Table 3: Carbon budget derived in PDRs (this work), diffuse ISM (from Bell et al. 1983; Federman et al. 1994; Shuping et al. 1999; Lucas & Liszt 2000; Roueff et al. 2002; Cernicharo et al. 2002; Rachford et al. 2002) and dark clouds (from Pratap et al. 1997; Bell et al. 1998; Dickens et al. 2000; Turner et al. 2000; Fossé et al. 2001; Fossé 2003). Note that, for a given number of carbon atoms, the molecules found in the different regions may differ (e.g. no Cn 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. (1996) and assumed an H/H2 abundance ratio of 0.6 as observed in the $\zeta $ Oph line of sight (e.g. Rachford et al. 2002).
Number of carbon atoms $N_{\rm Tot}$/N(H2)
  PDRs Diffuse ISM Dark clouds
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 (C2, C2H, C2S,..) $8.0\times10^{-8}$ $8.0\times10^{-8}$ $1.0\times10^{-8}$
3 (C3, C3H, C3H2, HC3N,CH3C2H,..) $5.0\times10^{-9}$ $4.5\times10^{-9}$ $1.2\times10^{-8}$
4 (C4, C4H, C4H2,..) $2.0\times10^{-9}$ $1.5\times10^{-9}$ $1.7\times10^{-8}$
5 (C5, C5H, HC5N,..) - - $1.2\times10^{-9}$
6 (C6H, C6H2,..) $1.1\times10^{-10}$ - $4.2\times10^{-10}$
7 (HC7N,..) - - $5.5\times10^{-10}$
8 (C8H,..) - - $1.1\times10^{-11}$


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