Raindrops kick up soil chemicals

Researchers suss out possible explanation for rain-driven aroma

BUBBLING UP Soon after a water droplet hits the ground, tiny air bubbles form. The bubbles ascend through the squashed drop and pop, releasing jets of water and chemicals (yellow arrows) into the air.

Courtesy of Youngsoo Joung

By Andrew Grant

February 10, 2015 at 3:00 pm

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A raindrop doesn’t just go splat when it hits the ground. A fizz emanates from each drop, a new study published January 14 in Nature Communications reveals, transporting chemicals from the ground into the air. This mechanism may create the earthy aroma after a rainstorm.

Using high-speed cameras on water droplets falling onto synthetic materials, MIT mechanical engineers Cullen Buie and Youngsoo Joung were surprised to see a cascade of tiny air bubbles that rose through each compressed droplet just after contact and burst. Buie immediately thought of the smell after a quick rainstorm and wondered whether these little bubbles could carry that aroma. So Buie and Joung imaged droplets striking soil and, sure enough, observed the same fizz. Subsequent experiments demonstrated that when the air bubbles pop, they release chemicals from the soil.

A 1964 study proposed that the earthy smell following a storm, called petrichor, comes from chemicals in the soil. Until now, nobody had explained how those chemicals got released into the air. Buie and Joung are now exploring whether rain could spread soilborne microbes or pesticides.

SPARKLING RAIN High-speed imaging reveals a fizz of water and gases generated after a water droplet lands on a permeable surface such as soil.

Credit: MIT

Editor’s note: This story was updated on March 13, 2015, to clarify that the 1964 study noted that the smell comes from chemicals in the soil. The paper did not, as previously stated, link the smell to chemicals from plants and bacteria in soil.