Table 3.

External validation statistics of the historical PM_2.5 models using CHS, CARB dichot, and IPN data by estimated temporal trend and year.

Validation data	Year	na	FRM/IMPROVEPM2.5		CASTNet PM2.5 sulfate		WBAN visibility
			R2	RMSE (μg/m3)	R2	RMSE (μg/m3)	R2	RMSE (μg/m3)
CHS	Allb	13 (120)	0.76	4.00	0.76	3.98	0.81	3.59
	1994	12	0.71	5.19	0.69	5.34	0.80	4.33
	1995	12	0.66	5.97	0.63	6.31	0.75	5.17
	1996	12	0.77	4.40	0.75	4.56	0.82	3.86
	1997	12	0.83	3.12	0.84	3.01	0.88	2.64
	1998	12	0.83	2.87	0.87	2.55	0.87	2.54
	1999	12	0.73	4.30	0.75	4.13	0.74	4.16
	2000	12	0.80	3.43	0.82	3.24	0.82	3.31
	2001	12	0.82	3.79	0.85	3.44	0.86	3.27
	2002	12	0.81	3.20	0.82	3.12	0.79	3.31
	2003	12	0.88	2.39	0.90	2.22	0.89	2.30
CARB dichot	Allb	33 (162)	0.55	5.54	0.48	5.98	0.61	5.17
	1988	8	0.09	9.70	0.00	10.52	0.15	9.40
	1989	12	0.25	9.07	0.10	9.94	0.33	8.55
	1990	11	0.68	4.77	0.53	5.74	0.76	4.08
	1991	12	0.31	9.24	0.16	10.16	0.43	8.35
	1992	14	0.51	5.35	0.40	5.91	0.63	4.68
	1993	15	0.54	3.88	0.33	4.67	0.66	3.30
	1994	13	0.77	4.08	0.69	4.72	0.84	3.37
	1995	12	0.71	3.46	0.63	3.91	0.70	3.54
	1996	15	0.52	4.00	0.66	3.37	0.57	3.81
	1997	15	0.41	3.19	0.59	2.66	0.45	3.08
	1998	16	0.31	4.11	0.37	3.94	0.30	4.14
	1999	12	0.85	2.39	0.84	2.50	0.82	2.64
	2000	6	0.53	2.41	0.46	2.59	0.41	2.69
	2001	3	0.00	9.41	0.00	9.34	0.00	9.19
IPN	Allb	16 (18)	0.16	6.15	0.02	6.63	0.00	7.40
	1980	6	0.40	5.11	0.27	5.62	0.00	6.96
	1981	12	0.11	6.61	0.00	7.09	0.00	7.61
Notes: CARB dichot, California Air Resources Board dichotomous sampler monitoring; CASTNet, Clean Air Status and Trends Network; CHS, Children’s Health Study; FRM, Federal Reference Method; IMPROVE, Interagency Monitoring of Protected Visual Environment; IPN, Inhalable Particulate Network; 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 for 1994–2003 from CHS, for 1988–2001 from CARB dichot, and for 1980–1981 from IPN.