Right now our main effort is directed toward oxides of nitrogen which comes out of automobile tail pipe and cause the photochemical reactions which color the air a muddy brown. There is no question they are a problem in areas like L. A. where we have a more or less constant temperature inversion trapping the air. But Dr. [John] McKetta lists the findings in his field as his no. 3 shock & surprise. Nature it seems also produces oxides of nitrogen. As a matter of fact nature produces 97% of them.

Radio commentary (August 1975)