TCS Daily

Damn the Science, Full Speed Ahead

By Joel Schwartz - August 20, 2003 12:00 AM

A funny thing happens on weekends in most cities. Nitrogen oxide emissions go down by 10 to 40 percent, but ozone smog levels stay the same or rise.


Ozone accumulates when nitrogen oxides (NOx) and volatile organic compounds (VOC) react in the presence of sunlight on hot, calm days. But under the right conditions -- conditions that now exist in many American cities -- reducing NOx can actually make ozone worse. This is challenge enough for areas trying to comply with EPA's tough new eight-hour ozone standard. But EPA and the California Air Resources Board (CARB) are compounding the problem by making believe it doesn't exist.


Indeed, despite the risks, during the last few years these agencies have promulgated regulations specifically intended to achieve large, nationwide NOx reductions. These regulations will cost billions of dollars per year, but risk worsening smog for millions of Americans.


The "Weekend Ozone Effect"...


Think of it as a natural test of an ozone reduction strategy. Each weekend NOx emissions go way down because of large declines in diesel traffic, but ozone stays the same or goes up. The phenomenon has been observed for decades in some areas, but ozone-monitoring data show that it is increasing in magnitude and has become the norm at almost all monitoring sites.1


Several studies have concluded the weekend decline in NOx emissions is, paradoxically, the cause of increased ozone.2-7 Ozone formation depends on the ratio of VOC to NOx. When VOC/NOx is high -- greater than about 10 -- ozone formation is limited by the availability of NOx, and VOC reductions have no effect on ozone levels.


But when VOC/NOx falls below 10, VOC reductions begin to reduce ozone. The rub is that under strongly VOC sensitive conditions, reducing NOx increases ozone. Urban areas tend to have the lowest VOC/NOx ratios and are therefore the most VOC sensitive. During the last few decades the VOC/NOx ratio has been declining in most areas, increasing the weekend effect.  


That emissions happen to change on weekends provides the opportunity to study the effect of NOx and VOC reductions on ozone levels. But the conclusion that NOx reductions are ineffective or detrimental applies to ozone formation on all days of the week. Thus, the conclusions of weekend effect research have general implications for air pollution policy.


Recent modeling studies have concluded that NOx reductions of up to 50 percent would increase ozone levels in many major cities, including New York, Chicago, Philadelphia, southern California, and the San Francisco Bay area. Beyond about 50 percent, the VOC/NOx ratio would become high enough that further NOx reductions would reduce ozone.3, 8, 9


In contrast, VOC reductions would reduce ozone at least to some extent almost everywhere, and would be very effective in some urban areas. However, to attain EPA's 8-hour ozone standard, VOC reductions would be insufficient, and NOx reductions on the order of 70 to 90 percent would ultimately be necessary in most metropolitan areas. Such large NOx reductions are unattainable during the next five to ten years-the amount of time allotted for non-attainment areas to meet the new eight-hour standard. This calls into question whether attaining the eight-hour standard is even feasible.


...And Its Discontents


There are other hypotheses for the origin of the weekend effect, including a change in the timing of emissions on weekends, or carryover of pollution from increased driving on Friday and Saturday evenings. If either of these were correct, NOx reductions might not be detrimental after all. However, based on observations of ambient pollution levels and emissions, combined with computer modeling of air chemistry and transport, a series of recent reports concludes that lower weekend NOx emissions are the major culprit.2-7


This isn't what regulators want to hear. Admitting that NOx reductions have become detrimental to ozone control would be a major embarrassment for both EPA and CARB. Both agencies have promulgated stringent regulations that will eliminate most NOx emissions from automobiles and diesel trucks during the next 20 to 30 years, as the fleet turns over to vehicles built to the tougher standards. EPA also requires a 60 percent reduction in NOx from coal-fired power plants starting next year.


Regulators are thus vigorously resisting the conclusions of independent researchers. But without an analytical leg to stand on, the agencies have resorted to red herrings that mainly obfuscate the debate. For example, in an opinion article in the magazine Environmental Manager, CARB's research staff argued that the correlation between declining ozone and declining NOx during the last 20 years shows that NOx controls are effective.10


But if correlation equals causation, presumably CARB would also agree that increased driving reduces air pollution, since driving has increased about 75 percent during the last 20 years. Of course, we can't make causal inferences on the basis of correlation alone. We have to look at underlying mechanisms to see whether our conclusions are plausible.


Increased driving obviously didn't reduce air pollution. Instead, the average emission rate of the vehicle fleet decreased much more rapidly than driving increased, so emissions declined overall in spite of increased driving. Likewise, we know from chemical mechanism studies that NOx has a non-linear relationship to ozone formation, with a key parameter being the VOC/NOx ratio. This ratio is low enough that marginal NOx reductions are counterproductive or ineffective.


CARB has also called into question the reliability of the models used to assess ozone formation and to test different control strategies, arguing that their output is too uncertain for drawing conclusions about the effect of NOx reductions on ozone levels.10 The irony here is that these are the same models that EPA and CARB use to set regional emission reduction requirements with ton-per-day precision, and to justify multi-billion-dollar pollution control regulations and plans.


Like many models of complex, real-world phenomena, urban airshed modeling is subject to uncertainties. But one might question the sincerity of regulators who ignore these uncertainties when implementing their favored programs, and highlight them in the context of politically unpalatable research results.


