The pollutants that are contained in the air varies depending on where you are; near major sources of pollutant emissions, such as power plants, automobiles, smelters, etc, the amount and type of pollutant can be quite large, whereas in remote areas of the world, these same pollutants are much smaller. The following links should help you in your search once you define what location you are interested in. Most of these are located on the Environmental Protection Agency's (EPA) website.
This EPA web site was designed for younger kids:
http://www.epa.gov/kids/
This link is geared for slightly older kids, and has information on a wider variety of air pollution issues.
http://www.epa.gov/students/
This page is a list of air pollution issue links created by the EPA. It has even more information, if you follow the links...
http://www.epa.gov/ebtpages/air.html
The following page is a link to the EPA's Office of Air and Radiation... with even more information...
http://www.epa.gov/air/
In particular, one group within OAR, the Office of Air Quality Planning and Standards (OAQPS), has a lot of very useful information, e.g., the following pages for air quality and emissions information, respectively:
http://www.epa.gov/air/oaqps/cleanair.html
http://www.epa.gov/air/oaqps/emissns.html
Finally, two sites that have a series of documents produced annually by OAQPS dealing with the nation's emissions and ambient air quality...
http://www.epa.gov/ttn/chief/trends/trends98/
I hope this helps in your studies.
Glenn Rolph
NOAA Air Resouces Laboratory