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

Atmospheric Chemistry and Physics Discussions

article

Zhang, Bo¹  Considine, David B.²  Sulprizio, Melissa P.³  Yantosca, Robert M.³  Liu, Hongyu¹  Crawford, James H.⁴  Chen, Gao⁴  Fairlie, T. Duncan⁴  Chambers, Scott⁵  Kang, Chang-Hee⁶  Williams, Alastair G.⁵  Zhang, Kai⁷ 

[1] National Institute of Aerospace;NASA Headquarters;John A. Paulson School of Engineering and Applied Sciences, Harvard University;NASA Langley Research Center;Australian Nuclear Science and Technology Organization;Department of Chemistry, Jeju National University;Pacific Northwest National Laboratory

DOI  :  10.5194/acp-21-1861-2021

学科分类：大气科学

来源: Copernicus Publications

【 摘 要 】

Radon-222 ( 222 Rn) 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 222 Rn using the GEOS-Chem chemical transport model to improve our understanding of 222 Rn 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 222 Rn 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 222 Rn concentrations at Northern Hemisphere midlatitudes and high latitudes due to an oversimplified formulation of 222 Rn 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 222 Rn emission scenario based on Zhang et al. (2011) and demonstrate its potential to improve simulated surface 222 Rn 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 222 Rn emission flux rates. However, large model underestimates of surface 222 Rn concentrations still exist in Asia, suggesting unusually high regional 222 Rn emissions. We therefore propose a conservative upscaling factor of 1.2 for 222 Rn emission fluxes in China, which was also constrained by observed deposition fluxes of 210 Pb (a progeny of 222 Rn). 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 222 Rn 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 222 Rn 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 222 Rn 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 222 Rn, which is partially compensated for by its weaker large-scale vertical advection, resulting in similar global vertical distributions of 222 Rn 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.

【 授权许可】

CC BY