The phrases "a long time", "some time", and "at least a bit" are doing a lot of heavy lifting there. Oxygen has been in the Earth's atmosphere for over 2 billion years. How long, roughly, is "some time"?
How big, roughly is the planet in question? How thick, roughly, is the atmosphere? What, roughly, is the composition of the atmosphere? What is the median temperature of the planet? List all species capable of metabolizing pollution back into harmless compounds. What are their lifespans? How do they reproduce?
...hopefully my point is made, now. I can't give details because they don't exist.
If a species produces sufficient pollution that it kills that species, it is a reasonable assumption that it was also produced at a rate far higher than the planets natural ecosystem could absorb it. It is also reasonable that the pollution also kills one or more species which were consuming or absorbing the pollution, slowing the rate at which it can be removed, or maybe even stopping that process completely.
Pollution does not disappear at the same time pollutants stop being introduced into the atmosphere.
There will be a time after which no pollutants are produced and before no pollutants are detectable. That is a non-zero amount of time. That is why I say that pollution could one day be detected on a planet that has not had a living civilization for "a long time." "A long time" being "more than five minutes" if you really want me to nail it down.
I understand. And my argument is that nonzero isn't a good enough lower bound to justify investing in something like that. Oxygen is produced by even very simple organisms, and thus has been present in our atmosphere in significant concentrations for billions of years, currently makes up 20+ percent, and its residence time is 4,500 years [1]. On the other hand, many CFC concentrations are measured in parts per trillion, and have a residence time measured in years or decades [2], and have only existed at all for a century or so at most. Even if every single living thing on this earth died tomorrow, oxygen would be detectible long, long after many pollutants were gone. And chances are, if anything were to survive the kind of disaster that would wipe out humanity, it would be something like the single-celled organisms that excrete oxygen. I'm prepared to believe that you're correct, but you have to present some more compelling evidence.
...hopefully my point is made, now. I can't give details because they don't exist.
If a species produces sufficient pollution that it kills that species, it is a reasonable assumption that it was also produced at a rate far higher than the planets natural ecosystem could absorb it. It is also reasonable that the pollution also kills one or more species which were consuming or absorbing the pollution, slowing the rate at which it can be removed, or maybe even stopping that process completely. Pollution does not disappear at the same time pollutants stop being introduced into the atmosphere.
There will be a time after which no pollutants are produced and before no pollutants are detectable. That is a non-zero amount of time. That is why I say that pollution could one day be detected on a planet that has not had a living civilization for "a long time." "A long time" being "more than five minutes" if you really want me to nail it down.