The keywords is "presumably". There should only be a drop if your hypothesis is correct which given population growth, car ownership trends and overall economic

The keywords is "presumably". There should only be a drop if your hypothesis is correct which given population growth, car ownership trends and overall economic growth is doubtful. I think it's safe to say, that, in the West, air pollution has dropped dramatically: https://en.wikipedia.org/wiki/Great_Smog https://en.wikipedia.org/wiki/Great_Smog