Simulation of radon-222 with the GEOS-Chem global model: emissions, seasonality, and convective transport
Authors: Zhang Bo , Liu Hongyu , Crawford James H. , Chen Gao , Fairlie T. Duncan , Chambers Scott , Kang Chang-Hee , Williams Alastair G. , Zhang Kai , Considine David B. , Sulprizio Melissa P. , Yantosca Robert M. 
Published: February 2021

Abstract. Radon-222 (222Rn) is a short-lived radioactive gas
naturally emitted from land surfaces and has long been used to assess
convective transport in atmospheric models. In this study, we simulate
222Rn using the GEOS-Chem chemical transport model to improve our
understanding of 222Rn emissions and surface concentration seasonality
and characterize convective transport associated with two Goddard Earth
Observing System (GEOS) meteorological products, the Modern-Era Retrospective
analysis for Research and Applications (MERRA) and GEOS Forward Processing (GEOS-FP). We
evaluate four global 222Rn emission scenarios by comparing model
results with observations at 51 surface sites. The default emission scenario
in GEOS-Chem yields a moderate agreement with surface observations globally
(68.9 % of data within a factor of 2) and a large underestimate of winter
surface 222Rn concentrations at Northern Hemisphere midlatitudes and
high latitudes due to an oversimplified formulation of 222Rn emission
fluxes (1 atom cm−2 s−1 over land with a reduction by a factor of 3 under
freezing conditions). We compose a new global 222Rn emission scenario
based on Zhang et al. (2011) and demonstrate its potential to improve
simulated surface 222Rn concentrations and seasonality. The regional
components of this scenario include spatially and temporally varying
emission fluxes derived from previous measurements of soil radium content
and soil exhalation models, which are key factors in determining 222Rn
emission flux rates. However, large model underestimates of surface
222Rn concentrations still exist in Asia, suggesting unusually high
regional 222Rn emissions. We therefore propose a conservative
upscaling factor of 1.2 for 222Rn emission fluxes in China, which was
also constrained by observed deposition fluxes of 210Pb (a progeny of
222Rn). With this modification, the model shows better agreement with
observations in Europe and North America (> 80 % of data within
a factor of 2) and reasonable agreement in Asia (close to 70 %). Further
constraints on 222Rn emissions would require additional concentration
and emission flux observations in the central United States, Canada, Africa, and
Asia. We also compare and assess convective transport in model simulations
driven by MERRA and GEOS-FP using observed 222Rn vertical profiles in
northern midlatitude summer and from three short-term airborne campaigns.
While simulations with both GEOS products are able to capture the observed
vertical 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 with
GEOS-FP, MERRA leads to stronger convective transport of 222Rn, which
is partially compensated for by its weaker large-scale vertical advection,
resulting in similar global vertical distributions of 222Rn
concentrations between the two simulations. This has important implications
for using chemical transport models to interpret the transport of other
trace species when these GEOS products are used as driving meteorology.