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  • Petrichor is the smell of dust after rain. It's a combination of plant oils, bacteria

  • and ozone. But this doesn't tell the whole story. Petrichor smells, well dusty. How does

  • the smell get from the ground to our noses?

  • A recent study from researchers at MIT suggests an aerosol effect is a main cause of the smell

  • of dust after a light rain.

  • We all know the smell. On a summer day when the air has been stale and hot for days and suddenly

  • a summer shower provides some relief. When we venture out to greet the world anew, we're met

  • with one of the most unique smells on the planet. It's a smell as old as time itself,

  • but it went without a name for a surprisingly long time. Petrichor was coined by two scientists,

  • Isabel Joy Bear and R. G. Thomas of Australia's Commonwealth Scientific and Industrial Research

  • Organization, in 1964. The word uses Greek term, Petra-meaning stone, and ichor, the

  • special substance that flows in the veins of gods.

  • But the mechanism behind Petrichor was a bit of a mystery until MIT scientists captured

  • slow motion images of falling water drops. The videos revealed that rain drops trap air

  • bubbles. When they hit the ground, the bubbles capture small molecules from the soil. These

  • molecules bubble up and release aerosols, not unlike the bubbles in a glass of champagne.

  • Capturing this common occurrence took a little while. The scientists performed over 600 experiments

  • dropping water on 28 different surfaces. 12 artificial surfaces and 16 soil samples. To

  • recreate the different types of rainfall, light and heavy and everything in between,

  • they varied the height. The higher the drop, the faster it fell. They found a Goldilocks

  • effect. The most aerosols were produced when the porous surface wasn't too wet or too

  • dry and when the rain wasn't too fast or too slow. If the rain falls too fast, there's

  • not enough time for the bubbles to form. So that's how the smell gets in the air, but

  • what is the smell made up of?

  • Previous studies suggest that the smell comes from oil given off by plants during dry periods

  • that falls on top clay and dirt.

  • Speaking of dirt, another part of petrichor comes from bacteria from the genus

  • actinomycetes. The same bacteria that are responsible for a recent break in

  • antibiotics. When these bacteria die, they release an organic compound called Geosmin,

  • which comes from the greek "earth smell". This is the damp, musty, strong well, earthy

  • smell after a storm. Human noses are extremely sensitive to the compound, we sniff it out

  • it just 5 parts per TRILLION. It's a harmless chemical, while we love it outside, it can

  • reduce the quality of drinking water and wine.

  • The smell of storms comes from ozone. A word which also has it's roots in greek for "smell".

  • Ozone is an alternative form of oxygen. While ground level Ozone is an icky, harmful smog,

  • atmospheric Ozone helps block some of the sun's most damaging rays. Storm-related

  • ozone is created when an electrical charge splits atmospheric nitrogen and oxygen molecules

  • into separate atoms. A few chemical reactions later, the oxygen reorganizes itself. Most

  • atmospheric oxygen is made up of two atoms. the typical O2. Yet during a storm, the reorganized

  • oxygen forms O3. The O3 is pushed down from the atmosphere by downdrafts, allowing us

  • to sniff this stormy scent.

  • Scientist still haven't found a way to bottle Petrichor. They've tried. How many candles

  • and laundry detergents have names like "spring rain". Most artificial scents haven't

  • come close because Petrichor is so complex and some compounds exist only in small traces

  • that can't be detected by some machines. I wish they'd hurry up though, I'm tired

  • of waiting.

  • So do you love the smell of rain? Let me know in the comments below

Petrichor is the smell of dust after rain. It's a combination of plant oils, bacteria

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