Characterization of Ozone Episodes in Urban Air

Air pollution and meteorological data were collected in Chicago during August and early September for 54 and 39 consecutive days in 1976 and 1977, respectively. Regular measurements were also made during these periods of the ozone-forming potential of ambient air. Rise in ozone levels was related to passage of a weather front. Minimum and maximum values of maximum hourly ozone concentrations 1-2 days and 3-5 days, respectively, after frontal passage. All O₃ concentrations ≥0.08 ppm occurred when the center of a high pressure system was over or east of the Chicago region and only on the second or subsequent day after frontal passage. A rough association appeared to exist between the average ozone-forming potential (the average of values just before, during, and just after a particular time period) and the maximum hourly ozone concentrations 3 days and 4 days later, when these variables were compared for the same frontal system.