Earlier quoted context omitted.
I disagree that no one can own the air. To clarify, airspace can be homesteaded like any other piece of property. Obviously this is not necessarily the ownership of every air molecule in a given airspace as that would be quite impractical. But just the same, no one can pollute someone else's airspace.
Airspace is not air. Air flows freely across airspace. How can you guarantee that your air will not flow into my property?
From a libertarian perspective, at least when discussing air pollution, it is unimportant that air molecules flow from one property to another. What is important is that I do not pollute your airspace.