Characterization of the 222Rn family turbulent transport in the convective atmospheric boundary layer J.-F. Vinuesa and S. Galmarini European Commission – DG Joint Research Centre, Institute for Environment and Sustainability, Italy
Abstract. The combined effect of turbulent transport and radioactive decay on the
distribution of 222Rn 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 222Rn and its progeny.
Their mean concentrations are directly correlated with their half-life,
e.g. 222Rn and 210Pb are the most abundant whereas 218Po show
the lowest concentrations. 222Rn 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 222Rn exhibits the typical
bottom-up behavior, the maximum flux location of the daughters is moving
upwards while their rank in the 222Rn progeny is increasing leading to a
typical top-down behavior for 210Pb. We also found that the relevant
radioactive decaying contributions of 222Rn short-lived daughters
(218Po and 214Pb) 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 222Rn daughters'
Under unsteady conditions, the same behaviors reported under steady state
conditions are found: deviation of the fluxes from the linear shape for
218Po, enhanced discrepancy in height of the radioactive transformation
contributions for all the daughters. In addition, 222Rn and its progeny
concentrations decrease due to the rapid growth of the CBL. The analysis
emphasizes the crucial role of turbulent transport in the behavior of
222Rn n morning concentrations, in particular the ventilation at the top
of the boundary layer that leads to the dilution of 222Rn by mixing with
radon low concentration air.
Citation: Vinuesa, J.-F. and Galmarini, S.: Characterization of the 222Rn family turbulent transport in the convective atmospheric boundary layer, Atmos. Chem. Phys., 7, 697-712, doi:10.5194/acp-7-697-2007, 2007.