²²²Radon (Rn) can be used to distinguish marine and continental air masses or to validate transport models. The Heidelberg Radon Monitor (HRM) measures ²¹⁴polonium (Po), a progeny of Rn. This study presents Po-based Rn activity concentrations measured with the HRM at eight stations in Germany with guidelines for estimating Rn from Po measurements. Comparison between modeled and measured activity concentrations shows that at high relative humidity Po measurements cannot be interpreted as Rn.