High-temporal resolution measurements of radiocarbon in atmospheric CO2 for source sector attribution of urban emissions

Urban areas dominate anthropogenic CO₂ emissions and play a key role in climate change mitigation. Efficient emissions reduction requires improved knowledge of the attribution of carbon emissions. Measuring the ¹⁴C (radiocarbon) content of atmospheric CO₂ is the most direct method to distinguish fossil CO₂ emissions (ffCO₂) from biogenic and natural fluxes, due to their lack of ¹⁴CO₂ content. Hence, CO₂ produced from the combustion of fossil fuels causes a measurable decrease in the atmospheric ¹⁴CO₂/CO₂ isotope ratio. Results from flask sampling campaigns indicate the potential of high time resolution ¹⁴C measurements to attribute flux estimates using urban-scale inversions.

We present a novel analytical platform for autonomous and semi-continuous analysis of Δ¹⁴C-CO₂ in atmospheric air samples with sub-hourly time resolution. The core of the analytics is based on a saturated-absorption cavity ring-down (C14-SCAR) spectrometer (ppqSense). This is coupled to a custom-developed compact quantum cascade laser absorption spectrometer (C13-QCLAS) to provide CO₂ purity and δ¹³CO₂. The C14-SCAR and C13-QCLAS share an automated pre-concentration device (NC Technologies), to purify CO₂ from air applying a temperature-swing zeolite adsorbent trap and remove N₂O interferences. We showcase performance characteristics of the coupled C14-SCAR / C13-QCLAS system for sensitive, i.e. ppm-level detection of fossil CO₂ contributions in urban environments, and present first time series data for a monitoring site in the vicinity of Zürich (Dübendorf). Results will be discussed in conjunction with city-wide observations of CO₂ and high-resolution simulations of ffCO₂ variability.

This work is supported by the SNSF project RADIANCE (206021_220392) and part of the projects 24GRD03 MetHIR and 24GRD06 MetCTG.