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. The top-down atmospheric inversion method that couplesatmospheric CO2 observations with an atmospheric transport model hasbeen used extensively to quantify CO2 emissions from cities. However,the potential of the method is limited by several sources of misfits betweenthe measured and modeled CO2 that are of different origins than thetargeted CO2 emissions. This study investigates the critical sources oferrors that can compromise the estimates of the city-scale emissions andidentifies the signal of emissions that has to be filtered when doinginversions. A set of 1-year forward simulations is carried out using theWRF-Chem model at a horizontal resolution of 1 km focusing on the Paris areawith different anthropogenic emission inventories, physicalparameterizations, and CO2 boundary conditions. The simulated CO2concentrations are compared with in situ observations from six continuousmonitoring stations located within Paris and its vicinity. Results highlightlarge nighttime model–data misfits, especially in winter within thecity, which are attributed to large uncertainties in the diurnal profile ofanthropogenic emissions as well as to errors in the vertical mixing near thesurface in the WRF-Chem model. The nighttime biogenic respiration to theCO2 concentration is a significant source of modeling errors during thegrowing season outside the city. When winds are from continental Europe andthe CO2 concentration of incoming air masses is influenced by remoteemissions and large-scale biogenic fluxes, differences in the simulatedCO2 induced by the two different boundary conditions (CAMS andCarbonTracker) can be of up to 5 ppm. Nevertheless, our results demonstratethe potential of our optimal CO2 atmospheric modeling system to beutilized in atmospheric inversions of CO2 emissions over the Parismetropolitan area. We evaluated the model performances in terms of wind,vertical mixing, and CO2 model–data mismatches, and we developed afiltering algorithm for outliers due to local contamination and unfavorablemeteorological conditions. Analysis of model–data misfit indicatesthat future inversions at the mesoscale should only use afternoon urbanCO2 measurements in winter and suburban measurements in summer.Finally, we determined that errors related to CO2 boundary conditionscan be overcome by including distant background observations to constrainthe boundary inflow or by assimilating CO2 gradients of upwind–downwindstations rather than by assimilating absolute CO2 concentrations.

atmospheric chemistry and physics

Sensitivity to the sources of uncertainties in the modeling of atmospheric CO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; concentration within and in the vicinity of Paris