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The "Piston Effect"

Drawing by Karen Lefohn
Copyright held by A.S.L. & Associates

The previous 1-hour primary and secondary federal standards for ozone were each set at a level of 0.12 ppm. The standards were attained when the expected number of days per calendar year with maximum hourly average concentrations above 0.12 ppm was equal to or less than 1, averaged over 3 years (i.e., no more than 3 exceedances in 3 years). Current emissions control strategies focus on reducing the highest hourly average concentrations.

For developing appropriate emission control strategies for meeting the 8-hour standard, it is important to identify the unique patterns of hourly average concentrations that make up the 8-hour violations and to investigate the importance of mid-range concentrations (i.e., 0.06-0.09 ppm) in defining these violations. For those sites whose 8-hour violation is mostly defined by hourly average concentrations below 0.09 ppm, control strategies will have to change from focusing on the higher hourly average concentrations to the mid-level (i.e., 0.06 to 0.09 ppm) hourly average values.

Using the EPA's Aerometric Information Retrieval System (AIRS), renamed the Air Quality System (AQS), A.S.L. & Associates found that for the period 1993-1995, approximately 50% of the areas that violated the 8-hour standard were influenced by 4 or more occurrences of mid-level hourly average concentrations (i.e., less than or equal to 0.09 ppm).

In addition, A.S.L. & Associates identified those sites that demonstrated a significant reduction in ozone levels for the period 1980-1995. Using the data from the sites that experienced reduced ozone levels over the period of time, A.S.L. & Associates investigated whether the rate of decline of the mid-level hourly average concentrations was similar to the rate experienced by the high hourly average concentrations. The analysis indicated that there is a greater resistance to reducing the hourly average concentrations in the mid range than the hourly average concentrations above 0.09 ppm. Figure 1 below is an example that shows that the higher hourly average concentrations (i.e., above 90 ppb) decreased at a faster rate (greater negative rate per year) than the hourly average concentrations in the mid-level range. The numbers of hourly average concentrations in the low end of the distribution also decreased. Both the high and low ends of the distribution are moving toward the center of the distribution.

As control strategies are implemented, the resistance to lower the higher hourly average concentrations will be low, but the resistance will increase as one attempts to reduce the mid-level values. This experience is similar to a piston compressing against a gas. The resistance is initially low; however, the resistance increases as the piston continues to compress against the gas.

Based on this analysis, which used empirical data, it appears that when control strategies are implemented, the "piston effect" will cause higher hourly average concentrations to be reduced faster than the mid-level values. Similar to the results obtained for the hourly values, the rate of decline for the 8-hour daily maximum values in the mid range will be much slower than the higher 8-hour values. Figure 2 illustrates the slowing down process for Fairfield County, Connecticut. A rapid decrease in the early years occurred, but less of a decline occurred in the later years.

The year-by-year figure below illustrates the changes in the 4th highest 8-hour daily maximum concentration. A rapid decrease in the early years occurred, but less of a decline occurred in the later years. For this monitoring site, a weaker statistically significant downward trend was observed for the 1994-2008 period than for the 1980-2008 period (-1.07 %/year versus -1.82 %/year, respectively) (Lefohn et al., 2010).

For some sites that violate the 8-hour primary standard, the attainment of this standard may be extremely difficult. A peer-reviewed paper describing the piston effect and its consequences was published in the June 1998 issue of Environmental Science & Technology (Lefohn et al., 32(11):276A-282A). In addition, in 1997, the "Piston Effect" was discussed in an Atmospheric Environment New Directions Column.

EPA has commented on the effects of the "piston effect". In its October 5, 1998 report "Use of Models and Other Analyses in Attainment Demonstrations for the 8-Hour ozone NAAQS," the Agency noted

"Analyses of regional modeling results suggest that relative reduction factors tend to have lower values (i.e., greater reductions occur) if the starting concentrations are higher (ECR, 1998; Meyer et al., 1997). Lefohn, et al., (1998) report similar findings in their review of recent trend data."

