见代码：

documentclass[10pt]{article}
usepackage{booktabs}
usepackage{threeparttable}
begin{document}

begin{table}[htbp]
centering
small
begin{threeparttable}
caption{label{tab:results}Effect of Trade Openness on Environment (Air Pollution)}
begin{tabular}{lccc}
toprule
& NO $ _2 $ & SO $ _2 $ & PM \midrule
$ln(y/pop)$ & 408.74* & 287.25* & 566.65 \& (121.79) & (118.81) & (336.19)\
$ln(y/pop)^2$ & $-$22.85* & $-$16.58* & $-$35.57**
\& (6.90) & (6.78) & (19.06)
\$(X+M)/Y$ & $-$.29** & $-$.31* & $-$.37
\& (.17) & (.08) & (.34)
\$Polity$ & $-$3.20* & $-$6.58* & $-$6.70**
\& (1.47) & (2.05) & (3.42)
\$ln(LandArea/pop)$ & $-$5.94 & $-$2.92* & $-$13.02*
\& (5.93) & (1.39) & (6.29)
\Obs. & 36 & 41 & 38
\$R^2$ & 0.16 & 0.68 & 0.62
\
bottomrule
end{tabular}
%尾注
small Note: Robust standard errors in parentheses. Intercept included but not reported.
%脚注
begin{tablenotes}
item[*] significant at 5
% level
item[**] significant at 10
% level
end{tablenotes}
end{threeparttable}
end{table}

end{document}

注意：要用到的包。

结果图如下：

http://s16/mw690/531bb763gd28b2df5b5bf&690