First Do No Harm: EPA’s New Rules Will Worsen Smog

On May 13, 1999, the Environmental Protection Agency proposed new rules tightening motor vehicle emissions, including those from light trucks, a category of vehicles that includes the popular SUVs and minivans. In conjunction, the agency also proposed sharply lower standards for sulfur content in gasoline. EPA has attempted to justify these so-called “Tier 2/sulfur” rules by stating that the expected emissions reductions in oxides of nitrogen (NOx) would reduce ozone (the primary constituent of smog) and bring more areas of the country into attainment with the federal ozone standard. In fact, precisely the opposite is true – and EPA knows it. Despite its assertions to the contrary, the evidence shows that the new rules, if finalized in their current form, would actually increase smog in most major cities, and put as many as 12 areas out of attainment with the federal standard for ozone.

NOx Reductions – For Better And For Worse. Unlike other pollutants, ozone is not directly released into the atmosphere, but is created through a complex series of reactions involving NOx and volatile organic compounds (VOCs) in the presence of sunlight. In its recent proposal, EPA simplistically characterizes NOx as an ozone precursor, and thus implies that NOx reductions always yield ozone reductions.

The truth is far more complex, especially now that both NOx and VOC emissions have already been reduced under the Clean Air Act. In fact, the National Academy of Sciences has concluded that “NOx reductions can have either a beneficial or detrimental effect on ozone concentrations, depending on the locations and emissions rates of VOC and NOx sources in a region.” In particular, smog in many urban areas increases when NOx concentrations are further reduced, while declines generally occur in less heavily populated downwind areas. The inverse relationship between NOx and ozone in cities is particularly strong when the ratio of VOCs to NOx is relatively low, and thus will become more pronounced as VOC emissions continue to decline under the Clean Air Act.

These so-called NOx disbenefits have been documented in many cities by what is known as the weekday/weekend effect. Ozone levels in cities actually rise on weekends relative to weekdays, despite less automotive traffic and thus lower emissions of both VOCs and NOx. Simply put, in an environment of reduced VOC emissions, further NOx declines cause a measurable rise in ozone. If implemented, the Tier 2/sulfur proposal will cause these weekend-like effects to occur throughout the week in most urban areas.

Indeed, until a few years ago, EPA’s ozone National Ambient Air Quality Standards (NAAQS) attainment strategy wisely focused on reductions of VOCs because NOx reductions were viewed as largely counterproductive. The NOx reductions which have occurred were primarily technology-driven. Now, EPA has done a complete reversal, and is specifically targeting automotive NOx emissions.

A New Source of Smog – The EPA. EPA’s own evidence relied upon to support its rules shows that smog will increase in some urban centers, and decline mostly in rural areas. The real result will be increased pollution precisely where populations are concentrated. The agency has deceptively obscured the magnitude of this effect by using a regionally-based model which does not zero in on the negative effects in metropolitan areas. In effect, by blending in cities with the outlying areas, EPA has tried to hide the extent that the air will be made worse in urban centers. Even so, EPA’s evidence predicts increasing smog levels in several counties containing large cities.

However, the negative effects of Tier 2 /sulfur are far more extensive. Hidden behind EPA’s pollution gerrymandering is evidence that nearly every city will see a smog increase under Tier 2/sulfur. Table 1 focuses on 14 metropolitan areas likely to otherwise be borderline with respect to ozone attainment (0.124 parts per million) in 2007, when the new rules will be fully phased in. By taking the NOx/ozone relationship established by the weekday/weekend effect data, and plugging in EPA’s estimates of NOx reductions generated by Tier 2/sulfur, we calculate that smog will increase in each area as a result of the new rules.

Worse yet, as many as 12 cities (Atlanta, New Orleans, Boston, St. Louis, Kansas City, Memphis, Charlotte, Pittsburgh, Dallas, San Antonio, Chicago and Tulsa) would likely be pushed into non-attainment by Tier 2/sulfur. Not only would these areas suffer increased smog, they also would endure the economic penalties imposed by EPA on localities not meeting the Clean Air Act’s ozone standard – all for complying with the agency’s ill-conceived proposal.

Ozone Projected Increases from Tier2/Sulfur Rule


Baseline Ozone in 2007 (PPM)

Increase due to Tier 2/Sulfur Rule

Ozone in 2007 with Tier 2/Sulfur Rule




.120 – .124




.127 -.133

New Orleans



.125 – .128




.122 – .126

St. Louis



.121 – .126

Kansas City



.127 – .132




.129 – .134




.129 – .140




.127 – .134




.127 – .130

San Antonio



.122 – .126




.128 – .132




.121 – .124




.125 – .128

Source: Calculations by the authors, based on EPA data.

This evidence also shows that, without Tier 2/sulfur, only Los Angeles and Houston would not be in compliance with the ozone standard by 2007. Furthermore, neither of these cities would be helped by EPA’s proposal.

Conclusion. In sum, the proposed NOx controls are a counterproductive solution to the already-marginal problem of urban smog. A reassessment of Tier 2/sulfur is warranted.

1 Dr. Jones is the head of Zephyr Consulting, Seattle-based environmental consulting firm. Dr. Jones was a senior advisor on air quality to the President’s Council on Environmental Quality during the Ford and Carter administrations. He also served as a senior technical advisor and research manager at the Environmental Protection Agency. Ben Lieberman ([email protected]) is a policy analyst with the Competitive Enterprise Institute.

2 EPA, “Tier 2 Motor Vehicle Emission Standards and Gasoline Sulfur Control Requirements (Tier 2/sulfur)”, Federal Register, vol. 64, pp. 26,003-26,142 (June 30, 1999); EPA, Clarification of Proposed Rule, Provision of Supplemental Information and Request for Comment (EPA Supplement), Federal Register, vol. 64, pp. 35,112-35,119 (June 30, 1999).

3 National Academy of Sciences, “Rethinking The Ozone Problem In Urban And Regional Air Pollution,” National Academy Press, 1992, p. 12.

4 See, Altshuler, et al., “Weekday vs. Weekend Ambient Ozone Concentrations: Discussion and Hypothesis With Focus On Northern California,” JAWMA, vol. 45, December 1995, pp. 967-972; Vukovich et al., “A Preliminary Study of the Weekday/Weekend Differences in Ozone and its Precursors in Large Urban Regions,” Submitted to Atmospheric Environment, October 1998; Tran et al., “An Analysis of the Weekday-Weekend Behavior of Ambient Concentrations of Ozone and its Precursors,” Draft, California Air Resources Board, November 1996.

5 Abt Associates Inc., “Tier II Proposed Rule: Air Quality Estimation, Selected Health and Welfare Benefits, Methods, and Benefit Analysis Results,” April 1999, prepared for the Environmental Protection Agency; EPA Supplement.

6 Given its adverse impact on urban centers relative to outlying areas, Tier 2/sulfur also raises environmental justice concerns. President Clinton’s Executive Order 12898 encourages federal agencies to identify and address disproportionate environmental impacts on minority and low-income populations.

7 Jones, Kay H., and Lieberman, Ben, “EPA has Struck out its Tier II Proposal” Submitted to Journal of Environmental Management, Air and Waste Management Association, 1999.