Multiple proxies for continuous atmospheric fossil CO2 estimates: A comparison of the CO, NOx and APO proxies

For decades, radiocarbon has been the gold standard for separating fossil and non-fossil CO₂ (ffCO₂) contributions in ambient atmospheric CO₂. Although continuous ¹⁴CO₂ measurements are promising, they are still unavailable with sufficient precision for atmospheric applications. Therefore, ongoing attempts are to use alternative continuously measurable proxies to quantify fossil CO₂ admixture at a high temporal resolution. Proxies such as CO or NO_x have the advantage of being observed by remote sensing as well. The ICOS Central Radiocarbon Laboratory was conducting an intensive research campaign on ffCO₂ proxies for two and a half years at its urban pilot station in Heidelberg, Germany. The campaign aimed to investigate the potential and synergies of the three different ffCO₂ proxies: CO, NO_x and APO. All proxies have been observed continuously, and more than 400 flask samples have been collected and analysed for ¹⁴CO₂. This study examines and compares proxy-specific properties resulting in first order from their different atmospheric lifetimes. In addition, we quantify variabilities in the proxy/ffCO₂ ratio and deduce uncertainties of proxy-based ffCO₂ estimates.