Table 4.

The three metrics relating modeled concentrations (M) with the observed values (O) used for evaluating model performance in the smile plots and standard deviation for the ensemble.

NMSE	$\mathrm{NMSE}=\frac{\overline{{(O-M)}^2}}{\overline{O}\overline{M}}$	< = 1.5
FB	$\mathrm{FB}=\frac{2(\overline{M}-\overline{O})}{(\overline{O}+\overline{M})}$	|FB| < = 0.3
FAC2	Fraction of model estimates within a factor of 2 of the observed values 0.5 ≤ $\frac{M}{O}$ ≤ 2.0	FAC2> = 0.5
SD	$\mathrm{SD}=\sqrt{\frac{1}{N-1}{\sum }_{i=1}^N{(M_i-\overline{M})}^2}$	N: number of models in the ensemble $\overline{M}$ : ensemble, mean of models