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What Causes the After Smell of Rain

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What Causes the After Smell of Rain?
The scent is unmistakable, and it is most pronounced after a rain that comes at the end of a long dry spell. Many people appreciate those fresh, earthy and powerful post-rain smells. But other odors may be less fragrant, even foul. The scent is the same all over the world because it’s created by the same conditions everywhere. In 1964, Australian mineralogists Isabel Joy Bear and R.G. Thomas coined the word “petrichor” to collectively name the mix of after-rain scents. The word has Greek roots for stone and the blood of gods.
Bacteria
Probably most responsible for the scent after a rain is bacteria known as actinomycetes that live in the soil. Actinomycetes are a filamentous bacteria, which means they grow in long strands, not unlike some fungi or algae. When conditions are warm and moist, actinomycetes grow. As the soil dries, the bacteria produce spores. Once it rains again, the droplets strike the bacteria with enough force to dislodge those spores. Moist air acts as an aerosol, sending the aroma from those spores to people’s nostrils. Actinomycetes are present in soils everywhere but are especially plentiful in forests.
Acid
Pollution and naturally occurring chemicals in the atmosphere mix with raindrops, causing the rainwater to become acidic. Acidic raindrops react with organic matter in yards, on pavement or on the forest floor. The acid also reacts with chemicals on the ground. These reactions produce a sweet-smelling scent.
Oils and Chemicals
Plants naturally emit oils that eventually collect on their surfaces. Rain striking these oils releases them into the air. The acid in rain also reacts with some oils, turning them into a gas, which then wafts to human noses. Geosmin is a byproduct of the metabolism of some bacteria and algae. Rain sends some of the geosmin airborne, leaving an aroma that may resemble sweet, rich soil to some people.
Ozone
Ozone is more associated with the scents present before a storm rather than during or after a storm passes. The clean, chlorinelike smell, however, can mix with other aromas during a rain. Storms that are charged with electricity create ozone. Lightning has enough force to split oxygen and nitrogen molecules, which bond together as nitric oxide. Further chemical reactions in the atmosphere pull away some oxygen molecules, bonding them together to form ozone (O3).

Source: Opposing Views

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