The Zurich Low-cost CO₂ sensor network (ZiCOS-L): data processing, performance assessment and analysis of spatial and temporal CO₂ dynamics

Abstract. As part of the ICOS Cities project, a network of low-cost NDIR (non-dispersive infrared) CO₂ sensors was set up across the city of Zurich (Switzerland), known as ZiCOS-L. The network consists of 56 sites with paired low-cost sensors spread out over the urban area of Zurich. This publication focuses on the period from August 2022 to July 2024. The sensors require in-field training for model calibration before deployment and further post-processing steps to account for drift and outliers. After data processing, the hourly mean root mean squared error (RMSE) was 13.6±1.4 ppm and the mean bias 0.75±1.67 ppm when validated against parallel reference measurements from the mid-cost sensor network ZiCOS-M. CO₂ concentrations were highly variable with site means in Zurich ranging from 438 to 465 ppm. These differences can largely be explained by the nearby surroundings, with vegetation, traffic density and human activity being dominant factors while altitude and distance from the city centre had a minor effect. Vegetation (mainly grassland) amplified the morning concentration in summer by up to 20 ppm due to ecosystem respiration, while heavy traffic increased the morning rush hour concentration by 15 ppm. Human activity was shown to locally enhance CO₂ concentrations during two public events. Despite its lower measurement accuracy, the ZiCOS-L network enables the study of concentration dynamics at a spatial and temporal scale that could only be achieved at much higher cost with mid-cost or high-precision instrumentation. The observations generated by ZiCOS-L will be further used in ICOS Cities activities to validate CO₂ emission inventories with inversion modelling systems.