Tuesday, September 25, 2018

When It Rains It Smells


In English, the smell of rain didn’t have a name until 1964. Some mineralogists from Australia really wanted to figure out where the smell came from, so they did some experiments. In order for them to describe their results, they took the liberty of changing the pretty useless term for the smell of rain, “Argillaceous odor,” to “Petrichor.”

Argillaceous refers to things made of clay, and petrichor in Latin means rock blood. “Argillaceous” is not exactly the smell of rain, but the smell of wet clay (an odor I know very well as a potter). The word refers to the clay (argilla, Latin) and is used in the absence of the other smells that come from soil, like that which comes from grass and other plant matter as well as the microorganisms and fungi that live in fertile soil.

The main organic ingredient we aren’t mentioning here is “Geosmin.” This doesn’t come from rocks, but from a certain bacteria that lives in the soil, and we all know it as the smell of freshly disturbed earth (geosmin: earth+smell, Greek). Coincidentally, geosmin was named at almost the exact same time as petrichor, in 1965.

With Petrichor, the study authors wanted the semantic identity of the smell to be no longer limited to soil, but all the things that make it what it is. (They also used the phrase “osmic latitude” to refer the general smell-space of a smell, which is another valuable term for the olfactosphere.)

Their main purpose was to pinpoint the source of this ephemeral entity, detailing the variables that make it so – porosity of the clay/soil medium, makeup of the minerals, dryness of the medium prior to wetting, and the presence of both living and dead biological components, like geosmin.

In their pursuit they concocted all combinations of minerals, torched them to eradicate any organic matter contamination, and then literally sniffed the results (using only wetware, i.e., their human noses). Ultimately, the only thing I can tell you from their work is that soil with the most silica produced the sought-after results prodigiously.

An Ancient Recipe for the Smell of Silica
Then there’s that little village in India that’s centuries ahead of our Australian scientists – they bottle Petrichor and sell it as perfume. Four months of the hot, dust-blown summer in northern India does a good job of preparing a fragrant medium for extraction.

In the case of Petrichor, the drier the medium the stronger the scent released upon being wetted. This is because the dirt is absorbing the essential oils of the biosphere (terpenes floating in the air) as well as harboring micro-organisms, fungal colonies, and other plant matter in general. The longer it has to absorb, the more it accumulates – a ticking scent-bomb in a place like India where it doesn’t rain for quite a long portion of the year.

And then come the monsoons, releasing a warm, mineral-rich, and momentary fragrance. It’s a scent that has so possessed a population that they go to great lengths to capture it. Before the monsoons arrive in July, they remove a bunch of this sun-baked earth and fire in a kiln to get it super-dry. Then they distill the essence out of it.

The most interesting part is that this is the only attar they brew that does not come from a plant. And it’s true – what the Australian scientists proved was that it’s mostly silica dust.


Osmic Vernacular
The smell of rain speaks in many different tongues. Because the smell of rain depends so much upon what it wets, it’s different everywhere you go. Surely, one archetype of this scent comes from the parts of India where it’s very dry for a very long time, and then all of the sudden the monsoons come. Personally, I’ve never been to India, so if I smelled a bottle of “rain attar” it wouldn’t exactly send me into a nostalgic reverie. City rain, my kind of rain, smells like steaming asphalt. In lush and verdant West Africa, it smells different again.

This has a lot to do with the dynamics of aerosolization, which isn’t too hard to understand – millions and millions of raindrops smash into the earth, the impact sending tiny particles of whatever it hits shooting into the air. If you can picture it, this isn’t much different than an asteroid impact, only smaller. Argillaceous silica dust, bacterial metabolites, and all the other organic matter that covers the ground (including microscopic pulverized asphalt and other building materials) is sent flying so high into the air, that plumes of it can travel miles away, stirring thirsty cattle that can’t wait for the rains to come.

And let’s not forget ozone. Right before that rain falls, if there is an electrical charge in the air disturbing oxygen molecules, we get the smell of ozone (sharp, chlorine, burnt wires?). Oxygen molecules are usually paired together, but when the electrical activity in the air splits them apart, they can reform as three-molecule-oxygens, which are the only ‘version of oxygen’ that we can smell. (In fact I’m not certain that ozone-smell is oxygen; I’ll bet it’s more complicated than that, like how “chlorine” is not chlorine but the things it binds with, so that the more “chlorine” you smell in a pool, the dirtier it actually is).  

A concluding thought – the smell of rain has a lot to do with the smell of dirt and rocks and bacteria, and even with the smell of the air, and yet nothing to do with the smell of water.

Bear, I.J.; Thomas, R.G. Nature of argillaceous odour. Nature. 1964; 201(4923):993-995.

Geosmin, an Earthy-Smelling Substance Isolated from Actinomycetes. NN Gerber, HA Lechevalieh, Institute of Microbiology, Rutgers University, New Brunswick, New Jersey. Applied Microbiology, Vol 13, No 6, Nov 1965.

Making Perfume From the Rain, The Atlantic, 2015

Lexical Smell Network

New Jersey Turnpike during a rainstorm

Buy Geosmin on Sigma Aldrich!

This is the chemical name for geosmin:

Odor Descriptions of Geosmin from The Good Scents Co.
Fresh, humus, earthy, musty, freshly ploughed soil, the first rain after a dry period, dirty, weedy, wet, turnip, beet, the muddy smell in freshwater fish, the smell of the countryside

Some important bacterial players in this osmic drama:
Streptomyces coelicolor and Streptomyces griseus, of the Actinomycetes type mold

Post Script
Agrillaceous reminds me of Alliacious, another smell-word for things resembling or consisting of onions or garlic. It’s the first word in my lexical smell network, and I had certainly never heard of it until I began the project.

Geosmin can be made into argosmin, which is the same thing but doesn’t smell at all (Greek – argos =inactive…sound a little like argillaceous?)

And finally, because this is just a cool bit of trivia, geosmin has a hardline to the limbic system of the fruit fly – just the smell of it will activate an escape response, making sure they don’t lay their eggs in food spoiled by this potentially bad-news bacteria that creates geosmin.

Ovipositing btw, is the name for laying your eggs (de-positing , that is).

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