  The GERDA experiment searches for the neutrinoless double beta
($0\nu\beta\beta$) decay in $^{76}$Ge in order to probe whether the neutrino is
a Majorana particle and to shed light on the neutrino mass ordering. For
investigating such a rare decay it is necessary to minimize the background of
the experiment. In Phase II of the GERDA experiment the scintillation light of
liquid argon (LAr) is used as an additional background veto. In order to
estimate the efficiency of such a LAr veto it has to be known how far the
scintillation light, which peaks at 128 nm, can travel within the LAr. A
dedicated setup was built to measure the attenuation length of the
scintillation light in the LAr in-situ within the cryostat of GERDA. The setup
is composed of a stainless steel housing with a photomultiplier tube (PMT) at
one side and a moveable $^{90}$Sr source at the other side to measure the light
intensity at different distances between source and PMT.
  Furthermore, a sophisticated simulation was developed in order to determine
the solid angle correction as well as the background for this measurement. The
analysis results in an absorption length of $15.8 \pm 0.7$ (stat)
${}^{+1.5}_{-3.2}$ (syst) cm under the assumption of a scattering length of 70
cm at 128 nm. The obtained value of the absorption length is specific for the
LAr in GERDA at the time of the measurement.
