The Scent of Season
Emily Kwak (ISB G10)
Flowers. Grasses. Rain. We can usually smell those in spring. You can also smell the trees with fresh leaves. Not only the smell of spring, you might have smell summer, fall, and winter. Wait. How do you smell spring?
What is smell?
To smell means to perceive scents through the nose.
Whenever we smell something, our nose and brain work together to make sense of hundreds of very tiny invisible particles, known as molecules or chemicals, that are floating in the air. If we sniff, more of these molecules can reach the roof of our nostrils, making it easier to identify a smell.
The fact that we have two nostrils allows our brain to detect small differences in the number of molecules that reach each one, so we can follow a smell trail just like tracker dogs. Have you ever tried finding where a smell is coming from? See how hard it gets with one nostril blocked.
Inside your nostrils, there are tiny things called neurons that "talk” to each other using electrical messages.
What we perceive as smells are molecules, but smell is much more arbitrary than just that. It’s not that smell is more than molecules -- it’s how the molecules are interpreted in their context. Our kinship is even clearer when scientists study the noses of amphibians. There are even more olfactory receptor genes that match closely to our own. What’s more, some of the genes have changed structure, so that instead of capturing water-soluble molecules, they can capture airborne ones (amphibians still make receptors for grabbing water-soluble ones, since they spend time in the water). Mammals, to which we’re even more closely related, have many more olfactory receptor genes, which show even closer kinship to our own.
We are not smelling the season
Temperature has a big effect on aromatic stimulus. Spring and summer are typically hotter and have more “odorific” humidity levels. The warmer and more humid it is, the more molecules of odors are in the air and the more they move; therefore the more volatile they are — things have a greater smell intensity and we can smell more stuff. In cold air, there are fewer volatile molecules so we smell less. For example, to me, New York in the summer smells like urine mixing with smoky pretzels, hot dogs, and car exhaust. But if it’s 20 degrees colder, I perceive a difference in the intensity of those smells because the air is less humid. Day in, day out, we smell the same things, then we wake up one morning, and it smells like "fall" now. The temperature and humidity have dropped and the city smells less intense. In a way, we are more "change detectors" than anything else.
Before the rain begins, one of the first odors you may notice as winds pick up and clouds roll in is a sweet, pungent zing in your nostrils. That's the sharp, fresh aroma of ozone—a form of oxygen whose name comes from the Greek word ozein (to smell). Tropospheric chemist Louisa Emmons at the National Center for Atmospheric Research explains that ozone emanates from fertilizers and pollutants as well as natural sources. An electrical charge—from lightning or a man-made source such as an electrical generator—splits atmospheric nitrogen and oxygen molecules into separate atoms. Some of these recombine into nitric oxide, and this, in turn, reacts with other atmospheric chemicals, occasionally producing a molecule made up of three oxygen atoms—ozone, or O3. (Most atmospheric oxygen is made up of two atoms—O2.) The scent of ozone heralds stormy weather because thunderstorm downdrafts carry O3 from higher altitudes to nose level.
All these chemicals stirred up by the weather can carry messages. Some biologists suspect that petrichor running into waterways acts as a cue to freshwater fish, signaling spawning time. Microbiologist Keith Chater at the John Innes Center in England has proposed that geosmin’s fragrance may be a beacon, helping camels find their way to desert oases. In return, the bacteria that produce geosmin use the camels as carriers for their spores.
But do these smells send meaningful messages to humans? Anthropologist Diana Young at the University of Queensland has studied the traditions of aboriginal people in Australia's Western Desert. There, the first rains before wet winter and summer months are an important event, mingling the sweet odors of damp gum leaf oils, eucalyptus, animal waste, and dust. The rains bring games such as kangaroos and emus, quench thirst and transform the red desert into a verdant landscape. Young says that to these people the smell of rain is linked to the color green, a connection she calls "cultural synesthesia." In fact, many of these Aboriginal peoples manufacture their own perfume with plant and animal fats and rub it onto their bodies, a symbolic connection of body and landscape. The odor is believed to be protective and cleansing, linking present generations to their ancestors.
Psychologist Pamela Dalton, a member of the Monell Chemical Senses Center observes that, although humans don't appear to have innate responses to these odors, we do learn to associate them with our experiences. Flooding may forever scar us with moist, mildewy memories, but for many, the smell of rain is cleansing and refreshing—relief from the relentless summer heat.
Scent, memory, emotion
These types of tiny cells, called olfactory neurons (olfaction means smell), have long cable-like connections that send electrical messages to a spot at the front of the brain, known as the olfactory bulb. Each olfactory neuron connects with a different neuron in the olfactory bulb, which then sends this information to other areas of the brain.
The parts of the brain that get these signals also do other things, such as storing memories or provoking emotions. That is why some smells can bring back old memories.
Even some older adults can remember the smell of their kindy class or their grandparents’ house. Also, some smells can make us feel scared or happy, such as the smells of smoke or flowers. For example, the smell of freshly mowed lawn can help us relax.
The sense of smell is closely linked with memory, probably more so than any of our other senses. Those with full olfactory function may be able to think of smells that evoke particular memories; the scent of an orchard in blossom conjuring up recollections of a childhood picnic, for example. This can often happen spontaneously, with a smell acting as a trigger in recalling a long-forgotten event or experience. When we come into contact with an odor, or molecules from volatile substances drifting through the air, the neurons that make up your olfactory receptor cells send a signal to a part of your brain called the olfactory bulb. Axel and Buck found roughly 1,000 genes played a role in coding for different types of olfactory receptors, each of which focuses on a small subset of odors. Thus each receptor is not responsible for understanding all possible smells. Those signals are then passed to what are called microregions within the olfactory bulb where again, different microregions specialize in different odors. The olfactory bulb is then responsible for interpreting those signals into what we perceive as smells. Your olfactory bulb runs from your nose to the base of your brain and has direct connections to your amygdala (the area of the brain responsible for processing emotion) and your hippocampus (an area linked to memory and cognition). Neuroscientists have suggested that this close physical connection between the regions of the brain linked to memory, emotion, and our sense of smell may explain why our brain learns to associate smells with certain emotional memories.