After aircraft were grounded in the USofA from the 9/11 attacks, the boffins noticed marked improvements in various barometers of pollution (CO2, ozone, stuff like that) The skies over America weren't closed all that long (can't remember how long)
Actually no, there were not "marked improvements". It was a period of especially good/clear weather, particularly in the Eastern half of the country. Since it was outside of the norm (and would have been so with or without aviation), people with agendas have been bending the data ever since.