Los Angeles 40 years ago - every summer had smog alerts.
Now after strict auto pollution control measures, LA does not have any more smog alerts.
Paris does not have a locked in basin such as LA, which kept the smog hovering over the city, so Paris clean up of smog should be resolved sooner than LA.
Other world cities are facing worse problems, such as Beijing and Mexico City citing two with no record of any corrective city action.
Just some thoughts.
Read more about Paris at TIME.COM - World Europe by Sam Frizell.