\begin{tabular}{lrrrrr}
\toprule
Specification & NO$_2$ & CO & SO$_2$ & O$_3$ & PM$_{10}$ \\
\midrule
IDW baseline & -1.868$^{***}$ & -7.128 & -0.084 & 0.563 & -0.108 \\
 & (0.519) & (6.315) & (0.082) & (0.367) & (0.778) \\
\addlinespace[3pt]
Nearest baseline & -1.831$^{***}$ & 5.007 & -0.487$^{***}$ & 0.810 & -2.377$^{***}$ \\
 & (0.507) & (7.343) & (0.116) & (0.530) & (0.714) \\
\addlinespace[3pt]
IDW + cohort trends & 0.530 & 14.481$^{**}$ & 0.001 & -0.254 & 0.159 \\
 & (0.864) & (7.289) & (0.086) & (0.858) & (0.847) \\
\addlinespace[3pt]
Nearest + cohort trends & -0.415 & 21.603$^{*}$ & -0.132 & 0.524 & 0.332 \\
 & (1.026) & (11.097) & (0.119) & (1.162) & (0.907) \\
\addlinespace[3pt]
\bottomrule
\multicolumn{6}{l}{\footnotesize HAC(12) standard errors in parentheses.} \\
\multicolumn{6}{l}{\footnotesize $^{*}p<0.10$, $^{**}p<0.05$, $^{***}p<0.01$.} \\
\end{tabular}