Radon-222 monitoring at German ICOS atmosphere stations

Abstract. Atmospheric measurements of the short-lived radioactive noble gas ²²²Radon (²²²Rn) have many applications. Its concentrations are driven by atmospheric mixing as well as seasonal variations, which follow the seasonality of ²²²Rn exhalation from continental soils with lower values during the wet winter and spring months compared to more dry summer conditions. Hence, it can be used as tracer to distinguish marine from continental air masses or for transport model validation. The Heidelberg Radon Monitor (HRM) is a static filter detector measuring atmospheric ²¹⁴Polonium (²¹⁴Po), which is a progeny of ²²²Rn. These measurements can be used to infer atmospheric ²²²Rn activity concentrations if the radioactive disequilibrium between ²¹⁴Po and ²²²Rn at the measurement site is known. In this study, ²¹⁴Po activity concentrations measured with the HRM at 8 stations in the ICOS Germany network are presented, along with guidelines for evaluating these data to estimate atmospheric ²²²Rn activity concentrations. In addition to the established line-loss and disequilibrium corrections applied when sampling through long tubing or from air intake heights close to the ground, respectively, an upper limit for relative humidity (RH) is suggested, where secular equilibrium can still be assumed. At higher RH, aerosol scavenging effects can cause disequilibrium between ²¹⁴Po and ²²²Rn. Using comparison with the model this threshold is determined to be at about 98 % RH. A clear diurnal cycle of ²²²Rn is observed at all German tower stations during the summer and autumn months as well as seasonal cycle with maximum during summer and autumn months. Overall, our results demonstrate that the ²¹⁴Po-based ²²²Rn measurements with the HRM are reliable if the equilibrium conditions between ²¹⁴Po and ²²²Rn can be ensured, i.e. for air intake heights above 80–90 m a.g.l. at stations located in flat areas during conditions with RH < 98 %.