The ZiCOS-M CO₂ sensor network: measurement performance and CO₂ variability across Zürich

Abstract. As a component of the ICOS Cities project, a "mid-cost'' NDIR (nondispersive infrared) CO₂ sensor network was deployed across Zürich city (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 Zürich city with five 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) while the mean bias was -0.09 ppm (range of -0.72 and 0.66 ppm) when undergoing parallel measurements with a high-precision reference gas analyser. CO₂ concentrations (technically, dry air mole fractions), were highly variable with site means in Zürich ranging from 434 to 460 ppm and Zürich'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 a combination of strong plant respiration and very confined nocturnal boundary layers. High CO₂ episodes were detected outside Zürich's urban area demonstrating that processes acting on a variety of scales drove CO₂ levels. The ZiCOS-M network offered significant insights at an order of magnitude lower cost 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.