During the summer portion of the 1987 Southern California Air Quality Study (SCAQS), outdoor smog chamber experiments were performed on Los Angeles air to determine the response of maximum ozone levels, O3(max), to changes in the initial concentrations of hydrocarbons, HC, and nitrogen oxides, NO(x). These captive-air experiments were conducted in downtown Los Angeles and in the downwind suburb of Claremont. Typically, eight chambers were filled with LA air in the morning. In some chambers the initial HC and/or NO(x) concentrations were changed by 25% to 50% by adding various combinations of a mixture of HC, clean air, or NO(x). The O3 concentration in each chamber was monitored throughout the day to determine O3(max). An empirical mathematical model for O3(max) was developed from regression fits to the initial HC and NO(x) concentrations and to the average daily temperature at both sites. This is the first time that a mathematical expression for the O3-precursor relationship and the positive effect of temperature on O3(max) have been quantified using captive-air experiments. An ozone isopleth diagram prepared from the empirical model was qualitatively similar to those prepared from photochemical mechanisms. This constitutes the first solely empirical corroboration of the O3 contour shape for Los Angeles. To comply with the Federal Ozone Standard in LA, O3(max) must be reduced by approximately 50%. Several strategies for reducing O3(max) by 50% were evaluated using the empirical model. For the average initial conditions that we measured in LA, the most efficient strategy is one that reduces HC by 55-75%, depending on the ambient HC/NO(x) ratio. Any accompanying reduction in NO(x) would be counter-productive to the benefits of HC reductions. In fact, reducing HC and NO(x) simultaneously requires larger percentage reductions for both than the reduction required when HC alone is reduced. The HC-reduction strategy is the most efficient on average, but no single strategy is the optimum every day.
