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<p>Atmospheric transport inversions are a powerful tool for independently estimating surface CO<span class="inline-formula">₂</span> fluxes from atmospheric CO<span class="inline-formula">₂</span> concentration measurements. However, additional tracers are needed to separate the fossil fuel CO<span class="inline-formula">₂</span> (ffCO<span class="inline-formula">₂</span>) emissions from non-fossil CO<span class="inline-formula">₂</span> fluxes. In this study, we focus on radiocarbon (<span class="inline-formula">¹⁴</span>C), the most direct tracer of ffCO<span class="inline-formula">₂</span>, and the continuously measured surrogate tracer carbon monoxide (CO), which is co-emitted with ffCO<span class="inline-formula">₂</span> during incomplete combustion. In the companion paper by Maier et al. (2024), we determined discrete <span class="inline-formula">¹⁴</span>C-based and continuous <span class="inline-formula">Δ</span>CO-based estimates of the ffCO<span class="inline-formula">₂</span> excess concentration (<span class="inline-formula">Δ</span>ffCO<span class="inline-formula">₂</span>) compared with a clean-air reference for the urban Heidelberg observation site in southwestern Germany. The <span class="inline-formula">Δ</span>CO-based <span class="inline-formula">Δ</span>ffCO<span class="inline-formula">₂</span> concentration was calculated by dividing the continuously measured <span class="inline-formula">Δ</span>CO excess concentration by an average <span class="inline-formula">¹⁴</span>C-based<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M30" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><mi mathvariant="normal">Δ</mi><mi mathvariant="normal">CO</mi><mo>/</mo><mi mathvariant="normal">Δ</mi><msub><mi mathvariant="normal">ffCO</mi><mn mathvariant="normal">2</mn></msub></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="67pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="4b6d00de0d3fcd12a3e1f69c236907bf"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-24-8183-2024-ie00001.svg" width="67pt" height="14pt" src="acp-24-8183-2024-ie00001.png"/></svg:svg></span></span> ratio. Here, we use the CarboScope inversion framework adapted for the urban domain around Heidelberg to assess the potential of both types of <span class="inline-formula">Δ</span>ffCO<span class="inline-formula">₂</span> observations to investigate ffCO<span class="inline-formula">₂</span> emissions and their seasonal cycle. We find that, although they are more precise, <span class="inline-formula">¹⁴</span>C-based <span class="inline-formula">Δ</span>ffCO<span class="inline-formula">₂</span> observations from almost 100 afternoon flask samples collected in the 2 years of 2019 and 2020 are not well suited for estimating robust ffCO<span class="inline-formula">₂</span> emissions in the main footprint of this urban area, which has a very heterogeneous distribution of sources including several point sources. The benefit of the continuous <span class="inline-formula">Δ</span>CO-based <span class="inline-formula">Δ</span>ffCO<span class="inline-formula">₂</span> estimates is that they can be averaged to reduce the impact of individual hours with an inadequate model performance. We show that the weekly averaged <span class="inline-formula">Δ</span>CO-based <span class="inline-formula">Δ</span>ffCO<span class="inline-formula">₂</span> observations allow for a robust reconstruction of the seasonal cycle of the area source ffCO<span class="inline-formula">₂</span> emissions from temporally flat a priori emissions. In particular, the distinct COVID-19 signal – with a steep drop in emissions in spring 2020 – is clearly present in these data-driven a posteriori results. Moreover, our top-down results show a shift in the seasonality of the area source ffCO<span class="inline-formula">₂</span> emissions around Heidelberg in 2019 compared with the bottom-up estimates from the Netherlands Organization for Applied Scientific Research (TNO). This highlights the huge potential of <span class="inline-formula">Δ</span>CO-based <span class="inline-formula">Δ</span>ffCO<span class="inline-formula">₂</span> to validate bottom-up ffCO<span class="inline-formula">₂</span> emissions at urban stations if the <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M50" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><mi mathvariant="normal">Δ</mi><mi mathvariant="normal">CO</mi><mo>/</mo><mi mathvariant="normal">Δ</mi><msub><mi mathvariant="normal">ffCO</mi><mn mathvariant="normal">2</mn></msub></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="67pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="49b1ead5054eedd01003be55c0182cef"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-24-8183-2024-ie00002.svg" width="67pt" height="14pt" src="acp-24-8183-2024-ie00002.png"/></svg:svg></span></span> ratios can be determined without biases.</p>