To the dismay of environmental groups, the U.S. Environmental Protection Agency (“EPA”) on July 13 declined to adopt a long-anticipated new method of setting a combined secondary national ambient air quality standard (“NAAQS”) for nitrogen oxides (“NOx”) and sulfur oxides (“SOx”) recommended by EPA’s Clean Air Scientific Advisory Committee (“CASAC”) and instead proposed a five-year study pilot research effort on the issue.  The new combined NOx-SOx standard would have used the acid neutralizing capacity (“ANC”) of water bodies, rather than the concentration of pollutants in ambient air, to determine compliance with secondary NAAQS

Secondary NAAQS aim to protect plants from the direct effect of air pollutants.  Rejection of the new ANC-based combined secondary NAAQS standard essentially leaves the current longer-term secondary NAAQS standards unchanged – a limit of 0.053 parts per million (“ppm”) for NO2 averaged over a year and 0.5 ppm for SO2 averaged over three hours (not to be exceeded more than once a year).  EPA also proposed an additional set of shorter-term SO2 and NO2 secondary standards to match recent revisions to primary (health-based) standards for the two pollutants – 100 parts per billion (“ppb”) averaged over one hour for NO2 and 75 ppb averaged over one hour for SO2.

CASAC had recommended EPA adopt the ANC-based combined standards approach but had warned that the method could result in an overly conservative emissions limit.  While Earthjustice, an environmental advocacy group, sharply criticized EPA’s decision, the Agency defended studying the issue further, stating it would be “premature” to set an “unprecedented” ANC-based combined NOx-SOx standard given the great uncertainties and complexities inherent in setting such a standard.  A five-year research effort will take place in three-to-five locations in “selected acid-sensitive ecoregions” and create a monitoring network to inform the next NAAQS review of NOx and SOx, according to EPA’s proposal.