\begin{tabular}{lrrrrr}
\toprule
Specification & NO$_2$ & CO & SO$_2$ & O$_3$ & PM$_{10}$ \\
\midrule
$k=3$ & -1.917$^{***}$ & -14.676$^{**}$ & -0.191$^{**}$ & 0.712$^{**}$ & -0.322 \\
 & (0.522) & (7.153) & (0.080) & (0.361) & (0.706) \\
\addlinespace[3pt]
$k=7$ & -1.075$^{**}$ & -4.102 & -0.156$^{*}$ & 0.387 & -0.540 \\
 & (0.456) & (6.585) & (0.089) & (0.407) & (0.896) \\
\addlinespace[3pt]
Omit transport controls & -1.974$^{***}$ & -10.595 & -0.072 & 0.472 & 0.007 \\
 & (0.548) & (6.465) & (0.083) & (0.335) & (0.705) \\
\addlinespace[3pt]
Start in 2013 & -1.956$^{***}$ & -4.527 & -0.061 & 0.721$^{**}$ & -0.586 \\
 & (0.558) & (6.839) & (0.097) & (0.360) & (0.704) \\
\addlinespace[3pt]
Omit land-use controls & -2.042$^{***}$ & -7.936 & -0.076 & 0.546 & 0.024 \\
 & (0.525) & (6.687) & (0.081) & (0.371) & (0.738) \\
\addlinespace[3pt]
2018 cohort starts January & -1.840$^{***}$ & -10.333 & -0.075 & 0.464 & -0.445 \\
 & (0.502) & (6.287) & (0.081) & (0.363) & (0.832) \\
\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}