Turbulent transport of 222-Rn and its short-lived daughters in convective boundary layers

²²²Rn is a natural radioactive compound with a half-life of 3.8 days. Because of its noble gas nature, it is a suitable tracer in studies of atmospheric boundary layers. Ground-based measurements and vertical distributions of ²²²Rn and its daughters have been extensively studied in the past, e.g., to characterize the turbulent properties of the atmospheric boundary layer, to perform regional and global circulation model benchmarking and to estimate regional surface fluxes of air pollutant and in particular climatically sensitive compounds. In addition, radon progeny have been used to study the turbulent transport process since they have half-lifes of the same order of magnitude as the turnover time of the convective boundary layer (CBL). However, the distribution of these compounds can be affected by the turbulent mixing in the CBL.

The influence of turbulent mixing on the transport of ²²²Rn and its daughters is studied by analyzing the radioactive decay contribution to the governing equations. Large eddy simulation is used to simulate the reacting transport of ²²²Rn and its progeny in steady state convective boundary layers (CBL) and in unsteady conditions represented by the growth of a CBL under a pre-existing reservoir layer. Performing an exact decomposition of the flux budget equations allowed us to determine which physical processes are responsible for their vertical transport.

In the steady-state CBL, ²²²Rn flux decreases linearly with height. Its flux budget is similar to the one of inert emitted scalars, i.e., a balance between on the one hand the gradient and the buoyancy production terms, and on the other hand the pressure and dissipation at smaller scales which tend to destroy the fluxes. However, ²²²Rn short-lived daughters, i.e. ²¹⁸Po, ²¹⁴Pb and ²¹⁴Bi, have their radioactive decaying contributions acting as flux sources leading to deviations from the linear flux shape. The budget analysis reveals that the gradient contribution to the flux is the most affected term. In the unsteady boundary layer, ²²²Rn and its progeny concentrations collapse due to the rapid growth of the CBL. The analysis emphasizes the crucial role of turbulent transport in the behavior of ²²²Rn morning concentrations. In addition, the analysis of vertical distribution of the chemical contributions to the concentrations, i.e. the reacting zone, reveals a discrepancy in height of ²²²Rn daughters' radioactive transformations.