(1) Standards for nitrogen oxides (measured as nitrogen dioxide).

(a) The annual average concentration for nitrogen oxides in ambient air must not exceed 53 ppbv (100 µg/m3) measured in the ambient air as nitrogen dioxide.

(b) The three-year average of the ninety-eighthpercentile of the daily maximum one-hour average concentration of nitrogen oxides must not exceed 100 ppbv, as measured in the ambient air as nitrogen dioxide.

(2) Measurement method. The levels of nitrogen oxides must be measured as nitrogen dioxide by:

(a) A FRM based on 40 C.F.R. Part 50, Appendix F; or

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

(3) Interpretation method. The interpretation method found in 40 C.F.R. Part 50, Appendix S must be followed.