###### Abstract
Therefore, the study of anthropogenic CO fluxes is crucial to investigations of CO's atmospheric burden, trends, and its influences on radiative forcing, as well as to help verify anthropogenic CO\({}_{2}\) inventories through observations, and to provide supports for the global stocktake. Since 2000, the atmospheric CO burden has been monitored globally and continuously by space-borne instruments such as MOPITT [20], Atmospheric Infrared Sounder (AIRS) [21], Tropospheric Emission Spectrometer (TES) [22], Infrared Atmospheric Sounding Interferometer (IASI) [23], and TROPOspheric Monitoring Instrument (TROPOMI) [24]. These space-borne observations indicate atmospheric CO concentrations have been decreasing globally for the last two decades. This decrease is believed to be mainly due to the improvements in combustion efficiencies that have reduced the amount of anthropogenic CO emissions [10; 25; 26; 27; 28; 29; 30; 31; 32]. Because of large chemical CO sources and sinks in the atmosphere, the atmospheric CO concentrations are not only driven by the surface emissions. The declining trend of CO concentrations observed by satellites is inconsistent with global bottom-up inventories, which show that CO emissions have been rising since 2000 due to increased fossil fuel consumption [5; 33; 34]. Therefore, the chemical transport model and inversion method are needed to infer the surface-CO emissions from observations, as shown by [35; 36; 37; 15; 38]. [PERSON] [15] showed that the reduction in CO emissions in Europe since 2000, inferred from MOPITT observations, was mainly due to reduced anthropogenic emissions, which was consistent with local bottom-up inventories. [PERSON] [39] and [PERSON] [28] also obtained similar results in western Europe, while [PERSON]'s research on CO emissions, constrained by MOPITT observations [38], showed that the anthropogenic CO emissions in Europe had not changed since 2008. In this study, CO fluxes and concentrations in western Europe during four consecutive winter seasons from 2017 to 2020 were inverted by a regional CO flux inversion system, which was modified from [PERSON]'s WRF-Chem-/DART-based CO\({}_{2}\) flux inversion system [40] to assimilate MOPITT CO observations. The inversion system employed the same profile assimilation strategy as in [38; 41]. Western Europe has a well-developed and organized statistical infrastructure that enables it to have quite accurate emission inventories with uncertainty of less than 5% [42]. Western Europe is also a highly industrialized region. During the winter, the atmospheric CO concentrations and anthropogenic CO emissions are at their highest levels throughout the year [38; 43], and ecosystem activities are at the lowest levels, which means biogenic CO emissions and biogenic atmospheric sources of CO, such as CH\({}_{4}\) and NMVOC, are at their lowest levels in the year. Furthermore, biomass-burning CO emissions are also at their lowest levels during this period of a year [44; 45]. Therefore, it was assumed that the anthropogenic CO emissions accounted for the super majority of the winter CO fluxes in western Europe. Based on this assumption, emission inventories submitted to the UNFCCC [46] by western European countries were used to evaluate the accuracy of the winter CO fluxes inverted by the assimilation system. In addition, ground-CO observationsfrom ICOS sites [47] and external CAM-Chem results [48] were used to evaluate and analyze data assimilation (DA) and parallel simulation-only (SIM) experimental results.