  Nowadays there are several observational studies about the 13CO/C18O
abundance ratio (X) towards nearby molecular clouds. These works give
observational support to the C18O selective photodissociation due to the
interaction between the FUV radiation and the molecular gas. It is necessary to
increase the sample of molecular clouds located at different distances and
affected in different ways by nearby or embedded HII regions and OB
associations to study the selective photodissociation. Using 12CO, 13CO, and
C18O J=1-0 data obtained from the FOREST Survey, we analyze the filamentary
infrared dark cloud IRDC34.43+0.24 located at the distance of about 3.9 kpc.
This IRDC is related to several HII regions and YSOs. Assuming LTE we obtain:
0.8x10^16<N(13CO)<4x10^17cm^-2, 0.6x10^15<N(C18O)<4.4x10^16cm^-2, and 3<X<30
across the whole IRDC. Larger values of X were found towards portions of the
cloud related to the HII regions associated with the N61 and N62 bubbles and
with the photodissociation regions (PDRs), precisely the regions in which FUV
photons are strongly interacting with the molecular gas. Our result represents
an observational support to the C18O selectively photodissociation phenomenon
occurring in a quite distant filamentary IRDC. Additionally, based on IR data
from the Hi-GAL survey, the FUV radiation field was estimated in Habing units,
and the dust temperature and N(H2) distribution was studied. Using the average
of N(H2), values in close agreement with the canonical abundance ratios
[H2]/[13CO] and [H2]/[C18O] were derived. However, the obtained ranges in the
abundance ratios show that if an accurate analysis of the molecular gas is
required, the use of the canonical values may introduce some bias. Thus, it is
important to consider how the gas is irradiated by the far ultraviolet photons
across the molecular cloud. The analysis of X is a good tool to perform that.
