8 minute read
What's That Fall Smell?
The changing scents of the seasons help us to interpret our world in some very important ways.
By Linda Browne
Shane Howard photo
THERE’S NOTHING LIKE THE AROMA of a wild rose in bloom after a summer’s rain and the sweet scent of pine trees during a crisp autumn stroll through the woods. Scents colour our world and they’re all around us.
Smell is an important part of how we interpret and understand our world, and it’s inextricably linked to our memories and emotions. During this time of year, for instance, many of us associate that fall scent of musky leaf piles with back to school, which can bring up either positive or negative emotions, depending on the person and their experiences. Just as we can see and feel the change in the seasons, we can smell them, too. But what we smell and when we smell it also depends on our culture and geographic location (snow is as much a spring scent to those who live in Newfoundland and Labrador, for example, as the cherry blossoms that bloom in British Columbia the same time of year).
Shane Howard photo
All around us at any given time, organisms are releasing molecules that stimulate the olfactory nerves in our nose, which then send information to our brain so we can identify exactly what it is that we’re smelling. The molecules in the air that give rise to aroma change throughout the seasons and the temperature affects the way they move. During warm and humid conditions, for instance, aromatic molecules move through the atmosphere more quickly than when it’s cooler, at which time they are slower moving and so there are fewer of them in our breathing space.
Spring & Summer Smells
During springtime, we’re bombarded with smells that include flowers in bloom, which release a bouquet of aromatic molecules. But you might also associate this season with an earthy smell that comes from a variety of chemical compounds. One of these is geosmin (which comes from the Greek words for earth and smell), which is produced by streptomyces bacteria.
Linda Browne photo
“These bacteria are abundant in the soil. And as soon as the humidity level starts to go up, these bacteria go into overdrive... and they start to release these molecules that have a really earthy scent to them. And in fact, these geosmins, they give rise to the earthy taste of beetroots and a lot of root vegetables,” explains Dr. Chris Kozak, a professor in the department of chemistry at Memorial University.
Geosmin, Kozak says, also contributes to petrichor, which is the scent that arises just before or during a rain shower. The term, coined in 1964 by Australian scientists IsabelBear and Richard Thomas, comes from the Greek petra meaning “stone” and ichor, which, in Greek mythology, is the blood that flows through the veins of the gods. The oils from plants seep into the rocks and soil, and when the humidity rises and the rain hits the dry ground, the odour molecules get displaced and the oil gets released into the air, imbuing it with a distinctive scent.
The familiar smells of summer including rain and lightning, flowers and cut grass are each caused by their own unique chemical mechanisms.
Linda Browne photo
Sometimes, that rain might be accompanied by a thunder and lightning storm, which brings its own unique scent. When lightning strikes, Kozak explains, the oxygen in the air (O 2 or dioxygen) can get split into individual oxygen atoms, which can then combine with the dioxygen to form ozone (O 3 ), “which everyone knows from the ozone layer, and that protects the planet from UV radiation,” he says. so, for example, before a thunderstorm, if there’s thunder and lightning coming in the distance, we can smell that ozone in the air, which eventually rises up and enters the upper atmosphere.” Another smell that we commonly associate with the warmer months is the strong, sweet scent of a freshly cut lawn. Grass, Kozak explains, is made up of cellulose, oils and fats. “When the grass is cut, bits of the interior of the blade of grass are now exposed to oxygen, and that again triggers enzymes to start digesting the fats to try to heal the cut surface, to prevent any disease from entering the grass. So it’s the same thing as if we get a cut on our skin. We bleed, it turns into a clot, our body is trying to protect the laceration from getting [infected],” he says. Fatty acids, including linolenic and linoleic acids, Kozak adds, are broken down by enzymes, turning them into smaller molecules which can more easily evaporate from the surface of the grass.
Linda Browne photo
“And these molecules then start to oxidize a little bit and they get turned into what are called aldehydes. And aldehydes are very, very fragrant. So most of the smell that comes from fruits, a lot of the smells that come from flowers, they come from aldehydes… so they have that green grass smell.”
