An Alternative Accurate Tracer of Molecular Clouds: The "$X_{\rm
  CI}$-Factor"
  We explore the utility of CI as an alternative high-fidelity gas mass tracer
for Galactic molecular clouds. We evaluate the X$_{\rm CI}$-factor for the 609
$\mu$m carbon line, the analog of the CO X-factor, which is the ratio of the
H$_2$ column density to the integrated $^{12}$CO(1-0) line intensity. We use
3D-PDR to post-process hydrodynamic simulations of turbulent, star-forming
clouds. We compare the emission of CI and CO for model clouds irradiated by 1
and 10 times the average background and demonstrate that CI is a comparable or
superior tracer of the molecular gas distribution for column densities up to $6
\times 10^{23}$ cm$^{-2}$. Our results hold for both reduced and full chemical
networks. For our fiducial Galactic cloud we derive an average $X_{\rm CO}$ of
$3.0\times 10^{20}$ cm$^{-2}$K$^{-1}$km$^{-1}$s and $X_{\rm CI}$ of $1.1\times
10^{21}$ cm$^{-2}$K$^{-1}$km$^{-1}$s.
