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