What happened to all the smog that used to hang over our cities?

i remember growing up in the sixties and seventies, every major city was enveloped in smog and pollution, they are never like that now, perhaps l.a., but every city used to look like l.a. back then...and it was much cooler, ....hmmm. now wait a second, there was more smog back then and it was cooler, not much sun getting through....now, not so much smog and it's getting warmer....more sun getting THROUGH! OMFG! GET ME ALGORE ON THE HORN RIGHT FREAKING NOW!