Risk and exposure assessment for review of the secondary national ambient air quality standards for oxides of nitrogen and oxides of sulfur. Final Report Appendices 5-8 (5: Terrestrial acidification case study, 6
The U.S. Environmental Protection Agency (EPA) is conducting a joint review of the existing secondary (welfare-based) National Ambient Air Quality Standards (NAAQS) for nitrogen oxides (NOx) and sulfur oxides (SOx).1 A joint secondary review of these pollutants is being conducted because the atmospheric chemistry and environmental effects of NOx, SOx, and their associated transformation products are linked, and because the National Research Council (NRC) has recommended that EPA consider multiple pollutants, as appropriate, in forming the scientific basis for the NAAQS. This is the first time since the NAAQS were established in 1971 that a joint review of NOx, SOx, as well as of total reactive nitrogen, has been conducted. OVERVIEW OF NITROGEN AND SULFUR IN THE ENVIRONMENT - Under Section 108 of the Clean Air Act, the secondary standard is to specify an acceptable level of the criteria pollutant(s) in the ambient air that is protective of known or anticipated adverse effects to public welfare. For this review, the relevant atmospheric indicators are ambient NOx and SOx concentrations that can be linked to levels of deposition for which there are known or anticipated adverse ecological effects. The ecological effects of nitrogen and sulfur are caused both by the gas-phase and atmospheric deposition of the pollutants. The current secondary NAAQS were set to protect against direct damage to vegetation by exposure to gas-phase NOx or SOx, such as foliar injury, decreased photosynthesis, and decreased growth. Deposition of nitrogen- and sulfur-containing compounds that are derived from NOx and SOx may be wet (e.g., rain, snow), cloud and fog deposition, or dry (e.g., gases and particles) and can affect ecosystem biogeochemistry, structure, and function. Nitrogen and sulfur interactions in the environment are highly complex. Both are essential nutrients, and nitrogen can sometimes be limiting for productivity. Excess nitrogen (both oxidized and reduced forms) or sulfur can lead to acidification, and excess nitrogen can lead to nutrient enrichment and eutrophication. Acidification causes a cascade of effects that alter both terrestrial and aquatic ecosystems. When fully developed, acidification effects include lower biomass production rates, the injury and/or death of forest vegetation, and localized loss and extinction of fish and other aquatic species. In addition to contributing to acidification, NOx acts with other forms of reactive nitrogen (including reduced nitrogen) to increase the total amount of available nitrogen in ecosystems. Nitrogen deposition alone can alter numerous biogeochemical indicators, including primary productivity that leads to changes in community composition and eutrophication. In aquatic ecosystems, alterations in freshwater lake diatom communities and impaired water quality in the western United States have been observed. In estuarine ecosystems, additional nitrogen from anthropogenic atmospheric sources contributes to the total nitrogen loading and to increased phytoplankton and algal productivity, which leads to eutrophication. In terrestrial ecosystems, nitrate leaching is a well-documented phenomenon indicating that an ecosystem is receiving nitrogen in excess of biotic nutritional needs. Nitrogen deposition affects primary productivity, thereby altering terrestrial carbon cycling. This may result in shifts in population dynamics, species composition, community structure, and in extreme instances, ecosystem type. Lichens are the most nitrogen-sensitive terrestrial taxa, with documented adverse effects in the Pacific Northwest and in Southern California. Declining biodiversity within grasslands due to nitrogen deposition has also been observed in the central United States, along with changes in biodiversity in other ecosystems such as coastal sage scrub (CSS), mixed conifer forest (MCF) in California, and alpine ecosystems in the Rocky Mountains.