Behind petrichor’s fragrance

Petrichor, the first drop of rain that falls on the earth, brings the joy of water, along with the fragrance of the soil. Scientists speculate that humans may have developed an affinity for the smell of rain because it signalled the end of drought and the potential availability of fresh water.

Mughal rulers in the 16th century, such as Jahangir and his wife, Nur Jahan, were known for being patrons of Kannauj perfume, also known as ‘mitti attar’. Also known as the “smell of rain” or “baked earth” fragrance, this captures the aroma of wet earth after a rain.

Petrichor occurs when rainwater falls on dry soil, hence releasing a mixture of chemicals and oils into the atmosphere. The distinctive smell is a result of bacteria, plants, other organic matter, and even lightning to a certain extent. Generally, the characteristic smell is caused by bacteria.

Actinomycetes, a type of filamentous bacteria, typically develop in soil when conditions are warm and moist. Eventually, when the soil dries, the bacteria create spores in it. They release a chemical called geosmin, a type of alcohol molecule with a powerful scent. When it rains, the wetness and force of the raindrops propel these tiny spores into the air, where the moisture from the rain serves as an aerosol.

The moist air efficiently carries the spores to us, which we breathe in, and derive the smell. This is a cyclical process. The bacteria are ubiquitous, which accounts for the universality of this sweet “after-the-rain” smell.

Since the bacteria thrive in moist soil but release the spores once the soil dries out, the smell is most acute after a rain that follows a dry spell. 

In a 2015 study, using high-speed cameras, the scientists 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, much like bubbles in champagne.

These bubbles then shoot upward and burst from the raindrop, creating a cloud of aerosols. The team was able to predict the amount of aerosols released based on the raindrop’s velocity and the surface’s permeability. Researchers believe that in natural environments, these aerosols may transport aromatic compounds, as well as microorganisms and soil-based chemicals. Light or moderate rainfall may trigger the release of this aerosol, which the wind can then disperse.

The pre-rain smell is ozone. Lightning in the clouds splits nitrogen and oxygen gas into individual atoms, which then recombine to form compounds like nitric acid and ozone.

Downdrafts and the first drops of rain carry ozone to the ground, where we experience it as a sweet, lightly acidic smell. As the rain starts falling, drops of water strike plants and the ground, liberating organic compounds and aromatic oils that splash into the air as aerosols.