Table 2.

External validation statistics of the historical PM_2.5 models using PM_2.5 IMPROVE data for 1990–1998 by estimated temporal trend, year, and region.

Year/region	na	FRM/IMPROVE PM2.5		CASTNet PM2.5 sulfate		WBAN visibility
		R2	RMSE (μg/m3)	R2	RMSE (μg/m3)	R2	RMSE (μg/m3)
Allb	72 (423)	0.91	1.14	0.84	1.49	0.86	1.41
1990	30	0.85	1.04	0.78	1.26	0.70	1.48
1991	36	0.83	1.40	0.78	1.56	0.70	1.84
1992	37	0.91	1.19	0.84	1.59	0.85	1.57
1993	45	0.92	1.20	0.83	1.76	0.87	1.53
1994	50	0.92	1.03	0.84	1.45	0.89	1.20
1995	58	0.91	1.15	0.86	1.41	0.86	1.40
1996	56	0.93	0.93	0.88	1.26	0.91	1.10
1997	57	0.93	1.01	0.86	1.42	0.90	1.21
1998	54	0.90	1.28	0.83	1.70	0.87	1.46
Eastb	21 (120)	0.88	1.27	0.67	2.10	0.84	1.45
Mountain Westb	34 (202)	0.25	0.93	0.04	1.06	0.00	1.39
West Coastb	17 (101)	0.69	1.33	0.67	1.37	0.66	1.39
Notes: CASTNet, Clean Air Status and Trends Network; FRM, Federal Reference Method; IMPROVE, Interagency Monitoring of Protected Visual Environment; PM2.5, fine particulate matter; RMSE, root mean square error; WBAN, Weather Bureau Army Navy. aNumber of sites (number of observations when different from the number of sites). bAnnual averages from 1990 through 1998.