Secondary National Ambient Air Quality Standards (NAAQS) for Nitrogen Dioxide (NO2) and Sulfur Dioxide (SO2) and Particulate Matter (PM)

Rule Summary

On April 3, 2024, EPA proposed to replace the current secondary SO₂ standard with a new annual secondary standard of 10-15 parts per billion (ppb) and to retain the existing secondary standards for oxides of nitrogen (N oxides) and particulate matter (PM). The Clean Air Act directs EPA to set secondary NAAQS to protect the “public welfare” against adverse effects – including ecological effect such as damage to vegetation – caused by criteria air pollutants. Consistent with the scientific evidence indicating that SO_X, N oxides, and PM contribute to sulfur and nitrogen deposition, EPA conducted a concurrent review of the ecological effects of these pollutants with a focus on direct effects of these air pollutants on vegetation, as well as ecological effects related to atmospheric deposition of sulfur and nitrogen compounds in sensitive ecosystems.