Where to find Atmospheric data for various cities
Most cities used to have own pages about pollution data. But there are other sources. For example:

London: http://www.londonair.org.uk/LondonAir/Default.aspx

Bangkok: http://www.pcd.go.th/airquality/bangkok/

If you are looking for a single page, try this:

http://aqicn.org/city/all/