What is smog? What is the biggest source of smog today?

I know smog is a type of air pollution, but I am not aware of the biggest source of smog. Anything to help with that part of the question, would be great

2 answers

http://www.merriam-webster.com/dictionary/smog

http://en.wikipedia.org/wiki/Smog
Since this is not my area of expertise, I searched Google under the key words "smog" to get these possible sources:

http://www.google.com/search?client=safari&rls=en&q=smog&ie=UTF-8&oe=UTF-8

In the future, you can find the information you desire more quickly, if you use appropriate key words to do your own search. Also see http://hanlib.sou.edu/searchtools/.