\begin{tabular}{llrrrrr}
\toprule
Period & Estimator & NO$_2$ & CO & SO$_2$ & O$_3$ & PM$_{10}$ \\
\midrule
2012--2024 & reg & $<0.001^{***}$ & $<0.001^{***}$ & $<0.001^{***}$ & $<0.001^{***}$ & $<0.001^{***}$ \\
2012--2024 & dr & $<0.001^{***}$ & $<0.001^{***}$ & $0.0065^{***}$ & $<0.001^{***}$ & $<0.001^{***}$ \\
2013--2024 & reg & $<0.001^{***}$ & $<0.001^{***}$ & $<0.001^{***}$ & $<0.001^{***}$ & $<0.001^{***}$ \\
2013--2024 & dr & $<0.001^{***}$ & $<0.001^{***}$ & $0.0065^{***}$ & $<0.001^{***}$ & $<0.001^{***}$ \\
\bottomrule
\multicolumn{7}{l}{\footnotesize Entries are pre-trend test p-values.} \\
\multicolumn{7}{l}{\footnotesize Stars indicate rejection of the pre-trend test null.} \\
\multicolumn{7}{l}{\footnotesize $^{*}p<0.10$, $^{**}p<0.05$, $^{***}p<0.01$.} \\
\end{tabular}