  Context. We present our findings for the HCN/H13CN 1-0 line ratio in the
molecular outflow of Arp 220 west based on high-resolution ALMA data. Aims.
Molecular gas masses in the outflowing gas of galaxies driven by active
galactic nuclei (AGNs) or starbursts are important parameters for understanding
the feedback of these latter two phenomena and star-formation quenching. The
conversion factor of line luminosities to masses is related to the optical
depth of the molecular lines. Methods. Using H13CN 1-0, the isotopic line of
HCN 1-0, to obtain the line ratio of HCN/H13CN 1-0 is an ideal way to derive
the optical depth of HCN 1-0 in outflowing gas. Results. With the nondetection
of H13CN 1-0 in the outflowing gas, a 3{\sigma} lower limit of HCN/H13CN 1-0
line ratio is obtained, which is at least three times higher than that found in
the whole of the whole system of Arp 220. The high HCN/H13CN 1-0 line ratio
indicates low opacity of HCN 1-0 in the outflowing gas, even though the upper
limit of HCN 1-0 opacity obtained here is still not good enough to draw any
robust conclusions if HCN 1-0 is optically thin. A lower conversion factor of
HCN 1-0 luminosity to dense gas mass in the outflowing gas should be used than
that used for the host galaxy of Arp 220.
