I could have predicted this story. Here in LA, the sky can be as smoggy as all get out, but one decent sized storm cleans up the air. We have been getting zilch for rain, or even strong winds, so the air is just stagnating in the LA basin.
But never to worry,... we are supposed to have the opposite problem this fall. They are now anticipating a moderate sized El Nino, which equals flooding in California.
I drove through LA in 1990, I live here now. I can vouch for the fact that the air is about 99% cleaner now than 1990.