Preprints
https://doi.org/10.5194/acp-2022-106
https://doi.org/10.5194/acp-2022-106
 
18 Feb 2022
18 Feb 2022
Status: a revised version of this preprint was accepted for the journal ACP and is expected to appear here in due course.

Analysis of CO2, CH4 and CO surface and column concentrations observed at Reunion Island by assessing WRF-Chem simulations

Sieglinde Callewaert1, Jérôme Brioude2, Bavo Langerock1, Valentin Duflot2, Dominique Fonteyn1, Jean-François Müller1, Jean-Marc Metzger3, Christian Hermans1, Nicolas Kumps1, Emmanuel Mahieu4, and Martine De Mazière1 Sieglinde Callewaert et al.
  • 1Royal Belgian Institute for Spacy Aeronomy (BIRA-IASB), Brussels, Belgium
  • 2Laboratoire de l’Atmosphère et des Cyclones (LACy), UMR8105, Saint-Denis, Reunion Island, France
  • 3UAR 3365 - OSU Réunion, Université de La Réunion, Saint-Denis, Reunion Island, France
  • 4UR SPHERES, Department of Astrophysics, Geophysics and Oceanography, University of Liège, Liège, Belgium

Abstract. Reunion Island is situated in the Indian Ocean and holds one of the very few atmospheric observatories in the tropical Southern Hemisphere. Moreover, it hosts experiments providing both ground-based surface and column observations of CO2, CH4 and CO atmospheric concentrations. This work presents a comprehensive study of these observations made in the capital Saint-Denis and at the high-altitude Maïdo Observatory. We used simulations of the Weather Research and Forecasting model coupled with Chemistry (WRF-Chem), in its passive tracer option (WRF-GHG), to gain more insight in the factors that determine the observed concentrations. Additionally, this study provides an evaluation of the WRF-GHG performance in a region of the globe where it has not yet been applied.

A comparison of the basic meteorological fields near the surface and along atmospheric profiles showed that WRF-GHG has decent skill in reproducing these meteorological measurements, especially temperature. Furthermore, a distinct diurnal CO2 cycle with values up to 450 ppm was found near the surface in Saint-Denis, driven by local anthropogenic emissions, boundary layer dynamics and accumulation due to low wind speed at night. Due to an overestimation of local wind speed, WRF-GHG underestimates this nocturnal buildup. At Maïdo, a similar diurnal cycle is found but with much smaller amplitude. There, surface CO2 is essentially driven by the surrounding vegetation. The hourly column-averaged mole fractions of CO2 (XCO2) of WRF-GHG and the corresponding TCCON observations were highly correlated with a coefficient of 0.90. These observations represent different air masses than those near the surface, they are influenced by processes from Madagascar, Africa and further away. The model shows contributions from fires during the Southern Hemisphere biomass burning season, but also biogenic enhancements associated with the dry season. Due to a seasonal bias in the boundary conditions, WRF-GHG fails to accurately reproduce the CH4 observations at Reunion Island. Further, local anthropogenic fluxes are the largest source influencing the surface CH4 observations. However, these are likely overestimated. Further, WRF-GHG is capable of simulating CO levels on Reunion Island with a high precision. As to the observed CO column (XCO), we confirmed that biomass burning plumes from Africa and elsewhere are important for explaining the observed variability. The in situ observations at the Maïdo Observatory can characterize both anthropogenic signals from the coastal regions and biomass burning enhancements from afar. Finally, we found that a high model resolution of 2 km is needed to accurately represent the surface observations. At Maïdo an even higher resolution might be needed because of the complex topography and local wind patterns. To simulate the column FTIR observations on the other hand, a model resolution of 50 km might already be sufficient.

Sieglinde Callewaert et al.

Status: closed

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on acp-2022-106', Anonymous Referee #1, 16 Mar 2022
  • RC2: 'Comment on acp-2022-106', Anonymous Referee #2, 15 Apr 2022
    • AC1: 'Final author comments', Sieglinde Callewaert, 20 May 2022
  • AC1: 'Final author comments', Sieglinde Callewaert, 20 May 2022

Status: closed

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on acp-2022-106', Anonymous Referee #1, 16 Mar 2022
  • RC2: 'Comment on acp-2022-106', Anonymous Referee #2, 15 Apr 2022
    • AC1: 'Final author comments', Sieglinde Callewaert, 20 May 2022
  • AC1: 'Final author comments', Sieglinde Callewaert, 20 May 2022

Sieglinde Callewaert et al.

Sieglinde Callewaert et al.

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Short summary
A regional atmospheric transport model is used to analyze the factors contributing to CO2, CH4 and CO observations at Reunion Island. We show that the surface observations are dominated by local fluxes and dynamical processes, while the column data are influenced by larger-scale mechanisms such as biomass burning plumes. The model is able to capture the measured time series well, however the results are highly dependent on accurate boundary conditions and high resolution emission inventories.
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