US EPA

EPA has announced that the Second External Review Draft of the Integrated Science Assessment (ISA) for Oxides of Nitrogen and Sulfur - Environmental Criteria has been made available for independent peer review and public review. This draft ISA document represents a concise synthesis and evaluation of the most policy-relevant science and will ultimately provide the scientific bases for EPA's decision on retaining or revising the current secondary standards for NO₂ and SO₂.

Oxides of nitrogen and oxides of sulfur are two of six principal (or criteria) pollutants for which EPA has established national ambient air quality standards (NAAQS). The Clean Air Act requires EPA to periodically review the scientific basis for these standards by preparing an Integrated Science Assessment (ISA), formerly called an Air Quality Criteria Document (AQCD). The previous scientific review documents for oxides of nitrogen and sulfur were released in 1993 and 1982, respectively.

This draft ISA was prepared as part of the joint review of the secondary or welfare-based NAAQS for oxides of nitrogen and sulfur. The draft ISA evaluates and integrates the newly available environmental evidence, and contains the key information and judgments formerly found in the AQCD. A series of Annexes to the draft ISA provides more extensive and detailed summaries of the most pertinent scientific literature. This is EPA’s latest evaluation of the scientific literature on the potential adverse environmental effects resulting from exposures to oxides of nitrogen and sulfur. There are significant new data, particularly with regards to acidification and nitrogen enrichment, that strengthen the evidence for these effects since the last scientific review documents were released.

The current secondary NAAQS for SOX, set in 1973, is a 3-h average 0.5 ppm of SO2, not to be exceeded more than once per year. The secondary NOX NAAQS is identical to the primary standard set in 1971: 0.053 ppm NO2 as an annual average. These secondary standards are intended to protect against direct damage to vegetation by exposure to gas-phase NOX or SOX. Acute and chronic exposures to SO2 can have phytotoxic effects on vegetation, such as foliar injury, decreased photosynthesis and decreased growth. Similarly, exposure to sufficient concentrations of NO2, NO, PAN, and HNO3 can cause foliar injury, decreased photosynthesis and decreased growth. In addition, these gas-phase NOX may contribute to N saturation in some areas of the U.S. There is little new evidence overall for direct effects of exposure to gas-phase NOX or SOX on vegetation at current concentrations. However, there is some evidence that vegetation in regions with high concentrations of photochemical oxidants may be affected by HNO3 exposure.

U.S. EPA. Integrated Science Assessment (ISA) for Oxides of Nitrogen and Sulfur - Environmental Criteria (Second External Review Draft, Aug 2008). U.S. Environmental Protection Agency, Washington, DC, EPA/600/R-08/082, 2008.

Included for public review is the draft Integrated Science Assessment (ISA) for Oxides of Nitrogen and Sulfur – Environmental Criteria (Second External Review Draft). Comments on this document may be submitted and reviewed using the e-Government Regulations.gov Web site. From the site, select Environmental Protection Agency and the keyword EPA-HQ-OAR-2007-1145 (for the docket ID) to comment on this report. The deadline for comments is October 1, 2008.