  One major background in direct searches for weakly interacting massive
particles (WIMPs) comes from the deposition of radon progeny on detector
surfaces. The most dangerous surface background is the $^{206}$Pb recoils
produced by $^{210}$Po decays. In this letter, we report the first
characterization of this background in liquid argon. The scintillation signal
of low energy Pb recoils is measured to be highly quenched in argon, and we
estimate that the 103keV $^{206}$Pb recoil background will produce a signal
equal to that of a ~5keV (30keV) electron recoil ($^{40}$Ar recoil). In
addition, we demonstrate that this dangerous $^{210}$Po surface background can
be suppressed by a factor of ~100 or higher using pulse shape discrimination
methods, which can make argon dark matter detectors near background-free and
enhance their potential for discovery of medium- and high-mass WIMPs. We also
discuss the impact on other low background experiments.
