Measurement	Region	Annual Average, 2000–2002	Annual Average, 2015–2017	Percent Change	Number of Sites
Ambient Particulate Sulfate Concentration (µg/m³)	Mid-Atlantic	4.8	1.4	-71	13
	Midwest	4.3	1.4	-67	16
	North Central	1.3	0.70	-46	2.0
	Northeast	2.6	0.80	-69	6.0
	Pacific	0.82	0.55	-33	5.0
	Rocky Mountain	0.66	0.40	-39	10
	South Central	2.9	1.5	-48	2.0
	Southeast	4.2	1.3	-69	12
Ambient Sulfur Dioxide Concentration (µg/m³)	Mid-Atlantic	8.0	1.0	-88	13
	Midwest	6.8	1.1	-84	16
	North Central	1.0	0.50	-50	2.0
	Northeast	3.4	0.60	-82	6.0
	Pacific	0.37	0.34	-8.0	5.0
	Rocky Mountain	0.48	0.29	-40	10
	South Central	1.1	0.60	-45	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	0.90	-53	6.0
	Pacific	1.8	0.90	-50	5.0
	Rocky Mountain	0.78	0.48	-38	10
	South Central	1.5	0.90	-40	2.0
	Southeast	2.3	1.1	-52	12