Measurement Region Annual Average, 2000–2002 Annual Average, 2016–2018 Percent Change Number of Sites
Ambient Particulate Sulfate Concentration (µg/m³) Mid-Atlantic 4.8 1.2 -75 13
Midwest 4.3 1.3 -40 16
North Central 1.3 0.70 -46 2.0
Northeast 2.6 0.70 -73 6.0
Pacific 0.80 0.50 -35 5.0
Rocky Mountain 0.70 0.40 -44 10
South Central 2.9 1.4 -52 2.0
Southeast 4.2 1.2 -71 12
Ambient Sulfur Dioxide Concentration (µg/m³) Mid-Atlantic 8.0 1.0 -88 13
Midwest 6.8 0.80 -88 16
North Central 1.0 0.50 -50 2.0
Northeast 3.4 0.50 -85 6.0
Pacific 0.40 0.30 -16 5.0
Rocky Mountain 0.50 0.30 -48 10
South Central 1.1 0.50 -55 2.0
Southeast 3.4 0.50 -85 12
Ambient Total Nitrate Concentration (µg/m³) Mid-Atlantic 3.0 1.4 -53 13
Midwest 4.1 2.0 -51 16
North Central 1.2 0.80 -33 2.0
Northeast 1.9 0.80 -58 6.0
Pacific 1.8 1.0 -44 5.0
Rocky Mountain 0.80 0.50 -38 10
South Central 1.5 0.90 -40 2.0
Southeast 2.3 1.1 -52 12