  We report on a measurement of the attenuation length for the scintillation
light in the tonne size liquid argon target of the ArDM dark matter experiment.
The data was recorded in the first underground operation of the experiment in
single-phase operational mode. The results were achieved by comparing the light
yield spectra from 39-Ar and 83m-Kr to a description of the ArDM setup with a
model of full light ray tracing. A relatively low value close to 0.5 m was
found for the attenuation length of the liquid argon bulk to its own
scintillation light. We interpret this result as a presence of optically active
impurities in the liquid argon which are not filtered by the installed
purification systems. We also present analyses of the argon gas employed for
the filling and discuss cross sections in the vacuum ultraviolet of various
molecules in respect to purity requirements in the context of large liquid
argon installations.
