NCPA - National Center for Policy Analysis


June 16, 2006

The Environmental Protection Agency (EPA), which forces Americans to spend billions of dollars per year to address ozone air quality, is actually making the situation worse, says Joel Schwartz of the American Enterprise Institute.

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.
  • This is a problem because EPA and state regulators assume that reducing both VOC and NOx is necessary to attain the federal eight-hour ozone standard

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. 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. 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, says Schwartz.

Source: Joel Schwartz, "EPA Rule Is Making Ozone Smog Worse," Environment News, May 1, 2006.


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