That was a thought as well. Or else the elseworld equivalent of the same, depending on if I'm working with humanoids needing conditions close to Terran standard as far as air and water go. I'd hesitate to mess with a world's entire atmosphere in one until it's darn certain the place is truly uninhabited and lacking simpler life, because you do not know who might be living there already and what they might appear to be at first, especially to biased humans.

My first ideas all ran to contaminant removal from X lake, river or patch of sky, either via sifting or alteration, depending, which either way benefits the entire ecosystem. I'd need to set something up beforehand to contain whatever crud I filtered out of the lake, that much is for sure. I'd probably end up with a temporary liaison from the nearest town's waste treatment plant or garbage collection department, or else as local a private company with an interest in environmental cleanup as could be found.