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I say with air circulation

What you can smell is actually tiny amounts of vapour that are given off into the air by whatever it is you can smell. All the molecules in the air are constantly moving even when there is no breeze. The smell receptors in your nose and mouth are fantastically sensitive and can detect even a few molecules and your brilliant brain can tell what kind of molecules they are, so you know whether you can smell soup or roses. So when you are close to something it smells stronger because more molecules are getting into your nose. Dogs noses are much more sensitive than ours and they can detect scent molecules left behind by someone who has walked by even several days ago.