The ZiCOS-M CO ₂ sensor network: measurement performance and CO ₂ variability across Zurich

Abstract. As a component of the ICOS Cities project, a “mid-cost” NDIR (nondispersive infrared) CO2 sensor network was deployed across the city of Zurich (Switzerland), known as ZiCOS-M. The network was operational between July 2022 and July 2024 and consisted of 26 monitoring sites, 21 of which were located in or around the city of Zurich, with 5 sites outside the urban area. Daily calibrations using two reference gas cylinders and corrections of the sensors' spectroscopic response to water vapour were performed to reach a high level of measurement accuracy. The hourly mean root mean squared error (RMSE) was 0.98 ppm (range of 0.46 and 1.5 ppm) and the mean bias ranged between −0.72 and 0.66 ppm when undergoing parallel measurements with a high-precision reference gas analyser for a period of 2 weeks or more. CO2 concentrations (technically, dry-air mole fractions) were highly variable with site means in Zurich ranging from 434 to 460 ppm, and Zurich's mean urban CO2 dome was 15.4 ppm above the regional background. Some of the highest CO2 levels were found at two sites exposed to strong plant respiration in a very confined nocturnal boundary layer. High-CO2 episodes were detected outside Zurich's urban area, demonstrating that processes acting on a variety of scales drove CO2 levels. The ZiCOS-M network offered significant insights at a cost an order of magnitude lower compared to reference instruments, and the observations generated by ZiCOS-M will be used in additional ICOS Cities activities to conduct CO2 emission inventory validation with inversion modelling systems.