\begin{table}%t2
\caption{\label{XDRabundances}Abundances.}
%\centerline
{
\begin{tabular}{llll}
\hline
\hline
Species & $\mathcal{A}_i({\rm gas})$ & $\delta_i$ & $\mathcal{A}_i({\rm total})$\\
\hline
He$^{a,c}$   & $8.5 \times 10^{-2} $ & 1.0   &  $8.5 \times 10^{-2} $\\
C        & $1.4 \times 10^{-4} $ & 0.6   &  $2.5 \times 10^{-4} $\\
N        & $5.2 \times 10^{-5} $ & 0.7   &  $7.2 \times 10^{-5} $\\
O        & $3.4 \times 10^{-4} $ & 0.7   &  $4.7 \times 10^{-4} $\\
Si       & $1.7 \times 10^{-6} $ & 0.05  &  $3.4 \times 10^{-5} $\\
S        & $6.9 \times 10^{-6} $ & 0.5   &  $1.4 \times 10^{-5} $\\
Cl       & $5.4 \times 10^{-8} $ & 0.2   &  $2.4 \times 10^{-7} $\\
Fe       & $2.0 \times 10^{-7} $ & 0.007 &  $2.8 \times 10^{-5} $\\
\hline
P$^{b,c}$& $3.9 \times 10^{-8} $ & 0.1   &  $2.9 \times 10^{-7} $ \\
Na       & $5.9 \times 10^{-7} $ & 0.4   &  $1.5 \times 10^{-6} $\\
Mg       & $2.7 \times 10^{-7} $ & 0.08  &  $3.4 \times 10^{-6} $\\
\hline
Ne$^c$   &                       &       &  $6.9 \times 10^{-5} $\\
Al       &                       &       &  $2.3 \times 10^{-6} $\\
Ar       &                       &       &  $1.5 \times 10^{-6} $\\
Ca       &                       &       &  $2.0 \times 10^{-6} $\\
Cr       &                       &       &  $4.4 \times 10^{-7} $\\
Ni       &                       &       &  $1.7 \times 10^{-6} $\\
\hline
\end{tabular}}
\smallskip
Notes:\\
$^a$ Present in both PDR and XDR~chemical network.\\
$^b$ Present in PDR chemical network. \\
$^c$ Used to calculate $\sigma_{\rm pa}$ for XDR.\\
\end{table}