Characterization of the <sup>222</sup>Rn family turbulent transport in the convective atmospheric boundary layer

Abstract

Abstract. The combined effect of turbulent transport and radioactive decay on the distribution of 222 Rn and its progeny in convective atmospheric boundary layers (CBL) is investigated. Large eddy simulation is used to simulate their dispersion in steady state CBL and in unsteady conditions represented by the growth of a CBL within a pre-existing reservoir layer.The exact decomposition of the concentration and flux budget equations under steady state conditions allowed us to determine which processes are responsible for the vertical distribution of 222 Rn and its progeny. Their mean concentrations are directly correlated with their half-life, e.g. 222 Rn and 210 Pb are the most abundant whereas 218 Po show the lowest concentrations. 222 Rn flux decreases linearly with height and its flux budget is similar to the one of inert emitted scalar, 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 tends to destroy the fluxes. While 222 Rn exhibits the typical bottomup behavior, the maximum flux location of the daughters is moving upwards while their rank in the 222 Rn progeny is increasing leading to a typical top-down behavior for 210 Pb. We also found that the relevant radioactive decaying contributions of 222 Rn short-lived daughters ( 218 Po and 214 Pb) act as flux sources leading to deviations from the linear flux shape. In addition, while analyzing the vertical distribution of the radioactive decay contributions to the concentrations, e.g. the decaying zone, we found a variation in height of 222 Rn daughters' radioactive transformations.Under unsteady conditions, the same behaviors reported under steady state conditions are found: deviation of the fluxes from the linear shape for 218 Po, enhanced discrepancy in height of the radioactive transformation contributions for all the daughters. In addition, 222 Rn and its progeny concenCorrespondence to: J.-F. Vinuesa (jeff.vinuesa@jrc.it) trations decrease due to the rapid growth of the CBL. The analysis emphasizes the crucial role of turbulent transport in the behavior of 222 Rn morning concentrations, in particular the ventilation at the top of the boundary layer that leads to the dilution of 222 Rn by mixing with radon low concentration air.

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

“…Narrow vigorous thermals surrounded by relatively large subsidence motions generate turbulent mixing leading to a so-called mixed layer where, for instance, potential temperature is nearly constant with height. During the morning transition and the development of the unstable boundary layer, we find results similar to Vinuesa and Galmarini (2007). The concentration of all radio nuclei reduces abruptly in spite of the fresh emission of S 0 .…”

“…They showed that the turbulent properties of atmospheric convective boundary layers are of importance to study the dispersion and the transport of the 222 Rn family. In this present study, we broaden the recent work of Vinuesa and Galmarini (2007) by extending the study of the control exerted by turbulence on 222 Rn and its progeny atmospheric dispersion to the full range of stabilities characterizing atmospheric boundary layers. Not only do we corroborate their findings for the unsteady convective boundary layer but we generalize and complete the study by simulating a realistic diurnal cycle consisting of the three stability regimes i.e.…”

“…This has been mainly attributed to the slow development of detection technology of suitable accuracy that can practically be mounted on an aircraft. In the absence of such datasets, LES is an option and in a recent study, Vinuesa and Galmarini (2007) used for the first time LES to characterize the turbulent transport of the 222 Rn and its progeny in academic ABLs. They simulated a steady-state convective ABL and ABL under unsteady conditions, e.g., representing the morning transition.…”

Improved mixing height monitoring through a combination of lidar and radon measurements

“…Radon can then be used to compute an equivalent mixing height throughout the night. Then, after sunrise and the resumption of buoyancy-driven turbulent mixing, equivalent mixing height and actual mixing height become comparable for a while because the assumption of a wellmixed boundary layer becomes valid Vinuesa and Galmarini, 2007). For a period during the morning transition, the mixing layer can be observed with both lidar and radon.…”

Turbulent Dispersion of Non-uniformly Emitted Passive Tracers in the Convective Boundary Layer

“…Also none of the contributions shows a peak at the top of the boundary layer, and second, buoyancy is a source of flux throughout the ABL even in the entrainment layer where it is known to act as a sink for uniformly emitted scalars (for instance see Fig. 7a in Vinuesa and Galmarini (2007) where the same scaling approach is used). Over the sub-domain D, the contributions are more important when the emission is non-uniform.…”