The diurnal evolution of 222Rn and its progeny in the atmospheric boundary layer during the Wangara experiment

The diurnal atmospheric boundary layer evolution of the Rn-222 decaying family is studied using a state-of-the-art large-eddy simulation model. In particular, a diurnal cycle observed during the Wangara experiment is successfully simulated together with the effect of diurnal varying turbulent characteristics on radioactive compounds initially in a secular equilibrium. This study allows us to clearly analyze and identify the boundary layer processes driving the behaviour of Rn-222 and its progeny concentrations. An activity disequilibrium is observed in the nocturnal boundary layer due to the proximity of the radon source and the trapping of fresh Rn-222 close to the surface induced by the weak vertical transport. During the morning transition, the secular equilibrium is fast restored by the vigorous turbulent mixing. The evolution of Rn-222 and its progeny concentrations in the unsteady growing convective boundary layer depends on the strength of entrainment events.