A relaxed eddy accumulation flask sampling system for ¹⁴C-based partitioning of fossil and non-fossil CO₂ fluxes

<p><span id="page5350"/>A relaxed eddy accumulation (REA) system was developed and tested, enabling conditional sampling of air for subsequent <span class="inline-formula">¹⁴CO₂</span> analysis. This allows a <span class="inline-formula">¹⁴C</span>-based estimation of fossil fuel <span class="inline-formula">CO₂</span> concentrations in the collected air samples and, thus, an observation-based partitioning of total <span class="inline-formula">CO₂</span> fluxes measured in urban environments by eddy covariance into fossil and non-fossil components. This article describes the REA system, evaluates its performance, and assesses uncertainties in the concentration measurements. In the REA system, two separate inlet lines equipped with fast-response valves and loop systems adapted to the technical requirements enable the conditional collection of air in two sets of aluminum cylinders for updraft and downdraft samples, respectively. The switching between updraft sampling, downdraft sampling, and standby mode is thereby determined by the vertical wind measured at 20 Hz by a co-located ultrasonic 3D anemometer. A logger program provides different options for the definition of a deadband, which is used to increase the concentration differences between updraft and downdraft samples. After the sampling interval, the accumulated air is transferred by an automated 24-port flask sampler into 3 L glass flasks, which can be analyzed in the laboratory, and the cylinders are re-evacuated for the next sampling. The REA system was tested in the laboratory, as well as on a tall tower near the city center of Zurich, Switzerland. Between July 2022 and April 2023, 103 REA updraft and downdraft flask pairs for flux measurements and 9 flask pairs for quality control purposes were selected from the tall tower for laboratory analysis based on suitable micro-meteorological conditions. Uncertainties in the <span class="inline-formula">CO₂</span> concentration differences between updraft and downdraft flasks were estimated by simulations using 20 Hz in situ measurements of a closed-path gas analyzer and an open-path gas analyzer co-located with the ultrasonic anemometer. The measurements show that there is no significant bias in the concentration differences between updraft and downdraft samples and that uncertainties due to the sampling process are negligible when estimating fossil fuel <span class="inline-formula">CO₂</span> signals. In the Zurich measurements, the <span class="inline-formula">CO₂</span> concentration differences between the flask pairs agreed with the differences obtained from in situ measurements within <span class="inline-formula">−</span>0.005 <span class="inline-formula">±</span> 0.227 ppm. The largest source of uncertainty, as well as the main limitation, in the separation of fossil and non-fossil <span class="inline-formula">CO₂</span> signals in Zurich was the small signal-to-noise ratio of the <span class="inline-formula">Δ</span><span class="inline-formula">¹⁴C</span> differences measured by accelerator mass spectrometry between the updraft and downdraft flasks. The novel REA flask sampling system meets the high technical requirements of the REA method and is a promising technology for observation-based estimation of fossil fuel <span class="inline-formula">CO₂</span> fluxes.</p>