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<p>Atmospheric measurements of the <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M4" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><msub><mi mathvariant="normal">O</mi><mn mathvariant="normal">2</mn></msub><mo>/</mo><msub><mi mathvariant="normal">N</mi><mn mathvariant="normal">2</mn></msub></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="34pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="ccafc1cf4dcd8c0a9ea3d9498ae65a82"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-23-15767-2023-ie00001.svg" width="34pt" height="14pt" src="acp-23-15767-2023-ie00001.png"/></svg:svg></span></span> ratio and the <span class="inline-formula">CO₂</span> mole fraction (combined into the conceptual tracer “Atmospheric Potential Oxygen”, APO) over continents have been proposed as a constraint on <span class="inline-formula">CO₂</span> emissions from fossil-fuel burning. Here we assess the suitability of such APO data to constrain anthropogenic <span class="inline-formula">CO₂</span> emissions in western Europe, with particular focus on their decadal trends. We use an inversion of atmospheric transport to estimate spatially and temporally explicit scaling factors on a bottom-up fossil-fuel emissions inventory. Based on the small number of currently available observational records, our <span class="inline-formula">CO₂</span> emissions estimates show relatively large apparent year-to-year variations, exceeding the expected uncertainty of the bottom-up inventory and precluding the calculation of statistically significant trends. We were not able to trace the apparent year-to-year variations back to particular properties of the APO data. Inversion of synthetic APO data, however, confirms that data information content and degrees of freedom are sufficient to successfully correct a counterfactual prior. Larger sets of measurement stations, such as the recently started APO observations from the Integrated Carbon Observation System (ICOS) European research infrastructure, improve the constraint and may ameliorate possible problems with local signals or with measurement or model errors at the stations. We further tested the impact of uncertainties in the <span class="inline-formula">O₂:CO₂</span> stoichiometries of fossil-fuel burning and land biospheric exchange and found they are not fundamental obstacles to estimating decadal trends in fossil-fuel <span class="inline-formula">CO₂</span> emissions, though further work on fossil-fuel <span class="inline-formula">O₂:CO₂</span> stoichiometries seems necessary.</p>