Continuous 222Radon daughter observations at ATTO

²²²Radon (²²²Rn) is a short-lived radioactive noble gas (half-life time T_1/2 = 3.8 days), which is produced in all soils from the radioactive decay of ²²⁶Radium (²²⁶Ra), a member of the primordial ²³⁸Uranium decay series. ²²²Rn is the first gaseous constituent in this series and therefore has a chance of escaping from the (unsaturated) soil zone into the atmosphere. The exhalation rate of ²²²Rn from continental surfaces depends on the soil properties, mainly ²²⁶Ra content, grain size distribution, porosity, and moisture content. The ²²²Rn flux from (ocean) water surfaces is negligible compared to that from continental soils; therefore, the atmospheric ²²²Rn activity concentration can serve as a (qualitative) tracer to distinguish continental from marine air masses. If the continental ²²²Rn exhalation rate and its spatial and temporal distribution are known, ²²²Rn can also serve as a quantitative tracer for atmospheric boundary layer mixing and transport model validation.

Owing to its applicability as atmospheric transport tracer, ²²²Rn observations are recommended by the World Meteorological Organisation as supplementary measurement component at regional and global atmospheric observatories. At ATTO ²²²Rn is not measured directly but via its progeny ²¹⁴Polonium, using the Heidelberg Radon Monitor. The short-lived ²¹⁴Po is bound to aerosols, which are accumulated on a static filter from which its a-decay is measured in situ with a surface barrier detector. Atmospheric ²²²Rn activity concentration is then calculated based on assumptions on the secular equilibrium between ²²²Rn and its progeny in the atmosphere. ²¹⁴Po observations at ATTO started at the end of 2018.