EPA Proposes Secondary Standards for NOx and SOx

The U.S. Environmental Protection Agency (EPA) last week proposed secondary air quality standards for nitrogen oxides (NOx) and sulfur oxides (SOx), building on rules—like the recently finalized Cross State Air Pollution Rule—to reduce NOx and SOx emissions.

As part of its proposed action, announced on July 12, the EPA said it would combine a review of the secondary National Ambient Air Quality Standards (NAAQS) for oxides of nitrogen and sulfur. The EPA sets secondary standards to protect against environmental damage caused by certain air pollutants. “Consistent with the scientific evidence pointing to the interrelated impacts of NOx and SOx on plants, soils, lakes and streams, EPA assessed the environmental effects of these pollutants together,” it said, noting that NOx and SOx in the air can damage the leaves of plants, decrease their ability to produce food – photosynthesis – and decrease their growth.

“This is the first time EPA has reviewed the environmental impacts separately from the health impacts of these pollutants. It is also the first time the Agency has examined the effects of multiple pollutants in one NAAQS review,” the agency said.

Essentially, the agency is proposing to retain the existing secondary standards for NOx and SOx, concluding that the existing secondary standards protect plants from the direct effects of exposure to these pollutants in the air (e.g., decreased growth and foliar injury). The existing
secondary standards are:

  • For NO2: 0.053 ppm (parts per million) averaged over a year; and
  • For SO2: 0.5 ppm averaged over three hours, not to be exceeded more than once per year

But it is also is proposing to establish an additional set of secondary standards identical to the new health-based primary standards the agency set in 2010. The new secondary standards would be:

  • For NO2: 100 ppb (parts per billion) averaged over one hour; and
  • For SO2: 75 ppb averaged over one hour

“EPA has made significant progress in developing a multi-pollutant standard that would protect vulnerable ecosystems, including streams and lakes. To ensure any updated standard is effective, EPA is planning to conduct a field pilot program to collect and analyze additional data and information,” the agency said.

The EPA will accept comments for 60 days after the proposed rule is published in the Federal Register and will issue a final rule by March 2012.

Sources: POWERnews, EPA

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