The current system of monitoring and calculation ozone (O3) nonattainment needs to be revised so that meaningful area strategies to control ozone can be implemented. These revisions include: (1) using weighted composite readings from area monitors to determine general attainment status and (2) using eight-hour rolling averages to diminish the impact of uncontrollable transient conditions. Implementing these revisions will: (1) reduce the likelihood of a single monitor forcing an entire area into nonattainment, (2) employ specific strategies to mitigate the impact of localized high levels, and (3) encourage the placement of more air quality monitors across the state. As a result of this strategy, the ozone attainment status of three of the four nonattainment areas in Texas remains the same. Only El Paso, which was marginally above the existing standard for ozone, changes ozone attainment status. Using the criteria described below, the impact of this proposal on several areas of Texas is described in the attached bar graph.|
NOTE: The following provisions are linked together as joint components of a carefully formatted strategy and are not individually severable.
Composite Readings. Some of the nonattainment areas can be as large as 7,800 square miles with more than 14 monitors. Hourly composite readings (i.e. weighted averages of the hourly averages from all the monitors in the control area) provide a much clearer picture of the area air quality than a single monitor. By comparing these composite readings to established standards, we can more clearly determine when area controls are working or even warranted. To accommodate missing data and provide an incentive to maintain high monitor realiability and representative composite averages, values for monitors not achieving good data return during ozone forming hours (daylight) could be substituted using any one of a number of scientifically creditable methodologies developed over the past few years. This composite reading approach is recommended without regard to the level of the standard whether it its at the current 0.12 ppm or some other figure because it is more reflective of the areawide ozone levels.
Rolling Averages. The current practice of using one-hour averages from single monitors to determine levels is far too sensitive to meteorological conditions such as approaching weather systems. Because these weather systems are generally short-lived, it makes sense to the rolling average to smooth out the effect of such spikes. Current discussions concerning this issue focus on three components: (1) length of the rolling average period, (2) the level of the ozone standard for an exceedence, and (3) the number of
allowable exceedence days. Using an eight-hour rolling average reduces the impact of spikes and more accurately represents ozone formation. Health data indicate that adverse health effects are more likely to occur over longer periods of exposure. As is currenlty under consideration in a national forum and under this strategy, the ozone standard could be set as low at .070 ppm or as high as .090 ppm. However, the lower standard would require more allowable exceedence days than the higher one. Assuming the implementation of composite area readings, the standard should be no lower than .080 ppm ozone for an eight-hour rolling average with an allowable of at least five exceedences per year averaged over a three-year period. The most recent five years would be considered in each case, throwing out the highest and the lowest year. This standard is designed to be adequately protective of human health for the general population of the area. On the other hand, a higher ozone level at individual monitors may have the potential to affect members of the population living near the monitors. Specific programs to notify affected populations and address any such higher level will need to be developed and implemented.
Area Control Strategy. Given the size of attainment monitoring areas in Texas, subjecting vast areas to air quality controls based on readings of one monitor exceeding the existing standard only an exceptionally small percentage of the time is costly and provides marginal ozone protection benefits. Before spending millions of dollars, it makes sense to determine if the observed problem is an isolated occurrence of symptomatic of a larger area problem.
Individual Monitor Mitigation Strategy. To ensure that any affected populations are adequately protected, the following mitigation strategy is proposed to address those individual monitors that indicate a level of ozone above 0.12 ppm for more than one hour, appropriate notification procedures will be implemented. These procedures will be used to advise the public living near that monitor of the high ozone level and the appropriate precautionary measures to avoid unnecessary exposure. Should this level of ozone (0.12 ppm for more than one hour) be experienced at the same monitor more than five times in any year, specific programs to determine the cause of those levels of ozone and implement plans to reduce the localized high levels will be required.
Encouraging More Monitors. Presently, the approach of the relying solely on individual monitor readings to determine attainment status discourages the proliferation of monitors to uncovered areas. The potential costs generated by required follow-up to a few hours of exceedences on even a single monitor compel people to avoid additional monitors. Unfortunately, this avoidance frustrates attempts to monitor and model: (1) background or naturally occuring O3 precursor sources and (2) ozone transport from and to urban areas. By using composite monitor readings and eight-hour rolling averages and by negotiating with the U.S. Environmental Protection Agency to allow specific monitors to be designated for scientific research rather than attainment determination, we reduce the resistance to installing additional monitors. The data from additional monitors, would provide us with a more complete understanding if the cause-effect relationship involved and allow us to develop more effective area control strategies. It would also provide more information to allow evaluation of the causes of any higher levels of ozone at individual monitors and development of programs to reduce those levels consistent with providing protection to populations living near those monitors.
These changes to the current federally authorized system of monitoring and calculating O3 nonattainment need to be made so we can focus limited resources on fixing clearly defined air quality problems. By using composite area readings and eight-hour rolling averages, we can improve area control strategies and encourage putting in more monitors. Where monitors show hot spots, we can mitigate the impact of exposure through better notification and put our energies on identifying and fixing the cause of the localized high ozone level. All of these results will help us better understand the nature of and the remedy for ozone pollution.