Cities are responsible for about 70% of the global fossil fuel carbon dioxide (ffCO₂) emissions, which have led to a strong increase in the atmospheric CO₂ concentration. Therefore, it is important to estimate and validate the ffCO₂ emissions from urban hotspot regions. A powerful tool for independently estimating surface CO₂ fluxes from atmospheric CO₂ concentration measurements is the so-called atmospheric transport inversion. However, additional tracers like radiocarbon (¹⁴C) or carbon monoxide (CO) are needed to separate the ffCO₂ emissions from natural CO₂ fluxes.

In my presentation, I derive ∆ffCO₂ excess concentrations with respect to a less polluted background by using flask-based ∆¹⁴CO₂ observations and continuous CO measurements from the urban Heidelberg observation site. By putting the ¹⁴C-based and CO-based ∆ffCO₂ estimates into a regional atmospheric transport inversion system, I assess their potential to deduce the seasonal cycle of the ffCO₂ emissions in the Upper Rhine Valley metropolitan region. I reveal that the continuous CO-based ∆ffCO₂ concentration estimates provide robust seasonal cycles that show the distinct COVID-19 signal in 2020 and are suitable for validating the amplitude and phasing of the seasonal cycle of the emission inventories in the main footprint of Heidelberg.