  Estimating molecular abundances ratios from the direct measurement of the
emission of the molecules towards a variety of interstellar environments is
indeed very useful to advance in our understanding of the chemical evolution of
the Galaxy, and hence of the physical processes related to the chemistry. It is
necessary to increase the sample of molecular clouds, located at different
distances, in which the behavior of molecular abundance ratios, such as the
13CO/C18O ratio (X), is studied in detail. We selected the well-studied
high-mass star-forming region G29.96-0.02, located at a distance of about 6.2
kpc, which is an ideal laboratory to perform this kind of studies. To study the
X towards this region it was used 12CO J=3-2 data obtained from COHRS, 13CO and
C18O J=3-2 data from CHIMPS, and 13CO and C18O J=2-1 data retrieved from the
CDS database (observed with the IRAM 30m telescope). The distribution of column
densities and X throughout the molecular cloud was studied based on LTE and
non-LTE methods. Values of X between 1.5 to 10.5, with an average of 5, were
found, showing that, besides the dependency between X and the galactocentric
distance, the local physical conditions may strongly affect this abundance
ratio. We found that correlating the X map with the location of the ionized gas
and dark clouds allows us to suggest in which regions the far-UV radiation
stalls in dense gaseous components, and in which ones it escapes and
selectively photodissociates the C18O isotope. The non-LTE analysis shows that
the molecular gas has very different physical conditions, not only spatially
across the cloud, but also along the line of sight. This kind of studies may
represent a tool to indirectly estimate (from molecular lines observations) the
degree of photodissociation in molecular clouds, which is indeed useful to
study the chemistry in the interstellar medium.
