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

Time-Variable Aluminum Absorption in the Polar AR Ursae Majoris, and an Updated Estimate for the Mass of the White Dwarf