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  • Writer's pictureFrank DiGiovanni

Do you smell that?

Updated: Jan 4, 2023

The day was a rainy, drizzly one when I thought that it might be appropriate to answer the question (above), because as I was sitting on my balcony (on a Saturday in early September) the idea hit me as I was listening to the birds through the trees and the smell of oncoming rain came wafting on the breeze.

Lightning and a downpour seen over Berlin, Germany. Courtesy Wikimedia Commons (Matt Biddulph)

We've all be in the situation where we could smell the rain that was coming but hadn't started falling yet. But what is it? That sweet smell that precedes the start of the rain are a combination of chemicals produced in the atmosphere and from the Earth itself. The first smell associated with rain is ozone, yep, ozone. This combination of Oxygen (O3) is typically found high in the atmosphere (the ozone layer), between nine and 22 miles up, where it absorbs most of the ultraviolet radiation coming from the sun. Typically this gas stays up there but an oncoming thunderstorm can help create more of a local supply of this gas. Lightning or even an electrical generator can split atmospheric Nitrogen (N2) and atmospheric Oxygen (O2) into separate atoms. Some of these atoms recombine into Nitric Oxide (NO) which then react with other chemicals in the atmosphere, occasionally combining to form ozone. With the downdrafts associate with thunderstorms, these winds can push the accumulating ozone towards your nose as the storm approaches your location. Some folks aren't a fan of ozone while others think it smells clean, I'm in the camp of the latter and look forward to whenever nature provides with a bit of it down at ground level.

Another smell that you've undoubtedly come across in your rainy-day travels is one of my favorites and among some of my favorite Earthly smells (along with wild thyme). It was first described in 1964 by two Australian scientists who were searching for the source of the smell: Isabell (Joy) Bear and Richard Thomas. Although people have realized and known about this smell for centuries, the true chemical ID of the smell had eluded science for many years. Dubbing it "Petrichor" (and derived from Greek meaning "blood of the stones"), this chemical is actually a combination of compounds made from oils from plants and actinobacteria; a bacteria associated with decomposition when in soils. Their activity also creates an organic compound called geosmin, which is actually a type of alcohol and is closely associated with petrichor. Our friends at the American Chemical Society and PBS have a helpful video which gets into this sweet smell. Reactions is a video series produced by the American Chemical Society and PBS Digital Studios.

The smell of petrichor is also especially potent when the rain falls on dryer-than-typical dirt/earth. But that's rain-smell and it had me thinking further, about some of the more loved smells that Earth has, and this one, we all have memories associated with: cut grass.

Although I have no lawn to cut of my own now, the landscapers who come to my apartment complex always remind me of how much I love that smell. Like so many of us do, the memories alone, associated with cut grasses' aroma is enough to make us stop and remember sitting under a tree during the summer or remember walking across that freshly mowed lawn. But every time I hear the din of their mowers and blowers, I'm brought back to each time I found myself behind the mower. But what exactly is that smell?

Even though I'm in the air conditioning as I write this, I hope you're imagining that sweet smell of summertime as I am. Yeah, my sneakers always got a lovely shade of green on them when I cut the lawn (thanks chlorophyll) but that smell was always worth it. So what is it? Scientifically speaking these pleasant smelling chemicals belong to a group called Green Leaf Volatiles or GLVs. GLVs are an airborne mixture of these organic compounds that plants, grass in this case, produce when damaged by mechanical forces; this could be from an insect chewing the leaves or a lawn mower blade chopping the tops of the grass off. Ian Baldwin, a plant ecologist and founding director of the Max Planck Institute for Chemical Ecology in Germany, states "Plants manufacture slightly different forms of GLVs depending on what's happening to them." Like other GLVs, those associated with grasses and all plants are small enough that they can be carried aloft and find their way to your olfactory system, sometimes from over a mile away from their source! While this isn't vital for we humans, as we do not typically eat grass, it will indicate if someone has just cut down a basil (Ocimum basilicum) plant in order to make pesto, as my wife did last night. While this species of plant is one our noses might immediately recognize, Baldwin continues, "Just about all fresh vegetables have some GLV bouquet to them, and fruits may release the molecules as they soften and the membranes inside them break down. Throughout evolutionary history, we've used that information to know when something is ripe,"

Courtesy of Wikimedia Commons

It is also interesting to think that plants can and do respond to these GLVs. If you remember back to biology, plants react to stimuli in their environment, it's called "tropism" in case you forgot. California Poppies open with the proper amount of sunlight, Venus Flytraps close when their trigger-hairs are touched and grasses' stoma can detect their own GLVs and send signals to the rest of the plant. Grasses are not the only plants to respond to GLVs either, but what this means is the plant starts shuttling nutrients towards the roots rather than towards the tops of the grass blades in a process known as "bunkering". Essentially, the plant is getting ready to have part of it cut off!! An interesting thought the next time you find yourself behind the handle of one of these grass-decapitating devices.

Until next time something makes you start wondering, "What's that smell?", think of these two and I am hopeful you'll smile or have a happy memory associated with ozone and petrichor.

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