INTRODUCTION Photochemical smog results from photochemical reactions in the atmosphere that occur when nitrogen oxides and hydrocarbons are present. The reaction products include ozone@ peroxyacyl nitrates (PAN)@ aldehydes@ and a variety of secondary reaction products. To date@ knowledge of the photochemical smog reaction process is largely based on controlled experiments in laboratory test chambers and on statistical correlation analyses using atmospheric data on photochemical smog@ in the latter case@ most of the data have come from observations in the Los Angeles Basin. As yet not satisfactorily resolved in the smog-forming process is the question of the importance of specific types of hydrocarbons@ i.e.@ the olefins and other highly reactive types as compared to the less reactive aromatics@ Another question is the significance of the ratio of nitrogen oxides to hydrocarbons in the photochemical mixture. Chamber experiments are now being used to judge the significance of hydrocarbon and NOx reactivity to HC ratios in order to postulate smog formation mechanisms. Ultimately@ such studies could serve as a basis for air pollution control regulations and air quality standards on photochemical smog constituents such as NO2@ hydrocarbons@ and oxidant or ozone. Chamber studies cannot simulate all the variabilities of the real atmosphere@ and thus increased efforts have been made to carry out detailed aerometric analyses in Los Angeles@ A goal is a simulation model of photochemical smog. The interpretation of aerometric measurements from Los Angeles@ however@ presents major problems. One of the more important is the very great complexity of the area sources and their emissions. For example@ any sampling station in Los Angeles will be surrounded by sources of both hydrocarbons and NOx at distances varying from the street just outside the door to the freeway 20 miles away. As a result@ each sample will contain a wide mixture of emissions that have had various histories@ times for reaction@ and exposures to atmospheric influences. Although partial evaluations of the Los Angeles aerometric data have been published@ because of these and other difficulties@ the analyses leave unanswered many questions about the real photochemical smog system@