Atmospheric Chemistry and Physics
www.atmos-chem-phys.net
1680-7316
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7
3
2007
10.5194/acp-7-697-2007
http://www.atmos-chem-phys.net/7/697/2007/
http://www.atmos-chem-phys.net/7/697/2007/acp-7-697-2007.html
http://www.atmos-chem-phys.net/7/697/2007/acp-7-697-2007.pdf
697
712
2007-02-12
Characterization of the <sup>222</sup>Rn family turbulent transport in the convective atmospheric boundary layer
J.-F. Vinuesa
jeff.vinuesa@jrc.it
S. Galmarini
European Commission – DG Joint Research Centre, Institute for Environment and Sustainability, Italy
The combined effect of turbulent transport and radioactive decay on the
distribution of <sup>222</sup>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.
<br><br>
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 <sup>222</sup>Rn and its progeny.
Their mean concentrations are directly correlated with their half-life,
e.g. <sup>222</sup>Rn and <sup>210</sup>Pb are the most abundant whereas <sup>218</sup>Po show
the lowest concentrations. <sup>222</sup>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 <sup>222</sup>Rn exhibits the typical
bottom-up behavior, the maximum flux location of the daughters is moving
upwards while their rank in the <sup>222</sup>Rn progeny is increasing leading to a
typical top-down behavior for <sup>210</sup>Pb. We also found that the relevant
radioactive decaying contributions of <sup>222</sup>Rn short-lived daughters
(<sup>218</sup>Po and <sup>214</sup>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 <sup>222</sup>Rn daughters'
radioactive transformations.
<br><br>
Under unsteady conditions, the same behaviors reported under steady state
conditions are found: deviation of the fluxes from the linear shape for
<sup>218</sup>Po, enhanced discrepancy in height of the radioactive transformation
contributions for all the daughters. In addition, <sup>222</sup>Rn 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
<sup>222</sup>Rn n morning concentrations, in particular the ventilation at the top
of the boundary layer that leads to the dilution of <sup>222</sup>Rn by mixing with
radon low concentration air.
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