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

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.