The Smell of Spring

16th May 2018

Science

The Smell of Spring

Go outside. Take a deep breath. Smell that? The earthy aroma of plants and rain is everywhere. The weather may still be a little capricious this year, but these smells tell us that spring is definitely – and literally – in the air.

Smells like Spring Spirit

Seasons are detected by more than just a shift in temperature. Odour plays an important role in seasonal changes, and different times of year are characterised by their own distinct smell.

In winter, for example, the low temperatures mean that odour molecules move a lot more slowly than they do in summer. This is why in winter your nostrils are greeted with a rush of cold air rather than a cocktail of aromatic compounds.

Spring is no different. Hundreds of chemical reactions are taking place right beneath our very noses. Here are the scientific facts behind 2 of the most recognisable spring-time smells: rain and freshly cut grass.

Daisies in a field

The Smell of Rain

Spring is punctuated with frequent downpours. Some would describe the scent of rain as asphalt, wet pavement or even as foisty. But actually, it’s the redolent mixture of fatty acids, hydrocarbons, alcohols and other molecules being released into the air from a dry surface.

Mineralisation

Dry surfaces are covered in odoriferous molecules. These accumulate when decomposing organic matter releases airborne molecules that attach themselves to dry surfaces.

Known as mineralisation, organic matter decomposes by transitioning from complex organic compounds to simple mineral compounds.

Spring is particularly rife with mineralisation as the warmer weather helps to thaw out plants, insects and other organic matter after months of cold. This accelerates the mineralisation process and leads to an increase in airborne molecules.

Petrichor

When it rains, the water displaces these odoriferous molecules from dry surfaces and carries them into the air. The molecules can come from plants, trees or vegetation, as well as rocks, pavements and concrete.

These scents are called petrichor, but they are not solely responsible for the smell of rain. A damp spell also encourages the production of geosmin, a metabolic by-product of bacteria that is naturally found in algae and healthy soil.

Humans are extremely sensitive to the smell of geosmin, which releases an earthy scent. This is why rain and spring alike often smell muskier than other seasons.

A small puddle between moss-covered rocks after rain

When you look at this image, you can practically smell it. Rain displaces odoriferous molecules from dry surfaces which then mix with the air. Rain also increases the production of geosmin in soil which releases a wet, earthy scent. All of this combines to form the universally familiar smell of rain.

Freshly Cut Grass

While it’s a nightmare for hay-fever sufferers, the smell of freshly cut grass is one of the most adored fragrances of spring. But this conduit of nostalgia is actually the result of 6-carbon molecules getting released from the grass in the form of a distress signal.

Why Does Grass Smell Like That?

When grass is damaged – by your neighbour’s lawnmower or a grazing cow – it triggers the release of enzymes. These begin to breakdown the fats and phospholipids that are present in grass.

This breakdown of fatty acids eventually results in the production of Green Leaf Volatiles (GLV). This is the name given to a group of 6-carbon molecules that are released from the grass when it is damaged. These are also what emit that unmistakable scent of freshly cut grass.

The four most common GLVs are:

  • 2-hexenal (C6H10O)
  • 3-hexenal (C6H10O)
  • 3-hexenol (C6H12O)
  • 3-hexenyl acetate (C8H14O2)

3-hexenal is the key aroma compound released by grass. It is also very potent because only a small amount of it is needed for it to be noticeable to our senses – 0.25 parts per billion, to be exact.

However, while they may transport you to warm days playing rounders in the school field, GLVs are actually physiological distress signals.

Raising the Alarm

As soon as grass – or any plant really – is damaged, GLVs are released to warn nearby plants that danger is afoot. When other plants “smell” the GLV signal, they prepare themselves for the onslaught of herbivores or machinery by employing defence mechanisms.

As well as warning other plants by releasing these compounds, grass is also able to use GLVs to summon protection. Wasps, for example, are attracted to the smell of GLVs because it tells them that there are insects nearby to feed on. Their common targets are world-renowned plant and grass eaters, caterpillars.

The emission of GLVs lets the wasp know that there is a caterpillar nearby, and the wasp then parasitizes it. Without realising it, the wasp has just saved the day – from the grass’ point of view, at least.

Macro shot of blades of grass

The unmistakable smell of freshly cut grass is actually the result of Green Leaf Volatiles (GLV). These are 6-carbon molecules that are released from the grass when any damage is being done to it.

It may not be as romantic as spring fever, but the smell of spring is a phenomenal display of chemical reactions in nature that lets all animals know that the warmer months are finally here. 

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