Here's some good data for the Los Angeles area:
http://www.aqmd.gov/smog/Forecast.htm
http://www3.aqmd.gov/webappl/aqdetail/AirQualityParameterDat...
Click on "Historical Data" to create a graph.
Bottom line, LA seems to hover around 25 µg/m³.
it'd be even cooler if you could use data for some large, densely populated US cities as benchmarks, adjusted for time zone differences
I know anecdotally that the air in Chinese cities is bad, but I've never been really sure how much worse it is than say, LA, or NYC