(a) The level of the primary 1-hour annual ambient air quality standard for oxides of sulfur is 75 parts per billion (ppb, which is 1 part in 1,000,000,000), measured in the ambient air as sulfur dioxide (SO2). - (i) The 1-hour primary standard is met at an ambient air quality monitoring site when the three-year average of the annual (99th percentile) of the daily maximum 1-hour average concentrations is less than or equal to 75 ppb, as determined in accordance with Appendix T of 40 CFR part 50.
- (ii) The level of the standard shall be measured by a reference method based on 40 CFR part 50, Appendix A or A-1, or by a Federal Equivalent Method (FEM) designated in accordance with 40 CFR part 53.
(b) The level of the secondary 3-hour ambient air quality standard for oxides of sulfur is 0.5 parts per million (ppm, which is 1 part in 1,000,000), not to be exceeded more than once per calendar year. The 3-hour averages shall be determined from successive nonoverlapping 3-hour blocks starting at midnight each calendar day and shall be rounded to 1 decimal place (fractional parts equal to or greater than 0.05 ppm shall be rounded up). - (i) Sulfur oxides shall be measured in the ambient air as sulfur dioxide by the reference method described in Appendix A of 40 CFR part 50 or by an equivalent method designated in accordance with 40 CFR part 53.
- (ii) To demonstrate attainment, the second-highest 3-hour average must be based upon hourly data that are at least 75 percent complete in each calendar quarter. A 3-hour block average shall be considered valid only if all three hourly averages for the 3-hour period are available. If only one or two hourly averages are available, but the 3-hour average would exceed the level of the standard when zeros are substituted for the missing values, subject to the rounding rule of paragraph (b) of this section, then this shall be considered a valid 3-hour average. In all cases, the 3-hour block average shall be computed as the sum of the hourly averages divided by 3.
Amended, Eff. 10/13/2015.
Amended, Eff. 12/20/2016.