If you are going to ask “when will China clean up its air?”, you might wish to look at South Korea, a country with a broadly similar industrial profile, although of course Korea is much further along in terms of economic development.
As of 2002, South Korea was ranked 120th of 122 countries for air quality by the World Economic Forum. And at that time South Korea was pretty much a fully developed nation, economically speaking that is. South Korea was also already a democracy, and we know from Casey Mulligan (with Gil and Sala-i-Martin) that democracies tend to have cleaner air than autocracies, ceteris paribus.
Might we consider the possibility that China won’t clean up its air anytime soon? The good news, however, is that once Korea started its environmental clean-up, improvements came pretty rapidly. More recently, they come in at #43 on a more general index of environmental quality.
That fact is from Dong-Young Kim, The Challenges of Consensus Building in a Consolidating Democracy.