  We present spectra of the extreme polar AR Ursae Majoris (AR UMa) which
display a clear Al I absorption doublet, alongside spectra taken less than a
year earlier in which that feature is not present. Re-examination of earlier
SDSS spectra indicates that the Al I absorption doublet was also present
$\approx$8 years before our first non-detection. We conclude that this
absorbing material is unlikely to be on the surface of either the white dwarf
(WD) or the donor star. We suggest that this Al I absorption feature arises in
circumstellar material, perhaps produced by the evaporation of asteroids as
they approach the hot WD. The presence of any remaining reservoir of rocky
material in AR UMa might help to constrain the prior evolution of this unusual
binary system. We also apply spectral decomposition to find the stellar
parameters of the M dwarf companion, and attempt to dynamically measure the
mass of the WD in AR UMa by considering both the radial velocity curves of the
H$_\beta$ emission line and the Na I absorption line. Thereby we infer a mass
range for the WD in AR UMa of 0.91 $M_{\odot}$ $<$ $M_{\mathrm{WD}}$ $<$ 1.24
$M_{\odot}$.
