Moran, Joseph M., and M.D. Morgan. Macmillan College Publishing Co. Strahler, A.H., and A.N. Strahler. Modern Physical Geography . Inc. Photochemical Smog What follows is a brief description of photochemical smog formation, from a non-chemist. For those who want to pursue this topic, there are also some links to follow for more information.
Photochemical smog is brown smog, the gray-brown haze that fills the air in many cities.
It is especially a problem in warm, sunny regions where there are lots of cars burning gasoline. Researchers in the 1940's and 1950's in Los Angeles noticed that the kinds of pollutants in the air varied over the course of the day. Some pollutants increased in the morning, as people started driving their cars. Other pollutants, including the visible, brown smoggy haze, were most common in the middle of the day. The mix of pollutants changed again in the late afternoon and evening.