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<p>Providing timely information on urban greenhouse gas (GHG) emissions and their trends to stakeholders relies on reliable measurements of atmospheric concentrations and the understanding of how local emissions and atmospheric transport influence these observations.</p> <p>Portable Fourier transform infrared (FTIR) spectrometers were deployed at five stations in the Paris metropolitan area to provide column-averaged concentrations of <span class="inline-formula">CO₂</span> (<span class="inline-formula">XCO₂</span>) during a field campaign in spring of 2015, as part of the Collaborative Carbon Column Observing Network (COCCON). Here, we describe and analyze the variations of <span class="inline-formula">XCO₂</span> observed at different sites and how they changed over time. We find that observations upwind and downwind of the city centre differ significantly in their <span class="inline-formula">XCO₂</span> concentrations, while the overall variability of the daily cycle is similar, i.e. increasing during night-time with a strong decrease (typically 2–3&thinsp;ppm) during the afternoon.</p> <p><span id="page3272"/>An atmospheric transport model framework (CHIMERE-CAMS) was used to simulate <span class="inline-formula">XCO₂</span> and predict the same behaviour seen in the observations, which supports key findings, e.g. that even in a densely populated region like Paris (over 12 million people), biospheric uptake of <span class="inline-formula">CO₂</span> can be of major influence on daily <span class="inline-formula">XCO₂</span> variations. Despite a general offset between modelled and observed <span class="inline-formula">XCO₂</span>, the model correctly predicts the impact of the meteorological parameters (e.g. wind direction and speed) on the concentration gradients between different stations. When analyzing local gradients of <span class="inline-formula">XCO₂</span> for upwind and downwind station pairs, those local gradients are found to be less sensitive to changes in <span class="inline-formula">XCO₂</span> boundary conditions and biogenic fluxes within the domain and we find the model–data agreement further improves. Our modelling framework indicates that the local <span class="inline-formula">XCO₂</span> gradient between the stations is dominated by the fossil fuel <span class="inline-formula">CO₂</span> signal of the Paris metropolitan area. This further highlights the potential usefulness of <span class="inline-formula">XCO₂</span> observations to help optimize future urban GHG emission estimates.</p>