  The MicroBooNE liquid argon time projection chamber (LArTPC) maintains a high
level of liquid argon purity through the use of a filtration system that
removes electronegative contaminants in continuously-circulated liquid,
recondensed boil off, and externally supplied argon gas. We use the MicroBooNE
LArTPC to reconstruct MeV-scale radiological decays. Using this technique we
measure the liquid argon filtration system's efficacy at removing radon. This
is studied by placing a 500 kBq $^{222}$Rn source upstream of the filters and
searching for a time-dependent increase in the number of radiological decays in
the LArTPC. In the context of two models for radon mitigation via a liquid
argon filtration system, a slowing mechanism and a trapping mechanism,
MicroBooNE data supports a radon reduction factor of greater than 99.999% or
97%, respectively. Furthermore, a radiological survey of the filters found that
the copper-based filter material was the primary medium that removed the
$^{222}$Rn. This is the first observation of radon mitigation in liquid argon
with a large-scale copper-based filter and could offer a radon mitigation
solution for future large LArTPCs.
