Smog in New York City
Photochemical smog is composed of primary and secondary pollutants. Primary pollutants include nitrogen oxides and volatile organic compounds as a result of industrial processes, while secondary pollutants are created through the reaction of primary pollutants with ultraviolet light.
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"Photochemical smog."
http://en.wikipedia.org/wiki/Photochemical_smog%23Photochemical_smog
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