An intercomparison of wet precipitation scavenging schemes and the emission rates of 222Rn for the simulation of global transport and deposition of 210Pb

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Abstract
We simulated the transport of 222Rn and its progeny 210Pb using a global three‐dimensional atmospheric tracer transport model. The purpose of this study was to understand the effects of wet deposition scavenging processes and the emission rate of 222Rn on the global transport and deposition of 210Pb. We examined the deposition of 210Pb aerosols due to wet scavenging in stratiform and convective precipitation. Four schemes for wet precipitation scavenging removal, and two assumed distributions of the emission rate of 222Rn, were studied and tested. One was assumed to have a global constant rate and in the other the constant rate was assumed to have changed with latitude. The results of the model simulations were compared with archived radionuclide data measurements from the Environmental Measurements Laboratory's global network of sampling stations. The model‐calculated global distributions of the yearly mean of the surface air concentrations of 222Rn and 210Pb, and the yearly mean total deposition of 210Pb for 1986 data for each wet precipitation scavenging scheme studied are presented. For each scheme, we have also compared yearly mean total deposition and monthly averages of the surface air concentrations and the vertical distributions of 210Pb with the measurements. We found that the Walton et al. (1988) scheme produces the best results of the four schemes studied. Although this scheme, when utilizing a constant emission rate, gives satisfactory results, use of an emission rate that changes with latitudes improves the bias of this scheme, particularly at high northern latitude sites. The model comparisons with the measurements helped us to validate and improve the model. This atmospheric tracer transport model can be used for the simulation of other radionuclide and nonradionuclide tracers for future studies related to global climate change.