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 it. On a summer day when the air has been stale and hot for days and suddenly
a summer shower provides relief. When we venture outside 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 terms, 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 ACT TIN
O MY SEAT TEES 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 …