Our atmosphere is odorless. And no I dont mean the smells you get in polluted areas, or around agricultural land or near the sea where the salt and seaweed... these are chemicals floating through the air. I mean that nitrogen, oxygen and carbon dioxide (in its present minimal concentration) are odorless gases which constitute the majority of the atmosphere.

But is it really odorless? When you sit in a room with a certain smell, at first it is strong and after a while you adapt and cannot perceive it anymore. What if that room was a room you have been in your whole life. If nitrogen or oxygen or carbon dioxide or water vapour had a smell we wpuldnt even know. Our olfactory brain requires a "baseline" from which to detect other smells. This baseline I would imagine is perceived as "odorless" to maximise other smell sensations by contrast.

But, that does not mean this baseline is in fact without smell but rather this smell is undetectable.
My support for the argument is that;

a). If you increase the concentration of carbon dioxide (such as dry ice) to a quantity much greater than what is normally expired (roughly 4%) .... you can "smell" the carbon dioxide. But because oxygen and nitrogen are already present at high concentrations I would say it's hard to get the same effect.

b). Consider an alien lifeform that comes from a planet made of 60% ammonia gas, 25% chlorine and 15% CO2 (assuming hypothetical that a type of life could exist in such conditions). It arrives on earth having an olfactory "baseline" odourlessness from its atmosphere. It cannot smell ammonia or chlorine. Whilst to us ammonia and chlorine have very distinct and harsh smells. Would they perceive oxygen and nitrogen to have a strange smell?

I would imagine yes. I see no reason why at first encounter with a chemical that your brain has never registered before it will come as a sensation which can only be known as a sort of smell.