\begin{table}%T3
\caption{\label{carbon budget}Carbon budget derived in PDRs (this work), diffuse ISM (from Bell et~al. \cite{Bell1983}; Federman et~al. \cite{Federman1994}; Shuping et~al. \cite{Shuping1999}; Lucas \& Liszt \cite{Lucas2000}; Roueff et~al. \cite{Roueff2002}; Cernicharo et~al. \cite{Cernicharo2002}; Rachford et~al. \cite{Rachford2002}) and dark clouds (from Pratap et~al. \cite{Pratap1997}; Bell et~al. \cite{Bell1998}; Dickens et~al. \cite{Dickens2000}; Turner et~al. \cite{Turner2000}; Foss\'e et~al. \cite{Foss2001}; Foss\'e \cite{Foss2003}). Note that, for a given number of carbon atoms, the molecules found in the different regions may differ (e.g. no C$_n$ carbon clusters in PDRs and dark clouds). {For the diffuse ISM, we used the C/H~abundance ratio of~$1.4\times 10^{-4}$ reported by Cardelli et~al. (\cite{Cardelli1996}) and assumed an~H/H$_2$~abundance ratio of~0.6 as observed in the $\zeta$~Oph line of sight (e.g. Rachford et~al. \cite{Rachford2002}).}}
%\centering
\par
\begin{tabular}{l  c c c} 
\hline \hline  
Number of carbon atoms				& \multicolumn{3}{c}{$N_{\rm Tot}$/$N$(H$_2$)} \\
						& PDRs		 	& Diffuse ISM			& Dark clouds				\\ \hline
1 (C$^{\rm +}$, C, CO)				& $2.6\times10^{-4}$    & $2.3\times10^{-4}$    	& $8.0\times10^{-5}$    		\\
1 (CH$^{\rm +}$, CH, CS, HCN,..)		& $7.0\times10^{-8}$    & $5.0\times10^{-8}$    	& $\leq$$1.0\times10^{-7}$    		\\
2 (C$_2$, \cch, C$_2$S,..)			& $8.0\times10^{-8}$    & $8.0\times10^{-8}$    	& $1.0\times10^{-8}$    		\\
3 (C$_3$, \ccch, \ccchh, HC$_3$N,\chhhcch,..)	& $5.0\times10^{-9}$    & $4.5\times10^{-9}$    	& $1.2\times10^{-8}$    		\\
4 (C$_4$, \cccch, \cccch$_2$,..)		& $2.0\times10^{-9}$    & $1.5\times10^{-9}$    	& $1.7\times10^{-8}$    		\\
5 (C$_5$, C$_5$H, HC$_5$N,..)			& --                    & --                    	& $1.2\times10^{-9}$    		\\
6 (\csh, \csh$_2$,..)				& $1.1\times10^{-10}$   & --                    	& $4.2\times10^{-10}$    		\\
7 (HC$_7$N,..)       				& --                    & --                    	& $5.5\times10^{-10}$    		\\
8 (C$_8$H,..)					& --                    & --                    	& $1.1\times10^{-11}$    		\\
 \hline
\end{tabular}
\end{table}