Headlines
Secondary National Ambient Air Quality Standard
for Nitrogen Oxides (NOx) and Sulfur Dioxide (SO2)
The Environmental Protection Agency (EPA) is conducting a combined review of the secondary National Ambient Air Quality Standards (NAAQS) for Nitrogen Oxides (NOx) and Sulfur Dioxide (SO2). This is the first time since the NAAQS were established that EPA has assessed the impacts of multiple pollutants jointly. By jointly reviewing the impacts to environmental resources from NOx and SO2, EPA is setting precedence for multi-pollutant planning strategies.
North Carolina & Tennessee
NOx and SO2 are closely related pollutants with emissions from similar sources, common atmospheric transport, chemical transformation, and deposition processes, and related environmental impacts. NOx and SO2 contribute to deposition of nitrate (NO3) and sulfate (SO4) ions (also called acid deposition) that is adversely affecting sensitive aquatic and terrestrial ecosystems.
The Clean Air Act requires the secondary standard for NOx and SO2 to specify an acceptable level in the ambient air that is protective of known or anticipated adverse effects to public welfare. The current secondary standards for NOx and SO2 do not meet that requirement because there are documented adverse impacts at current NOx and SO2 concentrations.
EPA has completed a Risk and Exposure Assessment (REA) for NOx and SO2. As illustrated in the flow chart below, EPA takes a unique approach to link ambient air concentrations of NOx and SO2 to deposition of NO3 and SO4, and to indicators of ecological impacts. Acid deposition has been demonstrated to cause:
- acidification of sensitive lakes and streams and damage to sensitive aquatic species, including native trout in the eastern United States,
- acidification of sensitive forests in the eastern US, including sugar maple and red spruce in the Northeast and spruce and fir ecosystems in Southern Appalachia,
- changes to diatom populations in alpine lakes in the western US and contributing to eutrophication (reduced dissolved oxygen) of estuaries in the eastern US excess (from excess nitrogen deposition), and
- alterations to species composition and susceptibility to fire in sensitive forest ecosystems in California (from excess nitrogen deposition).
EPA intends to recommend levels of ecological indicators for the above impacts that would provide protection for specific resources and to link the ecological indicators back to a level of deposition that would not exceed that recommended levels of the ecological indicator. EPA is considering using flux, or transfer from the ambient air concentration to deposition to sensitive ecosystems, as the basis for the secondary standard. NPS and the Clean Air Scientific Advisory Committee endorse this novel approach to establishing secondary standards that are responsive to ecosystem risks.
NPS ARD contributed to the technical analyses and development of EPA’s assessment and reviewed the draft assessments. EPA highlighted case studies from Shenandoah and Rocky Mountain National Parks in the assessment.
EPA is under court order to recommend revised secondary standards by February 2010. Because there is insufficient time to develop policy recommendations, it is likely that EPA will not propose a fundamentally new form of the secondary standards this fall but will expedite revisions for the next NAAQS review for NOx and SO2. For more information on acid deposition and NPS studies to determine impacts in national parks see our acid deposition page.