NOTE

Regulatory Impact of Pollutant Concentration Fluctuations

Meteorology fluctuates from hour to hour, day to day, year to year, etc. So do many pollutant concentrations, being substantially influenced by meteorology. Ozone, in particular, is known to correlate significantly with ambient temperature. A well known example is the heat wave of 1988 that drove a large number of otherwise attainment areas in the eastern U.S. into nonattainment. The significance of concentration fluctuations cannot be ignored when the attainment status and/or control implementation of an area is at stake. The attainment test for the present ozone air quality standard stipulates that an area is violating the standard if any monitor in the area registers four or more exceedances of the level of the standard in three years. For many areas, because of year-to-year fluctuations of the ozone concentration, flip-flops in the attainment status will occur. To avoid flip-flops and secure a long-term attainment status, an area must effectively lower the design value well below the level of the standard to make room for the impact of meteorological fluctuation. There has been concern that if the effective target of the design value must approach the remote-area background ozone concentration, then achieving the standard may be impossible for many areas.

To illustrate the problem described above, Chock analyzed an EPA dataset covering the period of 1983- 1995. For the analysis to be meaningful, he considered only counties with no missing design values for all eleven consecutive three-year periods (designated 1985-1995). Two hundred and sixty nine counties satisfied this constraint. The long-term mean of the design values for each county is defined as the average of the eleven design values. Table 1 shows the statistics of violation of a given ozone air quality standard for counties whose long-term means of the corresponding design values are less than the level of the standard. The forms and levels of the standard considered are shown in the Table. Note that 40 to 60 percent of the counties with a long-term mean of the design values below the level of the standard actually violate the standard at least once over the eleven three-year periods. The long-term mean below which no violation of a given standard is observed is also presented in the Table. This value is the "effective" design value to assure compliance of the standard for the eleven consecutive three-year periods based on the present attainment test. As the length of time increases, this value will likely decrease and can get very close to the range of the observed natural or remote-area background ozone concentrations which is displayed in the last column of the Table. The range of the background concentrations corresponding to a given design value (e.g., the three-year average of the annual 3rd highest daily maximum 8h concentrations) are extracted from the data for monitors used recently to represent areas with background ozone concentrations. Note that the potential achievability problem can be illustrated only with a long- term data base. The current practice of looking at the number of nonattainment areas or counties in one given three-year period is insufficient to reveal the achievability problem.

Table 1 Statistics of violations of a given ozone air quality standard for counties whose long-term means of the corresponding design values are less than the level of the standard.*

Design Value

(Form of the std)

Total # of counties w/ mean < std

# of counties w/ 1 violation of the std

# of counties w/ 2 violations of the std

# of counties w/ 3 violations of the std

# of counties w/ >3 violations of the std

Total # of counties w/ > 1 violation of the std

% of counties w/ > 1 violation of the std

Long-term mean (ppb) below which no violation was observed Natural or remote-area background O3 concentration range (ppb, 3-yr avg) for the corresponding design value 4th in 3 yrs**

(1h, 1ex, >125 ppb)

164

14

8

25

21

68

41%

107.7

53-92 8h, ann. 5th, >85 ppb 116 12 14 12 24 62 53% 75.7 38-65 8h, ann. 2nd, >85 ppb 57 7 4 4 22 37 65% 76.8 44-68 8h, ann. 3rd, >85 ppb 76 4 9 11 17 41 54% 78.1 41-66 8h, ann. 3rd, >80.5 ppb 39 3 4 6 6 19 49% 74.3 41-66 *269 counties satisfy the requirement of having 11 consecutive 3-year periods of data. The long-term mean is the mean of the design values for the 11 consecutive, overlapping 3-year periods. The present attainment test is used. **The long term mean is the average of 10 overlapping 3-year periods in 268 counties. The design values for the 1992-1994 period are not available to us.