It has been, though it could be argued it's more of a climate change issue than a raw pollution issue.
The air pollution changes quite a lot with weather patterns. The pollution is the worst when the air becomes stagnant, hot and sunny. Wind doesn't blow pollutants away and the heat and sun make conditions prime for further chemical reactions to happen in the immediate air that makes smog worse.
The clearest days are right after it's rained. Rain and wind make a very large, noticeable improvement in an immediate sense. The weather patterns the last few years, especially in Southern California, have had even more hot, sunny and stagnant days than the average and less rain than usual. Some of it is cyclical, rain amounts can vary quite a bit year over year here, but there definitely has been an alarming trend, especially for SoCal, of much warmer winters and far less rain. Coastal areas have also had much less of the marine layer/fog than they used to in May and June as well, which is a significant departure from the normal climate out here as well. Part of it has to do with sea surface temperatures being higher than average for a significant amount of time right off the coast here.
Anyway, long story short, hotter, sunnier weather with less rain than usual = much higher air pollution accumulation, especially in the L.A. basin. But, given that vehicles are amazingly clean nowadays and we've already seen massive improvements in pollution due to the regulations, the problem now is the fact that the climate is no longer what it once was and the change in weather patterns is actually making it so there are more bad pollution days than California had 15-20 years ago.
By that observation, one could argue that curbing CO2 output is definitely of greater importance to pollution (even though it does not direct contribute to air quality issues) than further limiting of NOx, VOCs, HCs etc.
In other words, we need more efficiency, not cleaner exhausts... says the guy who just drove a 20 MPG V10 Touareg to Oregon and back... oops. Seriously, we could use a freeze on pollution regulations... they're clean enough. But if we could find other ways to encourage efficiency, that wouldn't be a bad thing.