Areas of Violation over the OTAG Region:

Air Quality Analysis Workgroup Input to Geographic Scenario Analysis.

November, 1996

The key role of the OTAG process is to determine whether ozone control measures need to be different from one region compared to another. For example, regions or areas of violations (AOVs) that are strongly influenced by transported ozone will require consideration of regional controls, while in other areas localized controls will suffice. In order to evaluate the regionality of the source impacts, the OTAG Policy Group has directed the Subgroups to develop approaches to the geographic evaluation of source impacts. The natural focal areas for geographic scenario analysis are those regions of OTAG that exhibit violations of the air quality standard, i.e. the AOVs.

In order to support the OTAG process the air quality analysis work group has conducted an evaluation and pattern analysis of areas of violation over the OTAG region. Violations were evaluated based on the current one-hour standard, as well as using the proposed 8-hour standard. The exceedence pattern analysis is described in more detail in the on-line report Pattern of 8-Hour Daily Maximum Ozone and Comparison with the 1-Hour Standard (

The pattern of exceedences for the 1-hour and 8-hour standard are shown in Figures 1 and 2, respectively. The 1-hour exceedances are most evident over the large urban-metropolitan areas: Washington-Boston corridor, Atlanta, GA, Houston, TX, Dallas/Ft. Worth, TX, Chicago, IL and St. Louis, MO. With the exception of Chicago, IL these areas of violation surround their respective metropolitan area. For Chicago, IL the area of violation is displaced to the eastern shores of Lake Michigan.

The areas of violation using the 8-hour standard, Figure 2 shows that in addition to the AOVs for the 1-hour standard there is broad region of exceedances, covering the industrial states just north of the Ohio River. For both measures of violations, the shape and size of AOVs for Dallas, TX and Houston, TX is rather uncertain because of low monitoring station density for the surrounding rural sites.

It is evident, that discernible differences exists in the exceedence pattern based on the 1 and the 8-hour standard. In particular, the ozone exceedences over the industrial states (from Illinois to Pennsylvania) are higher for the 8-hour standard. On the other hand, the 8-hour exceedences in the vicinity of New York City and Houston, TX. are reduced compared to the 1-hour standard.

The exceedance maps were used to delineate the areas of violation based on the 1-hour (Figure 1b) and 8-hour (Figure 2b) standard. The AOVs were drawn crudely by hand to indicate the somewhat arbitrary definitions.

Figure 1a. Spatial pattern of 1-hour exceedences. The contours are for 1 and 2 exceedences (>120 ppb)/year.
1b. Same with crude marking of areas of violations, AOVs

Figure 2. Spatial pattern of 8-hour exceedences. The contours are for 8 and 12 exceedences (>80 ppb)/year.
2b. Same with crude marking of areas of violations, AOVs

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