Table 3. Correlations between Predictions by Our Main SLR (Two-Step, step2) and RF (Two-Step, step1) Models at 41,936 Random Locations^a.

	PM2.5 Cu		PM2.5 Fe		PM2.5 K		PM2.5 Ni		PM2.5 S		PM2.5 Si		PM2.5 V		PM2.5 Zn
region	r	RMSEb	r	RMSE	r	RMSE	r	RMSE	r	RMSE	r	RMSE	r	RMSE	r	RMSE	N
all European countries	0.72	1.1	0.66	20.5	0.75	51.3	0.56	0.5	0.88	162.6	0.56	22.8	0.64	1.0	0.73	7.1	41,936
ELAPSE Countries
combined	0.79	1.0	0.77	19.1	0.70	56.1	0.53	0.5	0.89	142.0	0.75	12.5	0.66	0.8	0.73	7.0	27,411
Austria	0.77	1.1	0.80	16.2	0.80	35.0	0.28	0.1	0.95	69.5	0.08	9.5	0.82	0.2	0.89	4.4	1051
Belgium	0.82	0.9	0.89	11.9	0.05	21.0	0.83	0.4	0.79	47.4	0.71	8.2	0.76	0.8	0.70	11.4	355
Switzerland	0.80	1.1	0.89	13.1	0.76	24.1	0.36	0.1	0.48	95.0	0.38	9.2	0.49	0.2	0.65	5.8	500
Germany	0.74	1.0	0.80	14.9	0.43	30.6	0.66	0.3	0.67	60.1	0.59	8.2	0.80	0.4	0.58	6.9	4233
Denmark	0.77	0.3	0.77	9.3	0.68	15.0	0.21	0.2	0.73	36.0	0.45	7.2	0.24	0.3	0.49	2.0	522
France	0.62	1.1	0.75	14.2	0.31	26.9	0.39	0.4	0.72	69.0	0.48	9.0	0.77	0.6	0.59	6.8	6476
Italy	0.72	1.3	0.53	23.2	0.69	33.6	0.51	0.6	0.83	183.0	0.70	19.5	0.63	1.3	0.67	9.0	3550
Netherlands	0.74	0.9	0.83	14.2	0.55	17.2	0.64	0.4	0.60	45.0	0.68	8.9	0.48	0.7	0.66	14.2	451
Norway	0.37	0.0	0.61	5.6	–0.68c	8.6	0.43	0.2	0.25	104.9	0.03	4.3	0.80	0.3	0.46	4.0	2649
Sweden	0.56	0.2	0.77	5.8	–0.68c	22.6	0.68	0.1	0.88	86.7	0.31	4.9	0.73	0.4	0.38	3.6	4786
United Kingdom	0.85	0.6	0.87	12.0	–0.74c	25.9	0.59	0.3	0.86	72.1	0.12	9.6	0.57	0.5	0.68	4.1	2838
Non-ELAPSE Countries
Greece	0.55	1.0	0.57	16.8	0.38	28.7	0.44	0.8	0.18	123.5	0.34	25.6	0.46	1.9	0.54	7.0	1541
Finland	0.25	0.3	0.58	7.4	0.49	28.7	0.48	0.1	0.85	67.3	0.28	4.9	0.33	0.3	0.55	4.2	3208
Hungary	0.68	0.9	0.61	11.7	0.74	29.0	0.34	0.2	0.60	87.4	0.57	8.2	0.67	0.3	0.79	5.0	1123
Ireland	0.52	0.3	0.65	5.8	–0.70c	16.9	0.46	0.2	0.45	48.5	–0.08	4.8	0.41	0.4	0.26	1.8	844
Lithuania	0.63	0.8	0.68	7.0	0.65	25.3	0.26	0.1	0.16	52.5	0.59	4.7	0.29	0.2	0.89	1.8	783
Luxembourg	0.76	0.8	0.83	8.4	–0.08	16.2	0.82	0.1	0.82	26.5	0.77	4.9	0.80	0.2	0.83	2.1	33
Portugal	0.41	1.6	0.01	21.0	–0.03	23.5	0.71	0.4	0.72	72.9	0.22	7.3	0.80	0.7	0.60	6.6	1021
Spain	0.59	1.2	0.34	17.2	0.06	32.7	0.41	0.6	0.59	102.0	0.49	10.7	0.59	1.1	0.56	8.0	5972

^ar = Pearson correlation coefficient; RMSE = root-mean-square error.

^bUnit of RMSE: ng/m³.

^cWe do not have clear explanations of these high negative correlations. These values possibly reflect the poor performance of both models at low concentrations. Scatter plots documented the poor agreement between predictions by the two models with lots of scatters.