monitoring of levels of non metals oxides in the air has only been possible since the 60s/70s i believe, so in that time because of international accords on polution such as the clean air act there seems to be a decrease.
what you need to find is graphs showing metal oxides found in things like samples of glacial ice and dirt which can be dated, giving information on the pollutants in the air for millions of years... this indicates general increase in non metal oxide levels
hope that helps you