Differences between solids, liquids and gases:

http://www.bbc.co.uk/bitesize/ks2/science/materials/solids_liquids_gases/read/1/  

  
LAYERS OF THE ATMOSPHERE:
http://www.windows2universe.org/earth/Atmosphere/layers.html

AIR POLLUTION:
http://www.windows2universe.org/earth/Atmosphere/airpollution_intro.html