Integrated Science Assessment for Oxides of Nitrogen and Sulfur – Ecological Criteria

Authors

United States Environmental Protection Agency

Date of this Version

12-2008

Comments

EPA/600/R-08/082F
December 2008

Abstract

This Integrated Science Assessment (ISA) is a synthesis and evaluation of the most policy-relevant science that forms the scientific foundation for the review of the secondary (welfare-based) national ambient air quality standards (NAAQS) for oxides of nitrogen (NO_x) and oxides of sulfur (SO_x). The Clean Air Act definition of welfare effects includes, but is not limited to, effects on soils, water, wildlife, vegetation, visibility, weather, and climate, as well as effects on man-made materials, economic values, and personal comfort and well-being. The current secondary NAAQS for SO_x, set in 1973, is a 3-hour average of 0.5 ppm¹ sulfur dioxide (SO₂), not to be exceeded more than once per year. The secondary NAAQS for NO_x is identical to the primary standard set in 1971: an annual average not to exceed 0.053 ppm nitrogen dioxide (NO₂). The current secondary NAAQS were set to protect against direct damage to vegetation by exposure to gas-phase SO_x or NO_x.