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}}$.
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