WRF-Chem simulations of CO ₂ over Belgium and surrounding countries assessed by ground-based measurements

Abstract. The Weather Research and Forecasting model coupled with Chemistry (WRF-Chem), in its passive tracer option (WRF-GHG), was used to simulate CO2 concentrations over Western Europe during summer 2018. The model performance was evaluated against ground-based observations. Due to the large variety of anthropogenic emissions, we conducted five sensitivity tests using a combination of three different inventories (CAMS-REG-ANT, EDGAR, and TNO) and source-specific vertical emission profiles. Compared with observations from five Integrated Carbon Observation System (ICOS) atmospheric stations, the model captures diurnal CO2 variations at different heights. At the ICOS site in Karlsruhe, Germany, simulated near-surface CO2 mole fractions are highly sensitive to the choice of inventory, with discrepancies up to 14.99 ± 31.98 ppm, due to large nearby emission sources. Furthermore, incorporating source-specific vertical profiles notably improves accuracy, increasing the correlation coefficient from 0.53 to 0.78 when using EDGAR. The column-averaged dry-air mole fractions of CO2 (XCO2) from the Total Column Carbon Observing Network (TCCON) are well simulated by WRF-GHG. However, an overestimation of approximately 1.2 ppm was found at the Paris site, likely due to uncertainties in background fields and anthropogenic emissions. In addition, a negative bias was found in early June at most ICOS and TCCON sites, may be attributed to errors in simulated fluxes during the growing season. However, due to the lack of co-located flux observations, the exact cause remains uncertain. Overall, this study demonstrates the capability of WRF-GHG in simulating CO2 over Belgium and surrounding countries and provides insights into the regional-scale characteristics of atmospheric CO2.