\begin{tabular}{@{}lrrrrr@{}}
\toprule
& & \multicolumn{2}{c}{IDW} & \multicolumn{2}{c}{Nearest}\\
Variable & Holdouts & MAE & RMSE & MAE & RMSE\\
\midrule
NO$_2$ (ppb) & 53,342 & 3.486 & 4.630 & 3.624 & 4.961 \\
CO (ppb) & 52,839 & 73.451 & 98.241 & 89.349 & 121.845 \\
SO$_2$ (ppb) & 52,992 & 0.753 & 1.191 & 0.916 & 1.511 \\
O$_3$ (ppb) & 53,603 & 3.086 & 4.155 & 3.677 & 4.928 \\
PM$_{10}$ ($\mu$g/m$^3$) & 52,989 & 4.229 & 5.793 & 5.105 & 7.009 \\
Temperature ($^\circ$C) & 14,584 & 0.849 & 1.188 & 0.948 & 1.364 \\
Sunshine (h/day) & 14,551 & 0.462 & 0.617 & 0.568 & 0.759 \\
Low-wind equivalent (days) & 14,578 & 5.862 & 7.595 & 6.737 & 9.218 \\
Wind speed (m/s) & 14,578 & 0.565 & 0.886 & 0.674 & 1.019 \\
Relative humidity (\%) & 14,581 & 3.943 & 5.074 & 4.963 & 6.366 \\
Precipitation (mm/day) & 14,555 & 0.806 & 1.422 & 0.890 & 1.636 \\
\bottomrule
\end{tabular}