Emissions Down, Smog Up

Was 2003 the year we started losing the battle against ozone smog? You’d think so if you read the press and broadcast industry headlines. “Smog Woes Back on Horizon,” trumpeted the Los Angeles Times last July. “It’s One Smoggy Summer,” declared the Associated Press. And USA Today joined in with “Smoggy Skies Persist Despite Decade of Work.”

Unfortunately, these stories and others like them contain more errors than facts. According to USA Today, “One likely reason why the smog isn’t lifting is Americans are driving more miles than they did in the 1980s. And they’re driving vehicles that give off more pollution than the cars they drove in the ’80s.”

In reality, even with increased miles driven, total oxides of nitrogen emissions (NOx) from automobiles are down 56% since 1985, according to the most recent Environmental Protection Agency data, and emissions of volatile organic compounds (VOC) are down 67 percent.

NOx and VOCs work together to form ozone. Data from vehicle emissions inspection programs show that SUVs and pickups have had about the same VOC emissions as cars since the 1996 model year, while an already small NOx difference disappeared with the 2001 model year.

On-road pollution measurements show that average vehicle emissions are dropping about 10 percent a year as the fleet turns over to inherently cleaner and more durable vehicles, including modern sports utility vehicles. However, total driving is increasing only about 1 percent to 3 percent each year, giving net annual emissions declines of about 7 percent to 9 percent.

The EPA’s progressively tighter standards for diesel trucks and offroad vehicles are resulting in similar emissions declines.

You won’t learn much from environmental activists, who are no doubt getting ready to say, “I told you so!” Having complained that President Bush’s changes to New Source Review--a regulatory regime for industrial pollution sources like power plants--would increase pollution, the uptick in ozone in some areas looks like vindication. However, the changes to NSR have nothing to do with the recent ozone trends.

One reason is that the changes haven’t even taken effect yet. More importantly, large industrial polluters have been and continue to be under progressively declining pollution caps that are unaffected by NSR.

The EPA monitoring data shows large declines in industrial emissions during the last 20 years, and already adopted requirements ensure these gains will continue, regardless of NSR policy. The major press has thus overlooked a key story in environmental protection: If ozone-forming emissions are falling, why are ozone levels flat or rising in many areas?

The answer lies in a curious anomaly in the smog statistics: A disproportionate number of violations of the ozone standard occur on weekends, even though NOx emissions drop anywhere from 10 percent to 40 percent. And NOx declines are due to sharp reductions in diesel truck traffic and off-road diesel equipment use on weekends.

If ozone violations were equally likely on any day of the week, you’d expect two-sevenths, or about 29 percent, of ozone violations to occur on weekends. However, despite weekend NOx reductions, substantially more than 29 percent of ozone violations occur on weekends. In Los Angeles, for example, the fraction is nearly 50 percent.

This “weekend effect” is becoming increasingly prevalent in many other major cities such as Denver, Chicago, Philadelphia, and New York.The press and many environmental activists are oblivious, yet the phenomenon is well known to air-quality scientists, who have been giving the matter increasing scrutiny.

The findings of these efforts are startling. NOx reductions appear to be the chief cause of weekend ozone increases. Ozone forms from NOx and VOCs on warm, sunny days. The process isn’t linear, but depends on the ratio of VOC to NOx.

When that ratio is high, ozone formation is limited by the availability of NOx, and VOC reductions have no effect. However, at low ratios, VOC reductions begin to reduce ozone. The rub is that under this VOC-sensitive condition, reducing NOx increases ozone.

Urban areas tend to have the lowest VOC/NOx ratios and are therefore the most prone to harm from NOx reductions. VOC/NOx ratios have also been declining in most areas, exacerbating the problem.

To state this confusing matter in inverse terms, higher emissions of NOx on weekdays often have the paradoxical effect of inhibiting ozone formation. This runs opposite of what common sense would suggest: If there is less pollution coming out of tailpipes and smokestacks, there should be less pollution in the air we breathe. While true for most pollution, it is not always true for ozone.

Admitting that NOx reductions have become detrimental to ozone control would be a major embarrassment for the EPA, which has issued stringent regulations that will eliminate most remaining NOx emissions from motor vehicles, power plants, and other pollution sources during the next 20 years.Yet even the EPA has implicitly acknowledged the NOx problem.

When the EPA in 1999 required large NOx reductions from new automobiles, the agency’s own analysis concluded that the rule would increase ozone in many areas of the country.

Because of the quirky relationship between NOx, VOCs, and ozone formation, current regulatory policy is leading to the paradoxical world where emissions will continue to fall, but ozone levels will likely increase or remain stable for the next several years.

In the long term, already adopted requirements will lead to lower ozone levels, because the NOx disbenefit will begin to disappear once NOx is reduced more than about 50% below current levels.

Fortunately, there’s no need to trade short-term harm for long-term gain. The key is realizing that current EPA regulations will eliminate almost all remaining VOC and NOx during the next 20 years, as the vehicle fleet evolves into one composed of inherently cleaner models.

Thus, a more sensible strategy for both the short and long term would be for the EPA to seek more rapid VOC reductions, 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 the EPA’s ozone standard on the same schedule as currently planned, but with less harm in the interim. In addition, this change would give each nonattainment area flexibility to tailor its ozone reduction strategy based on the specifics of local emissions and air chemistry.

Rapid VOC reductions are readily available. Automobiles contribute 50 percent to 75 percent of VOC emissions in metro areas, and the worst 5 percent of cars account for 50 percent of the automobile contribution. Emissions inspection programs are supposed to repair these cars, but they suffer from noncompliance, leaving many “gross polluters” still on the road.

Instead of ineffective periodic tests, the few gross polluters can be identified on the road with an inexpensive technology called remote sensing, and their owners required to repair or voluntarily scrap their cars for a cash incentive.

The EPA and state regulators have resisted replacing their pet inspection programs with on-road testing. But there is no other means to more substantial, more rapid, or less expensive improvements in air quality.

Joel Schwartz is a visiting fellow at the American Enterprise Institute. Steven F. Hayward is the F. K. Weyerhaeuser Fellow at AEI.

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About the Author


Steven F.
  • Steven F. Hayward was previously the F.K. Weyerhaeuser Fellow at AEI. He is the author of the Almanac of Environmental Trends, and the author of many books on environmental topics. He has written biographies of Presidents Jimmy Carter and Ronald Reagan and of Winston Churchill, and the upcoming book, The Politically Incorrect Guide to the Presidents. He contributed to AEI's Energy and Environment Outlook series. 

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