calculations of cost-effectiveness of control measures (i.e., cost of the control measure in dollars divided by the expected emission reductions in tons).

Estimates of OPE, ranging from 7 to 10, were initially derived on the basis of linear relationships between O3 and the oxidation products of NOx at rural sites (Trainer et al. 1993). Chin et al. (1994) derived a lower limit for OPE of 1.7 and argued that the earlier estimates overstated OPE because NOx is removed from the atmosphere more rapidly than is O3. More recent studies involving direct, airborne measurements within power plant and urban plumes (Ryerson et al. 1998) and regional analyses of rural O3 monitoring data (Kasibhatla et al. 1998) yield OPE values in the range of one to three molecules of O3 per molecule of NOx.

The concentration of NOx and VOC/NOx ratios are the two main factors affecting the OPE. At low VOC/NOx ratios, HO reacts predominantly with NO2 to form HNO3, removing radicals and NOx from the photochemical cycle and retarding O3 formation. Under these conditions, a decrease in NOx concentration favors O3 formation (ozone formation is hydrocarbon limited). High VOC/NOx ratios favor HO reaction with VOCs that generate new radicals that accelerate O3 production. However, at a sufficiently low concentration of NOx, or a sufficiently high VOC/NOx ratio, a further decrease in NOx favors peroxy-peroxy reactions, which retard O3 formation by removing free radicals from the system (ozone formation is NOx limited). At a given level of VOC, there exists a NOx mixing ratio at which a maximum amount of ozone is produced. This optimum VOC/NOx ratio depends on the reactivity of HO to the particular mix of VOCs present. Because NOx is removed faster than hydrocarbons, VOC/NOx ratios tend to increase during transport, and ozone formation can change from hydrocarbon limited in the urban core to NOx limited in downwind suburban and rural locations. Accordingly, NOx reductions could lead to higher peak 1-hour average O3 levels in the urban locations that are currently hydrocarbon limited, but to lower 8-hour average O3 levels in downwind locations. Ozone formation is complex, and a thorough understanding of the response of ozone levels to specific changes in VOC or NOx emissions is the fundamental prerequisite to developing cost-effective ozone abatement strategies.



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