Classification of regions.

The District of Columbia plan was evaluated on the basis of the following classifications: Air quality control region Pollutant Particulate matter Sulfur oxides Nitrogen dioxide Carbon monoxide Ozone National Capital Interstate I I III I I [37 FR 10857, May 31, 1972, as amended at 39 FR 16346, May 8, 1974; 46 FR 61263, Dec. 16, 1981]