Creation and fate of ozone layers aloft in Southern California

Data obtained during the 1987 Southern California Air Quality Study (SCAQS) were used to examine the creation and fate of ozone layers aloft. The ultimate objective was to estimate the potential impact of the layers on surface ozone concentrations. The smog episode of 13–15 July 1987 formed the basis for a case study. Several ozone layers aloft were created over the northern part of the Los Angeles Basin (e.g. over El Monte) during the day on 13 July. These layers were attributed to the action of slope flows (including convergence), convective elements transported into the inversion layer and undercutting by the seabreeze flow. A peak value of 34 pphm was observed aloft in one of the layers. The principal layers aloft were still present intact over the northern part of the Basin on the morning of 14 July. Their movement eastward (from areas in the vicinity of El Monte and Upland) was estimated by trajectory and isentropic analysis. The results indicated that a likely impact area of the ozone layer aloft near Upland during the day of the 14th was near San Bernardino. The ozone layer above El Monte on the morning of 14 July appears to have been incorporated into the mixed layer near or to the southeast of Riverside. A maximum ozone impact of 3–4 pphm was estimated although a portion of this impact was probably due to an overnight reservoir of surface-based precursors of ozone.