Classification of regions.

The Arkansas plan was evaluated on the basis of the following classifications: Air quality control region Pollutant Particulate matter Sulfur oxides Nitrogen dioxide Carbon monoxide Ozone Central Arkansas Intrastate II III III III III Metropolitan Fort Smith Interstate II III III III III Metropolitan Memphis Interstate I III III III I Monroe (Louisiana)-El Dorado (Arkansas) Interstate II III III III III Northeast Arkansas Intrastate III III III III III Northwest Arkansas Intrastate III III III III III Shreveport-Texarkana-Tyler Interstate II III III III III [37 FR 10850, May 31, 1972, as amended at 39 FR 16346, May 8, 1974; 45 FR 6571, Jan. 29, 1980]