It's the smell of decay.

>>1401
I was thinking that it might be that, maybe something trees release as part of the de-leafing process? But, decay is usually a smell we find extremely repulsive

Maybe it's connected to heating? Here the air in the winter often smells smokey because some houses are still heated by burning wood (or garbage).

Well, I decided to look it up finally.

-Summer is more humid which allows odor to travel and linger better which means a lot of smells combine which becomes a basic indistinguishable smell

-In autumn as trees and plants go dormant the sugars and other components in leaves break down and gas is released

-Both of things together mean that in the dry air of autumn we can smell that gas from the plants in isolation

An article also mentioned that we pay more attention to it because of memories we've associated with it earlier in our lives, but I was more concerned with its direct cause.