Abstract
Abstract. The radioactive decay of radon and its progeny can lead to ionization of air molecules and consequently influence aerosol size distribution. In order to provide a global estimate of the radon-related ionization rate, we use the global atmospheric model ECHAM5 to simulate transport and decay processes of the radioactive tracers. A global radon emission map is put together using regional fluxes reported recently in the literature. Near-surface radon concentrations simulated with this new map compare well with measurements. Radon-related ionization rate is calculated and compared to that caused by cosmic rays. The contribution of radon and its progeny clearly exceeds that of the cosmic rays in the mid- and low-latitude land areas in the surface layer. During cold seasons, at locations where high concentration of sulfuric acid gas and low temperature provide potentially favorable conditions for nucleation, the coexistence of high ionization rate may help enhance the particle formation processes. This suggests that it is probably worth investigating the impact of radon-induced ionization on aerosol-climate interaction in global models.
Highlights
In recent years the impact of atmospheric ions on aerosol formation and life cycle has attracted increasing attention
Kazil et al (2010) show that in the global aerosol-climate model ECHAM5-HAM, charged H2SO4/H2O nucleation induces a −1.15 W m−2 flux of shortwave radiation at the top of the atmosphere via direct, semi-direct and indirect aerosol effects. This value is considerably larger than the fluxes caused by cluster activation (−0.235 W m−2) and neutral H2SO4/H2O nucleation (−0.05 W m−2)
Galactic cosmic rays play a major role in the upper troposphere and lower stratosphere, over the oceans and in the polar regions, other natural processes cause ionization, the main contributors being the radioactive decay of radon (222Rn), thoron (220Rn) and Published by Copernicus Publications on behalf of the European Geosciences Union
Summary
In recent years the impact of atmospheric ions on aerosol formation and life cycle has attracted increasing attention (see, e.g., Yu and Turco, 2000; Lovejoy et al, 2004; Kulmala et al, 2004; Kazil et al, 2006, among others). Before our work presented in this paper, the global radon flux map by Schery and Wasiolek (1998) (hereafter SW1998) was the only one that includes detailed regional information and seasonal variation over land surfaces It has been used in several studies of transport modelling (see, e.g., Koch et al, 2006; Hirao et al, 2008). Lupu and Cuculeanu (1999) showed that even above vegetated ground (where the dry deposition velocity is supposed to be larger than above bare ground), the effect of dry deposition on the concentration of radon decay products above 5 m is relatively small compared to the effect of turbulent mixing Considering these results and the fact that scavenging happens at time scales much longer than the life-times of the progeny, we ignore this process in our simulations
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