From the EPA.gov site:

EPA’s scientific air quality information in real time is now available on Google Earth.  This information tool accesses the AIRNow database hourly so that the Air Quality Index (AQI) displays  the most current air quality conditions.  Public health officials, media outlets and the general public can now view timely air quality information, by city, on Google Earth.

Here you go: http://www.epa.gov/region09/air/airnow/

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