Virginia Law

A. The primary ambient air quality standards are as follows:

1. The annual ambient air quality standard is 80 micrograms per cubic meter (0.030 parts per million) -- annual arithmetic mean not to be exceeded in a calendar year. The annual arithmetic mean shall be rounded to three decimal places (fractional parts equal to or greater than 0.0005 ppm shall be rounded up).

2. The 24-hour ambient air quality standard is 365 micrograms per cubic meter (0.14 parts per million) -- maximum 24-hour concentration not to be exceeded more than once per calendar year. The 24-hour averages shall be determined from successive nonoverlapping 24-hour blocks starting at midnight each calendar day and shall be rounded to two decimal places (fractional parts equal to or greater than 0.005 ppm shall be rounded up).

3. The 1-hour annual ambient air quality standard is 75 parts per billion (ppb), measured in the ambient air as sulfur dioxide (SO₂). The 1-hour primary standard is met 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.

4. The annual and 24-hour ambient air quality standards set forth in subdivisions 1 and 2 of this subsection shall no longer apply to an area one year after the effective date of designation of that area pursuant to § 107 of the federal Clean Air Act for the 1-hour annual ambient air quality standard set forth in subdivision 3 of this subsection.

B. The secondary ambient air quality standard is 1,300 micrograms per cubic meter (0.50 parts per million) -- maximum three-hour concentration 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 one decimal place (fractional parts equal to or greater than 0.05 ppm shall be rounded up).

C. Sulfur dioxide shall be measured by the reference method described in Appendix A-1 or A-2 of 40 CFR Part 50, or other method designated as such, or by an equivalent method.

D. To demonstrate attainment of the annual and 24-hour primary standards set forth in subdivisions A 1 and 2 of this section, the annual arithmetic mean and the second-highest 24-hour averages must be based upon hourly data that are at least 75% complete in each calendar quarter. A 24-hour block average shall be considered valid if at least 75% of the hourly averages for the 24-hour period are available. In the event that only 18, 19, 20, 21, 22, or 23 hourly averages are available, the 24-hour block average shall be computed as the sum of the available hourly averages using 18, 19, and so on as the divisor. If fewer than 18 hourly averages are available, but the 24-hour average would exceed the level of the standard when zeros are substituted for the missing values, subject to the rounding rule of subdivision A 2 of this section, then this shall be considered a valid 24-hour average. In this case, the 24-hour block average shall be computed as the sum of the available hourly averages divided by 24. Data used for demonstrating attainment of the 1-hour annual primary standard set forth in subdivision A 3 of this section shall meet the requirements of Appendix T of 40 CFR Part 50.

E. To demonstrate attainment of the secondary standard, the second-highest 3-hour average must be based upon hourly data that are at least 75% 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 subsection 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 three.