Classification of regions.

The Massachusetts plan was evaluated on the basis of the following classifications: Air quality control region Pollutant Particulate matter Sulfur oxides Nitrogen dioxide Carbon monoxide Ozone Metropolitan Boston Intrastate I I III I I Merrimack Valley-Southern New Hampshire Interstate I I III III III Metropolitan Providence Interstate I I III III III Central Massachusetts Intrastate I II III III III Hartford-New Haven-Springfield Interstate I I III I I Berkshire Intrastate II III III III III [37 FR 10872, May 31, 1972, as amended at 39 FR 16346, May 8, 1974; 45 FR 61303, Sept. 16, 1980; 84 FR 29383, June 24, 2019]