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

Journal Article · Wed Feb 10 00:00:00 EST 2021 · Atmospheric Chemistry and Physics

DOI:https://doi.org/10.5194/acp-21-1861-2021· OSTI ID:1769628

Zhang, Bo ^[1]; Liu, Hongyu ^[1]; Crawford, James ^[2]; Chen, Gao ^[3]; Fairlie, Duncan ^[4]; Chambers, S ^[5]; Kang, Chang-Hee ^[6]; Williams, A ^[5]; Zhang, Kai  ^[7]; Considine, David ^[8]; Sulprizio, Melissa ^[9]; Yantosca, R ^[9]

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1. NATIONAL INSTITUTE OF AEROSPACE
2. Colorado School Of Mines
3. NASA Langley Research Center
4. NASA
5. Australian Nuclear Science and Technology Organization
6. Jeju National University, Republic of Korea
7. BATTELLE (PACIFIC NW LAB)
8. nasa
9. Harvard University

Radon-222 (222Rn) is a short-lived radioactive gas naturally emitted from land surface, 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 with aims to improve our understanding of 222Rn emissions and surface concentration seasonality, and to characterize convective transport associated with two Goddard Earth Observing System (GEOS) meteorological products, MERRA and GEOS-FP. We evaluate four available 222Rn emission scenarios by comparing model results with surface observations at 51 global surface sites. The default emission scenario in GEOS-Chem yields a moderate agreement with global surface observations (< 70% data within a factor of 2) and a large underestimate of wintertime surface 222Rn concentrations at Northern Hemisphere mid- 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 show its potential to improve simulated surface 222Rn concentrations and seasonality. The regional components of this emission scenario include spatially and temporally varying emission fluxes derived from previous measurements of soil radium content and soil exhalation models, which are key factors to 222Rn emission flux rates. However, large model underestimates of surface 222Rn concentrations still exist in Asia, suggesting unusually high regional 222Rn emissions. We propose a conservative up-scaling factor of 1.2 for 222Rn emission fluxes in China, as also constrained by the observed deposition fluxes of 210Pb (decay daughter of 222Rn). With this modification, the model shows better agreement with the observations in Europe and North America (>80% data within a factor of 2), and reasonable agreement in Asia (close to 70%). Further constraints on 222Rn emissions would require additional observations of surface 222Rn concentrations and emission fluxes in central U.S., 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 during northern mid-latitude summertime and from two short-term airborne campaigns. While the 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 to GEOS-FP, MERRA leads to stronger convective transport of 222Rn, which is partially compensated by its weaker large-scale vertical advection, resulting in similar global vertical distributions of 222Rn concentrations between the two simulations.

• Research Organization: Pacific Northwest National Lab. (PNNL), Richland, WA (United States)
• Sponsoring Organization: USDOE
• DOE Contract Number: AC05-76RL01830
• OSTI ID: 1769628
• Report Number(s): PNNL-SA-146982
• Journal Information: Atmospheric Chemistry and Physics, Vol. 21, Issue 3
• Country of Publication: United States
• Language: English