderate rain, while far fewer aerosols were released during heavy rain.
Buie said this mechanism may explain petrichor - a phenomenon first characterised by Australian scientists as the smell released after a light rain.