Although CARB disputed the weekend effect conclusions in an opinion article in a trade magazine, the agency's views were not able to pass the rigors of scientific peer review. The July 2003 issue of the Journal of the Air & Waste Management Association (JAWMA) devoted a special section to studies of the weekend effect, some of which are cited at the end of this column. The journal's reviewers rejected CARB's submission.


EPA also clearly sees the weekend effect conclusions as a threat to its regulatory strategy of large NOx reductions. The technical documentation for EPA's proposed off-road diesel rule, released in May, approvingly cites the CARB paper that was later rejected by JAWMA.11 Therein lies another irony: When EPA in 1999 promulgated a rule requiring a 90 percent reduction in NOx emissions from automobiles, the agency's own analysis concluded that the rule would increase ozone in many areas of the country.12


Desperately Seeking Net Benefits


EPA promulgated its stringent NOx requirements for vehicles and power plants specifically to aid attainment of the eight-hour ozone standard. The risks of NOx reductions aside, you still have to wonder why EPA is pursuing the eight-hour standard in the first place. Even EPA's own analysis concluded that the measures necessary to attain the standard would impose costs on the American public far greater than the benefits achieved.13 That should have stopped the agency in its tracks. It didn't and EPA plans to begin enforcing the standard next year.


Since we can't hope for net improvements in people's welfare, at least we can try to limit the damage. Given the risks of additional near-term NOx reductions, EPA should instead seek more rapid reductions in VOC, and, where possible, delay blanket national NOx reductions for several years.


What makes this strategy appealing is that VOC reductions will reduce ozone in most places, especially places where most people live. Furthermore, atmospheric modeling suggests that the detrimental effects of NOx reductions can be somewhat mitigated by front-loading VOC reductions to keep ahead of declines in NOx.


After substantial near-term VOC reductions, later NOx reductions would achieve eight-hour ozone attainment in the long term, but with less harm in the interim. In addition, this change would give each non-attainment area flexibility to tailor its ozone reduction strategy based on the specifics of local emissions and air chemistry.


Automobiles contribute 50 to 75 percent of all VOC emissions, and the worst five percent of cars accounts for half the automobile contribution. These cars can be identified on the road with remote sensing and their owners required to repair or voluntarily scrap their cars for a cash incentive. There is no other means to more substantial, more rapid, or less expensive reductions in VOC emissions.


Americans deserve regulations based on rigorous science that deliver net benefits to their welfare. It's unfortunate that environmental regulatory agencies are falling short on both of these key goals.


Joel Schwartz is an adjunct fellow with the American Enterprise Institute and the author of the new AEI study "No Way Back: Why Air Pollution Will Continue to Decline"


References Cited

1.     Heuss, J. M. et al. "Weekday/Weekend Ozone Differences: What Can We Learn from Them," Journal of the Air & Waste Management Association, 53, pp. 772-788 (2003).

2.     Lawson, D. R. "The Weekend Effect--the Weekly Ambient Emissions Control Experiment," Environmental Manager, pp. 17-25 (July 2003).

3.     Fujita, E. M. et al. "Evolution of the Magnitude and Spatial Extent of the Weekend Ozone Effect in California's South Coast Air Basin 1981-2000," Journal of the Air & Waste Management Association, 53, pp. 864-875 (2003).

4.                Yarwood, G. et al. "Modeling Weekday/Weekend Ozone Differences in the Los Angeles Region for 1997," Journal of the Air & Waste Management Association, 53, pp. 864-875 (2003).

5.     Marr, L. C. & Harley, R. A. "Spectral Analysis of Weekday-Weekend Differences in Ambient Ozone, Nitrogen Oxide, and Non-Methane Hydrocarbon Time Series in California," Atmospheric Environment, 36, pp. 2327-2335 (2002).

6.     Marr, L. C. & Harley, R. A. "Modeling the Effect of Weekday-Weekend Differences in Motor Vehicle Emissions on Photochemical Air Pollution in Central California," Environmental Science & Technology, 36, pp. 4099-4106 (2002).

7.     Pun, B. K. & Seigneur, C. "Day-of-Week Behavior of Atmospheric Ozone in Three U.S. Cities," Journal of the Air & Waste Management Association, 53, pp. 789-801 (2003).

8.                Reynolds, S. & Blanchard, C. L., "Understanding the Effectiveness of Precursor Reductions in Lowering 8-Hour Ozone Concentrations in the Eastern United States," Envair, San Rafael, CA, 2003.

9.                Reynolds, S. et al. "Understanding the Effectiveness of Precursor Reductions in Lowering 8-Hr Ozone Concentrations," Journal of the Air & Waste Management Association, 53, pp. 195-205 (2003).

10.   Croes, B. E. et al. "The O3 'Weekend Effect' and NOx Control Strategies: Scientific and Public Health Findings and Their Regulatory Implications," Environmental Manager, pp. 27-35 (July 2003).

11.   EPA, "Draft Regulatory Impact Analysis: Control of Emissions from Nonroad Diesel Engines. Chapter 2. Air Quality, Health, and Welfare Effects," Washington, DC, 2003,

12.   Abt Associates, "Tier II Proposed Rule: Air Quality Estimation, Selected Health and Welfare Benefits Methods, and Benefit Analysis Results," EPA, Research Triangle Park, NC, 1999.

13.   EPA, "Regulatory Impact Analyses for the Particulate Matter and Ozone National Ambient Air Quality Standards and Proposed Regional Haze Rule," Washington, DC, 1997,

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