Infrared spectroscopy with path lengths in air in excess of 100 meters was first applied to the study of air pollution at the Franklin Institute Laboratories in Philadelphia, Pa., in 1955. This paper is a personal account of the studies of the photochemistry of polluted air by this research group during the 1950s. In our first study, we verified the strange aspects of photochemical ozone formation as reported by A.J. Haagen-Smit. At the same time a new set of infrared bands appeared. It took several years to determine that these belonged to a previously unknown family of compounds called the peroxyacyl nitrates (PANs). They are toxic to plants, irritating to the eye, and (in liquid form) very explosive. We constructed a second infrared instrument in a laboratory trailer and took it to southern California in 1956. With this instrument, we verified laboratory findings on ozone and PAN chemistry in real polluted air, and it became evident that automobile exhaust was the dominant cause of photochemical smog. We then moved this infrared instrument to the University of California, Riverside, campus where an interdisciplinary study with plant scientists finally established PANs as the cause of “oxidant plant damage.”
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