May 29, 2024

With the first rains falling in Mumbai, the air is filled with this pleasing, earthy smell of the rain which I love. This smell is inherently loved by all humans and has inspired many poems and songs but what causes this smell?

This smell associated with rainfall is known as petrichor. Bacteria, plants and even lightning plays a role in the production of this divine smell.

Bacteria

Streptomyces, a common genus of bacteria, are found in both rural and urban areas and as well as marine environments. These tiny organism’s primary function is to decay dead and decaying organic and release nutrients for other plants and organisms. The byproduct of this process is an organic compound known as geosmin. This complex compound is a type of alcohol which has such a strong scent that our noses can detect just a few parts of geosmin per trillions of air molecules.

During the dry periods, due to the lack of moisture in the air and soil, the decomposition activity of these bacteria slow down. As the rains approach, the air and soil become more humid and the bacteria’s activity rate increase and so more geosmin is produced. During the rains, this geosmin is dissolved in the raindrops and is ejected in an aerosol form which is carried by the wind to our noses. 

Plants

Often chemicals and oils produced by plants which have an appealing fragrance are carried to our noses in a similar way as the geosmin. Additionally, rain also breakdown dry plant materials which release strong scents like when you crush dried herbs. This adds to the geosmin scent making it more alluring.

Lightning Bolts

Another component that makes up this earthy petrichor is ozone. During a thunderstorm, lightning or other electrical discharges in the air split up oxygen and nitrogen molecules in the atmosphere. Most of these atoms pair back when the air cools down while the rest reshuffle. Some oxygen atoms (O) combine with oxygen molecules (O2) producing ozone (O3) which has a very distinctive smell.

All these smells combined gives us our petrichor smell which we love. The smell is so pleasing that petrichor scented perfumes and air fresheners are also available.

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