Aerosol generation after drop impingement on porous media is a three-step process, consisting of bubble formation, bubble growth, and bubble bursting. (Image courtesy of Youngsoo Joung/MIT)
I don’t know about you but I have eaten sand/soil moistened by early rain after a harrowingly dry and hot Indian summer. Growing up in Ahmedabad and living through temperatures often touching between 112 to 114 degrees Fahrenheit the first monsoon rains always caused Earth to emit that irresistible fragrance. Now thanks to scientists at the Massachusetts Institute of Technology (MIT) we know what releases that fragrance.
In a story reported for MIT News by Jennifer Chu, it is revealed that a light rainfall in the aftermath of a hot dry summer releases aerosols trapped in the earth’s surface through a mechanism that the scientists have identified.
Cullen R. Buie, an assistant professor of mechanical engineering at MIT, and Youngsoo Joung, a post doctoralstudent in Buie’s lab, conducted some 600 experiments on 28 types of surfaces: 12 engineered materials and 16 soil samples, according to the story. “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.",” the report says.
The story does not quite say it but from firsthand experience I can tell you that this fragrant emissions happen only after the first spell of rains. It does not continue to happen as the monsoon/rainy season progresses. I think a prolonged dry spell is necessary for this effect to happen.
Apart from making you want to eat the top soil the earthy smell that people experience is also known to whet an unusual appetite for certain kinds of food. In the context of where I grew up the preferred food combination soon after the first wet spell was “uni uni rotli aney karela nu shakh”, fresh, hot tortilla-like thin Gujarati bread (rotli) made from wheat flour and curried bitter gourd. I am fairly sure other regions have their own combinations.
That earthy fragrance is one of those enduring ones that never really leave your olfactories. On reading the MIT story I could smell it this morning. Although snow is also essentially water, snowfall does not cause that smell because I presume it requires that the surface is hit with some force in order that aerosols are unlocked and released.