  We report measurements of the charged daughter fraction of $^{218}$Po as a
result of the $^{222}$Rn alpha decay, and the mobility of $^{218}$Po$^+$ ions,
using radon-polonium coincidences from the $^{238}$U chain identified in 532
live-days of DarkSide-50 WIMP-search data. The fraction of $^{218}$Po that is
charged is found to be 0.37$\pm$0.03 and the mobility of $^{218}$Po$^+$ is
(8.6$\pm$0.1)$\times$10$^{-4}$$\frac{\text{cm}^2}{\text{Vs}}$.
