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Ajit Johnson

Cancer biologist, University of Edinburgh

.
May 31, 2017 · 259 Reads

What gives rain the fresh earthly smell?

Do you love the fresh earthy smell of rain? Surprise!, it is not the smell of soil. The smell actually has a name and is called “petrichor,” from the Greek words “petra,” meaning “stone,” and “ichor,” which refers to the fluid that flows like blood in the veins of the gods.

The petrichor is formed due to a blend of three components 1) Compounds secreted by soil-dwelling bacteria, 2) Plant based oils and 3) Ozone.

Aroma produced by soil dwelling bacteria

Actinobacteria– a soil dwelling bacteria that secretes an organic compound called geosmin. Geosmin is responsible for the distinct earthy aroma and the earthy taste of beets. The bacteria secrete the compound when they produce spores (typically during long dry spells as a mechanism for survival).

When it rains, the force of rain landing on the ground carries these spores and the aromatic compounds up into the air, which is later inhaled by us. Geosmin is often most pronounced when it rains for the first time in a while because the largest supply of spores has collected in the soil. Studies have also revealed that the human nose is extremely sensitive to geosmin.

Plant based oils

Some plants secrete oils during dry periods, and when it rains, these oils are released into the air. It is thought that these oils are secreted to inhibit seed germination as a mechanism to limit competition for scarce water supplies during dry times.

Ozone

When someone says they can smell rain coming, it is probably because of this. A lightning bolt’s electrical charge can split oxygen and nitrogen molecules in the atmosphere, and they often recombine into nitric oxide (NO), which then interacts with other chemicals in the atmosphere to produce ozone, which has a sharp smell faintly reminiscent of chlorine. It may be that wind from an approaching storm has carried ozone down from the clouds and into the person’s nostrils.

Now you know the reason behind the sweet, fresh, powerfully evocative smell of fresh rain after a dry spell that invariably hits you.

References: Nature, Nature communication, Nature

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