Sulfur Dioxide State Planning

The Clean Air Act regulates sulfur dioxide as a criteria pollutant. The U.S. Environmental Protection Agency (EPA) sets National Ambient Air Quality Standards for sulfur dioxide (SO₂). These standards include both standards to protect public health and secondary standards to protect the environment. The annual and 24-hour standards were set in 1971, along with secondary annual and 3-hour standards.

EPA reviews pollutant standards every five years. Sometimes, a review results in a tightened standard. The secondary annual standard was revoked in 1973, but the existing 3-hours secondary standard of 500 parts per billion (ppb) was retained. When EPA revised the sulfur dioxide standard in 2010, they revoked the two previous standards, replacing them with a new 1-hour standard of 75 ppb. The form of the 1-hour standard is the 3-year average of the 99th percentile of the yearly distribution of 1-hour daily maximum SO₂ concentrations.

When EPA revised the 2010 sulfur dioxide standard, they also chose a different approach to establishing boundary designations. EPA decided to use a hybrid monitoring-modeling approach for sulfur dioxide, with a series of variable designation dates based on the designation approach selected. The initial round for boundary designations was based on available ambient air quality monitoring data at the time EPA established the standard. The second round was targeted at areas surrounding facilities that had the largest annual SO₂ emissions. The third round was targeted at areas surrounding facilities with emissions more than 2,000 tons per year that elected to characterize air quality using refined dispersion modeling analyses. The fourth and final round is targeted at areas surrounding facilities with emissions more than 2,000 tons per year that elected to characterize air quality using newly deployed ambient SO₂ monitors in the areas of maximum expected SO₂ concentrations surrounding such sources.

The SO₂ standard below includes the state implementation plan elements developed for the standard and the relevant documents associated with the boundary designations for each round. If you are looking for a document that is not available on our website, please submit a Open Records/ Sunshine Law Request.