Potential of 14C-based vs. ΔCO-based ΔffCO2 observations to estimate urban fossil fuel CO2 (ffCO2) emissions

Atmospheric transport inversions are a powerful tool for independently estimating surface CO2 fluxes from atmospheric CO2 concentration measurements. However, additional tracers are needed to separate the fossil fuel CO2 (ffCO(2)) emissions from non-fossil CO2 fluxes. In this study, we focus on radiocarbon (C-14), the most direct tracer of ffCO(2), and the continuously measured surrogate tracer carbon monoxide (CO), which is co-emitted with ffCO(2) during incomplete combustion. In the companion paper by Maier et al. (2024), we determined discrete C-14-based and continuous Delta CO-based estimates of the ffCO(2) excess concentration (Delta ffCO(2)) compared with a clean-air reference for the urban Heidelberg observation site in southwestern Germany. The Delta CO-based Delta ffCO(2) concentration was calculated by dividing the continuously measured Delta CO excess concentration by an average C-14-based Delta CO/Delta ffCO(2) ratio. Here, we use the CarboScope inversion framework adapted for the urban domain around Heidelberg to assess the potential of both types of Delta ffCO(2) observations to investigate ffCO(2) emissions and their seasonal cycle. We find that, although they are more precise, C-14-based Delta ffCO(2) observations from almost 100 afternoon flask samples collected in the 2 years of 2019 and 2020 are not well suited for estimating robust ffCO(2) 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 Delta CO-based Delta ffCO(2) 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 Delta CO-based Delta ffCO(2) observations allow for a robust reconstruction of the seasonal cycle of the area source ffCO(2) 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(2) 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 Delta CO-based Delta ffCO(2) to validate bottom-up ffCO(2) emissions at urban stations if the Delta CO/Delta ffCO(2) ratios can be determined without biases.