Table 2.

Stratified ensemble cross validation statistics

Region	Pollutant	Sample size	CVa R2	CV RMSE	CV RMSEIQR
California	NO2	56,961	0.88	3.62 ppb	0.28
	NOx	56,961	0.88	9.63 ppb	0.37
Northern California AQSb	NO2	21,037	0.84	2.56 ppb	0.29
	NOx	21,037	0.84	7.11 ppb	0.38
Southern California AQS	NO2	33,800	0.88	4.10 ppb	0.25
	NOx	33,800	0.88	10.81 ppb	0.35
AQS	NO2	54,837	0.88	3.59 ppb	0.28
	NOx	54,837	0.88	9.56 ppb	0.37
Non AQS	NO2	2,124	0.77	4.42 ppb	0.29
	NOx	2,124	0.81	10.06 ppb	0.36

Note:

^a.CV: cross validation;

^b.AQS: EPA’s Air Quality System archive of quality-assured routinely collected measurement data.