Since the first O3 NAAQS were set in response to the 1970 CAA Amendments, significant progress in O3 reduction has been made in some areas of the United States. These areas include Los Angeles, New York, and Chicago, where the average daily maximum 1-hr O3 concentration decreased by about 15% over the decade 1986–1996. Despite almost three decades of massive and costly efforts to bring the O3 pollution under control, however, the lack of O3 abatement progress in a number of areas of the country has been discouraging and perplexing (NRC 1991). The complex O3-formation process, which involves the interaction of NOx, VOCs, and dynamic atmospheric processes, has probably contributed to the difficulties encountered in abating O3 pollution in the United States.
Reduction of O3 concentrations requires control of either NOx or VOC emissions or a combination of both. For the first two decades of O3 pollution mitigation in the United States, VOCs were the primary targets for emission reductions. The initial decision to pursue VOC controls was based on results from federal smog chamber experiments and data collected at monitoring sites in several U.S. cities, as well as the recognition that VOC controls would reduce the concentrations of eye irritants. In 1971, EPA issued the so-called Appendix J curve (Figure 3-2) for use by state and local agencies in developing SIPs (Fed. Reg. 36 ). The curve (essentially from a modified rollback model) was derived from observations in six U.S. cities. On the basis of the maximum O3 concentrations observed at these cities and their estimated VOC emissions, the curve purported to indicate the percentage of VOC emission reduction required to reach attainment in an urban area as a function of the peak concentration of photochemical oxidants observed in that area.
In the late 1970s, a more sophisticated method involving the use of a photochemical box model was developed by EPA. The empirical kinetic modeling approach (EKMA) (Dimitriades 1977) used the improved chemical mechanisms that were under intense development in the late 1970s and early 1980s (Atkinson and Lloyd 1984) to simulate the production of O3 within an idealized air parcel as it advected over an urban area with VOC and NOx emissions. Figure 3-3 illustrates the typical output from EKMA—the so-called Haagen-Smit or EKMA diagram, where lines of constant peak O3 concentrations (called O3 isopleths) are plotted as a function of VOC and NOx emissions. Inspection of Figure 3-3 reveals distinct regions of VOC and NOx sensitivity. For VOC-to-NOx ratios less than about 4:1, the atmosphere is VOC limited, the most expeditious path to O3 control is VOC reduction, and a reduction in NOx could increase O3. However, for VOC-to-NOx ratios greater than about 15:1, NOx controls provide the best way to reduce O3.
EKMA plots, as shown in Figure 3-3, captured the major features and complexities of the NOx/VOC/O3 system. However, to effectively apply the information to an O3 abatement strategy for a given nonattainment area, one must be able to accurately characterize where on the diagram an area is situated in terms of the VOC:NOx emissions ratio. Based on the available emissions inventories in the late 1970s and early 1980s, it appeared that most urban areas were near or above the ridge of the diagram, suggesting that VOC controls were the most effective path to attainment. Results using more sophisticated models, such as the UAM (urban airshed model), based on essentially the same emission inventories, generally