NCPA - National Center for Policy Analysis

How Lower Emissions Standards Can Increase Smog

February 16, 2004

Ozone levels have recently increased in some metropolitan areas across the United States. This is surprising to the Environmental Protection Agency (EPA) because the vehicle emissions that cause ozone, nitrogen oxides (NOx) and volatile organic compounds (VOCs), are down 56 percent and 67 percent, respectively, since 1985.

The real cause of this problem may be the little-known "weekend effect," which is actually exacerbated by EPA regulations. Here's how it works:

  • Scientists have observed that high ozone levels occur primarily on weekends, when nitrogen oxide emissions are often 40 percent lower than weekdays due to the sharp decline in diesel truck traffic.
  • Most scientists believe that this is due to chemical interactions between NOx and VOCs -- depending on the ratio of these two chemicals, NOx can actually inhibit ozone formation.
  • However, when there are far more VOCs in the atmosphere than NOx, ozone levels increase -- thus, for example, at some monitoring locations in Los Angeles, weekends account for nearly 80 percent of total exceedances of the ozone levels required by clean air standards.
  • When EPA in 1999 proposed requiring a 90 percent reduction in automobile NOx emissions, the agency's own analysis concluded that it would increase ozone in many areas.

Thus, paradoxically, emissions will continue to fall rapidly, but ozone ambient levels may increase. It would be better, say scientists, to reduce VOC levels faster than NOx, such that they remain ahead of NOx emission declines for several years. This could be done at a low economic cost, because the worst 5 percent of vehicle emitters are responsible for half of automobile VOC emissions. By contrast, rapid compliance with the EPA's proposed rule to reduce NOx emissions from non-vehicular diesel engines is expected to be costly -- and could worsen ozone problems.

Source: Joel Schwartz and Steven F. Hayward, "Emissions Down, Smog Up. Say What?" Environmental Policy Outlook, No. 16242, January-February 2004, American Enterprise Institute.

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