Measurement Region Annual Average, 2000–2002 Annual Average, 2018–2020 Percent Change Number of Sites
Ambient Particulate Sulfate Concentration (µg/m³) Mid-Atlantic 4.8 1 -79 13
Midwest 4.3 1.1 -74 16
North Central 1.3 0.6 -54 2
Northeast 2.6 0.6 -77 6
Pacific 0.8 0.5 -37 5
Rocky Mountain 0.7 0.4 -45 10
South Central 2.9 1.2 -59 2
Southeast 4.2 1 -76 12
Ambient Sulfur Dioxide Concentration (µg/m³) Mid-Atlantic 8 1 -88 13
Midwest 6.8 0.6 -91 16
North Central 1 0.4 -60 2
Northeast 3.4 0.3 -91 6
Pacific 0.4 0.3 -32 5
Rocky Mountain 0.5 0.2 -60 10
South Central 1.1 0.4 -64 2
Southeast 3.4 0.3 -91 12
Ambient Total Nitrate Concentration (µg/m³) Mid-Atlantic 3 1.3 -57 13
Midwest 4.1 1.9 -54 16
North Central 1.2 0.8 -33 2
Northeast 1.9 0.8 -58 6
Pacific 1.8 0.9 -50 5
Rocky Mountain 0.8 0.5 -38 10
South Central 1.5 0.9 -40 2
Southeast 2.3 0.9 -61 12