On my one visit to China, I was in Guangzhou (used to be better known as Canton). We stayed in a hotel about a half-mile downriver from a large bridge over the Pearl River. The first day we couldn't see the bridge -- we didn't even know it was there. That night a cold front came through and cleared the air. We looked out the windows the next morning and said "Where'd that come from?"
"Where'd that come from?"
Ha!!
A few years ago, in China, we were travelling to Hong Kong by bus and passed thru the Canton/Guang Zhou area (big urban area). I could hardly wait to get thru there. Air stung my eyes and nose.