Atmospheric stability and secular equilibrium of the 222-Rn decaying family

A decaying chain is in secular equilibrium when the ratio of the activities of its nuclide is equal to 1. This corresponds to a balance between production and destruction of the nuclide. If the radon daughters' were subject only to the laws of radioactive decay, a secular equilibrium would prevail. In the atmospheric boundary layer, the temporal and vertical distribution of the radon decaying family is affected by turbulent transport. This leads to possible deviations from the secular equilibrium. The deviations are often used as an indicator of the residence time in the atmosphere or of the atmospheric stability.

Despite many experimental studies, there have been few of them reporting measurements of the vertical variation of radon and its daughters. Even fewer when the whole atmospheric boundary layer and a variety of stability conditions are considered. We used large-eddy simulation to study the dispersion of the nuclide. We simulated a full diurnal cycle consisting of the three stability regimes; stable, neutral and unstable. By imposing secular equilibrium as initial condition, the issue of its possible disruption has been addressed.

Departures from secular equilibrium prevail in the stable nocturnal boundary layer. This disequilibrium is attributed to the proximity of the radon source and the weak vertical transport. Since a significant fraction of the radon is fresh, the radon and progeny mixture is deficient in daughters. The mixing by convective turbulence induces a fast restoration of the equilibrium during the morning transition.

At first glance, it appears that the equilibrium levels can be correlated directly with stability regimes in the surface layer. However, our results revealed that other boundary layer characteristics are also influential. Most of all, we showed that the activity ratio between radon and its progeny cannot be used solely as an indicator of atmospheric stability for diurnal evolving atmospheric boundary layers.