Fall & Winter Aromas
When the air turns cooler, the leaves begin to transform into a brilliant display of red, orange, gold and brown before eventually cascading down to the earth. As they start to decay, bacteria tries to break down their components – including sugars, cellulose and lignins – into smaller molecules.
“What they’re doing is they’re releasing these things called terpenes and terpenoids, which are very, very fragrant... turpentine comes from the word terpene, and it’s got that classic sort of resiny smell. Some of these terpenes are called pinenes because it smells [like] pine. Others are called limonene because they’ve got a bit of a citrusy smell to them,” Kozak says, adding that fungus also feeds on the decaying matter, which contributes to the familiar fall aroma.
Fall leaves release a variety of odours as they rest on the ground. They are also fun to play in.
Dora-Leigh Eddy photo
During the winter, a quiet calm descends upon the earth and the scent of woodsmoke fills the frosty air. But really, Kozak says, what we’re noticing at this time of year is the absence of smell. There are less aromatic molecules in the air because of the colder temperatures, and less things to smell in general (especially when compared to a season like spring). However, there is one interesting aroma to note.
“Just like with ahead of an impending rainstorm, if you’ve got snowfall, we get this increase in humidity in the air, [which] allows more of these same scent molecules to travel to our nose. So therefore, we get a heightened sensation, which is what we associate with the feeling of ‘smelling like snow,’” Kozak says.
The smell a person experiences when they say 'smells like snow' can change from location to location. If you live near an ocean, you may smell seaweed and Sulphur.
Nola Snyder photo
What snow smells like to an individual, he adds, depends upon their location. For example, if a person lives in a big city and there’s pollution in the air, their nose will pick it up since the snow allows some of the pollutant molecules in the upper atmosphere to come down. If you live near the coast, the snow will contain hints of seaweed and sulphur.
“So to say ‘smells like snow’ really depends on where you live,” Kozak says, “because the snow sort of amplifies any small amounts of aromatic molecules in the atmosphere that are present.”
Scents & Sensibility
SMELL impacts our emotions and mood at any given time. Which odours bring about positive or negative feelings depends upon an individual’s personal experiences and their reactions to them.
The act of smelling is a chemical, biological and psychological process. When we sniff in odour molecules, even before our brain can label the particular scent, there’s a quick, hedonic response (which tells us whether something is pleasant or unpleasant).
says Dr. Pamela Dalton, a cognitive psychologist and member of the Monell Chemical Senses Centre in Philadelphia (an independent, non-profit, scientific institute that conducts and publishes research on taste and smell.
“But then you match that with past experience and this is where your own idiosyncratic smell memory comes in. And if you are living in the same place that you’ve lived in since you were a child, there are going to be repeated smell experiences that reinforce those smell memories.”
Smell, Dalton adds, impacts our emotions and mood at any given time. Which odours bring about positive or negative feelings depends upon an individual’s personal experiences and their reactions to them.
“So the person who likes the smell of roses, for another person, whether they’re allergic or they first experienced them in a negative context, that won’t be a pleasant smell. I have certain scents of summer in my garden that are very positive for me – some of them are my vegetable garden: the smell of tomato leaves or the smell of basil and herbs that I grow. But for someone else, they may have no association with that,” she says.
While we may not appreciate the aroma of everything that we come across in our lifetimes, our sense of smell is one that we should never take for granted. Regardless of how busy you may be or frustrated you feel, you should always take time to stop and literally smell the roses (or whatever you find pleasant).
“When somebody loses their smell on a more permanent basis, one of the things they report is a sadness about not being able to experience the world that they didn’t even realize they were picking up on. They may not have realized that when they hug their daughter or son or their husband or wife or a friend... the smell of that person is another piece of who they are, another piece of that connection. Or that walking into grandmother’s house, it always smells the same way when you go over there for dinner; it’s something that is very rewarding to you. And when you lose that ability, you can get very depressed about it. So it’s not trivial,” Dalton says.
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