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. Radon-222 (222Rn) is a short-lived radioactive gasnaturally emitted from land surfaces and has long been used to assessconvective transport in atmospheric models. In this study, we simulate222Rn using the GEOS-Chem chemical transport model to improve ourunderstanding of 222Rn emissions and surface concentration seasonalityand characterize convective transport associated with two Goddard EarthObserving System (GEOS) meteorological products, the Modern-Era Retrospectiveanalysis for Research and Applications (MERRA) and GEOS Forward Processing (GEOS-FP). Weevaluate four global 222Rn emission scenarios by comparing modelresults with observations at 51 surface sites. The default emission scenarioin GEOS-Chem yields a moderate agreement with surface observations globally(68.9 % of data within a factor of 2) and a large underestimate of wintersurface 222Rn concentrations at Northern Hemisphere midlatitudes andhigh latitudes due to an oversimplified formulation of 222Rn emissionfluxes (1 atom cm−2 s−1 over land with a reduction by a factor of 3 underfreezing conditions). We compose a new global 222Rn emission scenariobased on Zhang et al. (2011) and demonstrate its potential to improvesimulated surface 222Rn concentrations and seasonality. The regionalcomponents of this scenario include spatially and temporally varyingemission fluxes derived from previous measurements of soil radium contentand soil exhalation models, which are key factors in determining 222Rnemission flux rates. However, large model underestimates of surface222Rn concentrations still exist in Asia, suggesting unusually highregional 222Rn emissions. We therefore propose a conservativeupscaling factor of 1.2 for 222Rn emission fluxes in China, which wasalso constrained by observed deposition fluxes of 210Pb (a progeny of222Rn). With this modification, the model shows better agreement withobservations in Europe and North America (&gt; 80 % of data withina factor of 2) and reasonable agreement in Asia (close to 70 %). Furtherconstraints on 222Rn emissions would require additional concentrationand emission flux observations in the central United States, Canada, Africa, andAsia. We also compare and assess convective transport in model simulationsdriven by MERRA and GEOS-FP using observed 222Rn vertical profiles innorthern midlatitude summer and from three short-term airborne campaigns.While simulations with both GEOS products are able to capture the observedvertical gradient of 222Rn concentrations in the lower troposphere (0–4 km), neither correctly represents the level of convective detrainment,resulting in biases in the middle and upper troposphere. Compared withGEOS-FP, MERRA leads to stronger convective transport of 222Rn, whichis partially compensated for by its weaker large-scale vertical advection,resulting in similar global vertical distributions of 222Rnconcentrations between the two simulations. This has important implicationsfor using chemical transport models to interpret the transport of othertrace species when these GEOS products are used as driving meteorology.

Simulation of radon-222 with the GEOS-Chem global model: emissions, seasonality, and convective transport