Air quality data is available from several places depending on what you need and your preferred format.  

Air quality data from the AQS system is consolidated at the AirData website. That site has files of extracted AQS data at the annual, daily, and hourly level.  It also has several reports, graphs, and maps that you can generate based on specific selections.  These are available from the main AirData page.  It also has a REST query interface that allows you to query AQS raw data. If you need AQI (Air Quality Index) information, you can get it from the daily files on the download files page, or use the query daily data interface.

If you need real time data, go to the (AirNow Gateway / API page)

If you are interested in county AQI information, it is available here: AQI index

Discoverer is an Ad-Hoc Query tool for registered AQS users.