\begin{tabular}{llrrrr}
\toprule
Season & Outcome & Direct & Indirect & Total & Trend-adjusted total \\
\midrule
Winter & NO$_2$ & 0.157 & -1.340$^{**}$ & -1.183$^{*}$ & 1.326 \\
 &  & (0.135) & (0.588) & (0.663) & (1.272) \\
 & CO & 2.905 & -16.649$^{*}$ & -13.743 & 27.212$^{**}$ \\
 &  & (2.321) & (9.566) & (9.287) & (12.329) \\
 & SO$_2$ & -0.094$^{***}$ & -0.374$^{***}$ & -0.468$^{***}$ & -0.108 \\
 &  & (0.031) & (0.058) & (0.073) & (0.077) \\
 & O$_3$ & -0.164$^{**}$ & -0.486$^{*}$ & -0.650$^{**}$ & -0.013 \\
 &  & (0.079) & (0.287) & (0.303) & (0.542) \\
 & PM$_{10}$ & -1.079$^{***}$ & -0.167 & -1.246 & -0.834 \\
 &  & (0.341) & (0.926) & (0.828) & (1.241) \\
\addlinespace[4pt]
Spring & NO$_2$ & 0.213 & -2.189$^{***}$ & -1.976$^{***}$ & 0.235 \\
 &  & (0.168) & (0.577) & (0.525) & (0.923) \\
 & CO & 5.194 & -11.832$^{*}$ & -6.639 & 5.596 \\
 &  & (3.633) & (6.444) & (6.614) & (11.854) \\
 & SO$_2$ & -0.073$^{**}$ & -0.074 & -0.147 & -0.083 \\
 &  & (0.029) & (0.118) & (0.133) & (0.177) \\
 & O$_3$ & -0.106 & 1.472$^{***}$ & 1.366$^{***}$ & 0.628 \\
 &  & (0.215) & (0.502) & (0.492) & (0.988) \\
 & PM$_{10}$ & -0.614$^{*}$ & 1.362 & 0.749 & 1.208 \\
 &  & (0.357) & (1.188) & (1.372) & (2.330) \\
\addlinespace[4pt]
Summer & NO$_2$ & -0.042 & -2.134$^{***}$ & -2.176$^{***}$ & -0.561 \\
 &  & (0.106) & (0.427) & (0.388) & (0.587) \\
 & CO & 5.894 & -9.379 & -3.485 & 5.760 \\
 &  & (3.663) & (6.195) & (6.629) & (8.408) \\
 & SO$_2$ & 0.004 & 0.110 & 0.114 & -0.157 \\
 &  & (0.025) & (0.109) & (0.111) & (0.159) \\
 & O$_3$ & 0.211$^{*}$ & 1.672$^{***}$ & 1.882$^{***}$ & -0.378 \\
 &  & (0.126) & (0.543) & (0.559) & (1.217) \\
 & PM$_{10}$ & -0.500$^{*}$ & 0.845 & 0.345 & -0.036 \\
 &  & (0.270) & (0.866) & (0.735) & (1.209) \\
\addlinespace[4pt]
Autumn & NO$_2$ & 0.061 & -1.910$^{***}$ & -1.849$^{**}$ & 1.178 \\
 &  & (0.146) & (0.736) & (0.727) & (1.117) \\
 & CO & 7.166$^{***}$ & -16.037$^{*}$ & -8.871 & 21.140 \\
 &  & (2.519) & (8.260) & (8.861) & (13.578) \\
 & SO$_2$ & -0.037 & 0.007 & -0.030 & 0.056 \\
 &  & (0.024) & (0.069) & (0.079) & (0.091) \\
 & O$_3$ & -0.017 & 0.008 & -0.009 & -1.429 \\
 &  & (0.123) & (0.339) & (0.400) & (0.946) \\
 & PM$_{10}$ & -0.852$^{***}$ & 0.939 & 0.087 & -1.227 \\
 &  & (0.322) & (1.022) & (0.959) & (1.109) \\
\addlinespace[4pt]
\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}