Extended Carbon Line Emission in the Galaxy: Searching for Dark
  Molecular Gas along the G328 Sightline
  We present spectral data cubes of the [CI] 809GHz, 12CO 115GHz, 13CO 110GHz
and HI 1.4GHz line emission from an 1 square degree region along the l =
328{\deg} (G328) sightline in the Galactic Plane. Emission arises principally
from gas in three spiral arm crossings along the sight line. The distribution
of the emission in the CO and [CI] lines is found to be similar, with the [CI]
slightly more extended, and both are enveloped in extensive HI. Spectral line
ratios per voxel in the data cubes are found to be similar across the entire
extent of the Galaxy. However, towards the edges of the molecular clouds the
[CI]/13CO and 12CO/13CO line ratios rise by ~50%, and the [CI]/HI ratio falls
by ~10$%. We attribute this to these sightlines passing predominantly through
the surfaces of photodissociation regions (PDRs), where the carbon is found
mainly as C or C+, while the H2 is mostly molecular, and the proportion of
atomic gas also increases. We undertake modelling of the PDR emission from low
density molecular clouds excited by average interstellar radiation fields and
cosmic-ray ionization to quantify this comparison, finding that depletion of
sulfur and reduced PAH abundance is needed to match line fluxes and ratios.
Roughly one-third of the molecular gas along the sightline is found to be
associated with this surface region, where the carbon is largely not to be
found in CO. ~10% of the atomic hydrogen along the sightline is cold gas within
PDRs.
