Do you smell fall in the air? Here's why.

The days are getting shorter, which indicates fall is fast approaching. At high latitudes, like in northern Canada, that also means trees are beginning to shed their leaves.

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Leaves produce "food" for plants, in the form of glucose, or sugar. That occurs through photosynthesis, which requires sunlight. When the leaves are no longer useful to the plant as daylight wanes, the tree gets rid of them — they’re extracting more energy than they’re providing.

That means the leaves plummet to the ground and die. The first step to that entails leaves "exhaling" gases through their stomata, or small porous holes. 

Among these gases output are terpene and isoprenoids, ingredients in the oily residue that coats some plants. Terpenes are hydrocarbons, comprised primarily of hydrogen and carbon. Pinene, a type of terpene, smells like pine. It’s commonly found in the saplike resin that repairs the bark of conifers and pine trees.

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Some of these exhaled gases — known as volatile organic compounds — interact with different types of nitrous oxide in the atmosphere, producing ozone. The human nose is very sensitive to ozone, able to detect minute quantities. It sometimes smells "clean," like chlorine or fresh laundry. 

Our air this week is coming from northern Saskatchewan, Canada. That area is rich in pine and evergreen trees. Fall is in full force up there. The air eventually made a loop near the Hudson Bay, and has since surged south into the Lower 48. 

It's brought cool temperatures, dry air and, you guessed it, the smell of fall.