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