Measurement	Region	Annual Average, 2000–2002	Annual Average, 2017–2019	Percent Change	Number of Sites
Ambient Particulate Sulfate Concentration (µg/m³)	Mid-Atlantic	4.8	1.1	-77	13
	Midwest	4.3	1.2	-72	16
	North Central	1.3	0.7	-46	2
	Northeast	2.6	0.7	-73	6
	Pacific	0.82	0.54	-34	5
	Rocky Mountain	0.66	0.37	-44	10
	South Central	2.9	1.4	-52	2
	Southeast	4.2	1.1	-74	12
Ambient Sulfur Dioxide Concentration (µg/m³)	Mid-Atlantic	8	1	-88	13
	Midwest	6.8	0.8	-88	16
	North Central	1	0.5	-50	2
	Northeast	3.4	0.4	-88	6
	Pacific	0.37	0.3	-19	5
	Rocky Mountain	0.48	0.24	-50	10
	South Central	1.1	0.5	-55	2
	Southeast	3.4	0.4	-88	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	0.9	-2	2
	Northeast	1.9	0.8	-58	6
	Pacific	1.8	1	-44	5
	Rocky Mountain	0.78	0.49	-37	10
	South Central	1.5	0.9	-40	2
	Southeast	2.3	1	-57	12