(1) Standards for carbon monoxide.

(a) The eight-hour average concentration of carbon monoxide in the ambient air must not exceed 9 ppmv (10 milligrams per cubic meter) more than once per year.

(b) The one-hour average concentration of carbon monoxide in the ambient air must not exceed 35 ppmv (40 milligrams per cubic meter) more than once per year.

(2) Measurement method. The levels of carbon monoxide in the ambient air must be measured by:

(a) A FRM based on 40 C.F.R. Part 50, Appendix C and designated according to 40 C.F.R. Part 53; or

(b) A FEM designated according to 40 C.F.R. Part 53.

(3) Interpretation method. An eight-hour average must be considered valid if at least seventy-five percent of the hourly averages for the eight-hour period are available. In the event that only six (or seven) hourly averages are available, the eight-hour average must be computed on the basis of the hours available using six (or seven) as the divisor.

(4) Rounding of values. When summarizing data for comparison with the standards, averages must be stated to one decimal place. Comparison of the data with the levels of the standards in ppmv must be made in terms of integers with fractional parts of 0.5 or greater rounding up.