Using models, several investigators have commented on the difficulty in reducing the mid- level hourly average concentrations while reducing the fourth highest 8-hour average daily maximum concentration. Saylor et al. (1999) noted that for the Atlanta, Georgia area, NOx emissions reductions greater than 60 – 75% would be required to reduce the mid-level hourly average concentrations. Winner and Cass (2000) noted that the higher hourly average concentrations were reduced much faster than the mid-level values during simulation modeling for the Los Angeles area. Reynolds et al. (2003) analyzed ambient ozone concentrations used in conjunction with the application of photochemical modeling to determine the technical feasibility of reducing hourly average concentrations in central California. Various combinations of volatile organic compounds and oxides of nitrogen emission reductions were effective in lowering modeled peak 1-hour ozone concentrations. However, VOC emissions reductions were found to have only a modest impact on modeled peak 8-hour ozone concentrations. Reynolds et al. (2003) reported that 70 – 90% NOx emissions reductions were required to reduce peak 8-hour ozone concentrations to the desired level in central California.

Reynolds et al. (2004) used ozone measurements in conjunction with photochemical modeling to assess the feasibility of reducing hourly average concentrations in the eastern United States. The authors reported that various combinations of volatile organic compound and oxides of nitrogen emission reductions were effective in lowering modeled peak 1-hour O3 concentrations. VOC emission reductions alone had only a modest impact on modeled peak 8-hour ozone concentrations. Anthropogenic NOx emissions reductions of 46-86% of 1996 base case values were needed to reach the desired level of the 8-hour value in some areas.

Reynolds et al. (2003, 2004) have commented on possible chemical explanations for the observation that more prominent trends in peak 1-hour ozone levels than in peak 8-hour ozone concentrations or in occurrences of mid-level (i.e., 0.06 – 0.09 ppm) have been reported. The authors noted that when anthropogenic VOC and NOx emissions are reduced significantly, the primary sources of ozone precursors are biogenic emissions and CO from anthropogenic sources. Chemical process analysis results indicated that slowly reacting pollutant such as CO could be contributing on the order of 10 – 20% of the ozone produced. The authors recommended that further work focus on the need to confirm that biogenic emissions have not been significantly overestimated in the most recent emission inventories and on the examination of the effects of CO reductions.

In its review of possible changes to the federal ozone standard, EPA's recent results (2014) indicate that the highest concentrations are reduced faster than the mid-level concentrations as emission reductions are simulated. Thus, the "piston" effect has been simulated in the EPA's latest modeling results.

If you would like to read a summary of our May 1997 report describing our findings, please click here.

References

Lefohn, A.S., Shadwick, D., Oltmans, S.J. (2010). Characterizing Changes of Surface Ozone Levels in Metropolitan and Rural Areas in the United States for 1980-2008 and 1994-2008. Atmospheric Environment. 44:5199-5210.

Reynolds, S. D.; Blanchard, C. L.; Ziman, S. D. (2003) Understanding the effectiveness of precursor reductions in lowering 8-hr ozone concentrations. J. Air & Waste Manage. Assoc. 53: 195-205.

Reynolds, S. D.; Blanchard, C. L.; Ziman, S. D. (2004) Understanding the effectiveness of precursor reductions in lowering 8-hr ozone concentrations - Part II. The Eastern United States. J. Air & Waste Manage. Assoc. 54: 1452-1470.

Saylor, R. D.; Chameides, W. L.; Change, E. C. (1999) Demonstrating attainment in Atlanta using urban airshed model simulations: impact of boundary conditions and alternative forms of the NAAQS. Atmos. Environ. 33: 1057-1064.

US Environmental Protection Agency, US EPA, 2014. Policy Assessment for the Review of the Ozone National Ambient Air Quality Standards. EPA/452/R-14-006. Research Triangle Park, NC: Office of Air Quality Planning and Standards. August.

Winner, D. A.; Cass, G. R. (2000) Effect of emissions control on the long-term frequency distribution of regional ozone concentrations. Environmental Science & Technology. 34: 2612-2617.

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