1. a) 1971 Secondary Three-Hour NAAQS for Sulfur Oxides (as SO₂)

   1. 1) The level of the 1971 secondary three-hour NAAQS for sulfur oxides is 0.5 ppm, not to be exceeded more than once per calendar year. The three-hour averages must be determined from successive non-overlapping three-hour blocks starting at midnight each calendar day and must be rounded to one decimal place (fractional parts equal to or greater than 0.05 ppm must be rounded up).
   2. 2) Sulfur oxides must be measured in the ambient air as SO₂ by the FRM described in appendix A-2 to 40 CFR 50, incorporated by reference in Section 243.108, or by an FEM designated by USEPA and listed in List of Designated Methods, incorporated by reference in Section 243.108.

   3. 3) To demonstrate attainment, the second-highest three-hour average must be based upon hourly data that are at least 75 percent complete in each calendar quarter. A three-hour block average must be considered valid only if all three hourly averages for the three-hour period are available. If only one or two hourly averages are available, but the three-hour average would exceed the level of the standard when zeros are substituted for the missing values, subject to the rounding rule of subsection (b)(1), this must be considered a valid three-hour average. In all cases, the three-hour block average must be computed as the sum of the hourly averages divided by three.

      BOARD NOTE: This subsection (a) is derived from 40 CFR 50.5.

2. b) 2010 Primary One-Hour NAAQS for Sulfur Oxides (as SO₂)

   1. 1) The level of the 2010 primary one-hour NAAQS for sulfur oxides is 75 ppb, measured in the ambient air as SO₂.
   2. 2) The 2010 one-hour primary NAAQS for sulfur oxides is met at an ambient air quality monitoring site when the three-year average of the annual (99^th percentile) of the daily maximum one-hour average concentrations is less than or equal to 75 ppb, as determined in accordance with appendix T of 40 CFR 50, incorporated by reference in Section 243.108.

   3. 3) The level of the 2010 one-hour primary NAAQS for sulfur oxides must be measured by an FRM based on appendix A-1 or A-2 of 40 CFR 50, incorporated by reference in Section 243.108, or by an FEM designated by USEPA and listed in List of Designated Methods, incorporated by reference in Section 243.108.

      BOARD NOTE: This subsection (b) is derived from 40 CFR 50.17.

   4. c) National secondary ambient air quality standards for sulfur oxides (sulfur dioxide)

      1. 1) The level of the annual secondary national ambient air quality standard for oxides of sulfur is 10 parts per billion (ppb), measured in the ambient air as sulfur dioxide (SO2) by a reference method described in appendix A–1 and appendix A–2 to 40 CFR 50, incorporated by reference in section 243.108, or by a Federal Equivalent Method (FEM) designated by USEPA and listed in List of Designated Methods, incorporated by reference in Section 243.108.

      2. 2) The annual secondary standard is met when the 3-year average of the annual SO2 concentration is less than or equal to 10 ppb, as described in compliance with appendix T incorporated by reference in Section 243.108.

         BOARD NOTE: This subsection (c) is derived from 40 CFR 50.21

(Source: Amended at 50 Ill. Reg. 5364, effective April 2, 2026)