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Weekend Effect Makes Costly Measures Backfire
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The U.S. Environmental Protection Agency (EPA), which is forcing Americans to spend billions of dollars per year to address ozone air quality, is actually making the situation worse.
Visiting Fellow Joel Schwartz
The evidence is an air pollution phenomenon known as the “weekend effect.”
Ozone is formed in the atmosphere on sunny days from reactions between oxides of nitrogen (NOx) and volatile organic compounds (VOC). Every weekend in cities across America, ozone-forming emissions decline … but ozone levels stay the same or even rise.
Pollutants Fall, Ozone Rises
For example, in Los Angeles NOx and VOC decline, respectively, about 20 percent and 15 percent on Saturdays relative to weekdays. Nevertheless, ozone rises about 20 percent. On Sundays, NOx and VOC decline even more–about 35 percent and 20 percent, respectively, relative to weekdays. Yet ozone levels climb higher still, to about 30 percent above weekday levels.
Los Angeles has one of the worst weekend effects in the nation, but the pattern is similar all over the country. In Atlanta, NOx and VOC decrease, respectively, 57 percent and 17 percent on Sundays relative to weekdays, but ozone levels don’t change. In Cincinnati, NOx drops 40 percent on weekends, but with no change in ozone.
This is a problem because EPA and state regulators assume that reducing both VOC and NOx is necessary for attaining the federal eight-hour ozone standard, and they have built that assumption into NOx-reduction regulations that are costing Americans billions of dollars each year. But weekend-effect research suggests reducing NOx is at best slowing the pace at which ozone declines, and is even making ozone worse in some cities.
NOx Reductions Backfire
Scientists have observed the weekend effect for years, and numerous studies suggest NOx reductions are the culprit. Although NOx and VOC work together to form ozone, the effect is nonlinear and depends on the ratio of VOC to NOx in the atmosphere. At high VOC-to-NOx ratios–a condition referred to as being “NOx-limited”–reducing NOx reduces ozone, while reducing VOC has no effect.
On the other hand, at low VOC-to-NOx ratios–a “VOC limited” condition–reducing VOC reduces ozone, while reducing NOx increases ozone. (See sidebar for more on how this works.) Under VOC-limited conditions, if both VOC and NOx are reduced, the NOx reductions at best blunt the expected benefits of lower VOC–and at worst counteract them.
Over the past few decades, American metropolitan areas have been moving further into the VOC-limited regime, because VOC has been reduced more rapidly than NOx. Most VOC comes from gasoline engines, mainly automobiles, and total automobile VOC emissions have been dropping about 10 percent per year for more than a decade as the fleet turns over to inherently cleaner cars. NOx emissions, on the other hand, are about evenly spread among automobiles, diesel trucks, off-road diesel equipment, and power plants, and those emissions have been dropping more slowly.
Eight-hour ozone levels declined only slightly during the 1990s in most of the United States, and even rose in a few areas, despite large reductions in VOC and smaller reductions in NOx. A decade of VOC and NOx reductions had little effect on ozone levels. The NOx reductions are the leading explanation.
Regulators Reject Facts
Starting in the late 1990s, EPA began pursuing large NOx reductions. Automobile “Tier 2″ standards started phasing in with the 2004 model year, requiring an 80 to 90 percent reduction in automobile NOx. A 90 percent reduction of NOx emissions from new diesel trucks begins in 2007, with similar requirements for off-road diesel equipment beginning in 2010. NOx from coal-fired power plants declined about 60 percent between 1998 and 2004 and will decrease still further under EPA’s Clean Air Interstate Rule.
Many coal-fired power plants are in rural areas, and it is possible rural NOx reductions are effective in reducing rural ozone levels, because ozone formation is more likely to be NOx-limited there. But “mobile-source” NOx reductions mainly affect urbanized areas, and are therefore likely to further slow or even reverse progress in reducing ozone levels in the places where most Americans live.
Admitting that NOx reductions are actually harmful, however, would be a major embarrassment for federal and state regulators. Not surprisingly, they have vigorously resisted the implications of weekend-effect research.
VOC Is Key
Even during the Clinton administration, EPA concluded the measures necessary to attain the federal eight-hour ozone standard would impose costs on the American public far greater than the benefits achieved. And economists within the Clinton administration but outside EPA believed the agency had drastically low-balled the cost estimates. Nonetheless, the Bush administration plans to tighten the ozone standard still further.
Current federal ozone policy can only make Americans worse off. But by rationalizing ozone-control strategy, we can at least reduce the damage. Ideally, EPA should put the brakes on NOx reductions in urban areas by backing off on NOx requirements for new motor vehicles and retrofit programs.
At the same time, regulators should speed up VOC reductions. Automobiles contribute most VOC emissions, and the worst 5 percent of automobiles account for half the total VOC contribution by automobiles. 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.
What makes this strategy appealing is that VOC reductions will indeed reduce ozone in most places, especially the places where most people live. There is no other means to more substantial, more rapid, or less expensive improvements in ozone air quality.
How Ozone Is Formed
NOx, shorthand for the sum of NO2 + NO, is necessary for the formation of ozone (O3). Even when volatile organic compounds (VOC) are not present, ozone is formed through a series of sunlight-driven reactions among NO2, NO, and oxygen:
This cycle results in relatively low ozone levels. Ozone can’t build up because, although it is formed in reaction (2), it is destroyed in reaction (3).
But add VOC, and ozone builds up. VOC allow NO2 to be regenerated without destroying ozone. That is, VOC allow reaction (3) to be bypassed. OH radicals (also generated by various reactions among pollutants in the atmosphere) convert some VOC to peroxy radicals, which then regenerate NO2 as follows:
. . . where the two oxygen atoms (“OO”) are the peroxide group attached to a VOC.
Ozone formation depends on the ratio of VOC to NOx (VOC/NOx). At high VOC/NOx ratios, ozone formation is controlled by the amount of NOx available, and reaction (4) is the main route to regenerate NO2 from NO. Under this “NOx-limited” situation, decreasing NOx reduces ozone, while decreasing VOC has little or no effect on ozone.
But at low VOC/NOx, ozone formation is limited by the amount of VOC available for reaction (4), and reaction (3) becomes the main route to regenerate NO2 from NO. In addition, at low VOC/NOx, NO2 competes with VOC to react with OH radicals, slowing the rate at which VOC is converted to peroxy radicals, and thereby slowing the rate of reaction (4).
Under this “VOC-limited” or “VOC-sensitive” condition, reducing VOC reduces ozone, but reducing NOx increases ozone. The NOx reductions increase ozone through two means: First, by slowing down the rate of ozone destruction through reaction (3), and second, by speeding up the rate of NO2 regeneration through reaction (4), allowing each molecule of NOx to make ozone more rapidly.
Joel Schwartz is a visiting fellow at AEI.
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