Design, deployment, and exploitation of a high-density CO2 sensor network in Zürich City

Since July 2022, a high-density CO₂ monitoring network has been operating in and around Zürich City, Switzerland as part of the ICOS Cities project. The network is formed of 80 sites that have been equipped with CO₂ monitors with varying performance and cost points. Three high-precision CO₂ gas analysers, 20 mid-cost sensors (installed with reference gas cylinders), and 114 low-cost sensors are in use with some sites having multiple sensors installed. Combined with several modelling approaches, the observations from the sensor network will allow for the characterisation of the city's emissions and to validate the city's CO₂ emission inventory. The different types of CO₂ monitors are combined with different data processing strategies to ensure their observations are adjusted for sensor drift and changes in responses so they can be used for robust analysis in near-real time. Between July 2022 and early January 2023, the CO₂ network's mean concentration was 446 ppm and the difference between the network's background and urban-traffic sites was 31 ppm suggesting a significant urban enhancement during this period. Distinct daily and seasonal patterns were observed for different types of sensor locations, reflecting diverse emission regimes across the urban area. Despite careful efforts during the site selection process, local contamination has been observed for many rooftop sensors due to their proximity to heating stacks. However, the installation of wind sensors allows for the observations to be flagged for such situations, which will be important for downstream data users. Details on the sensor adjustment strategies, the technologies in use for the data processing pipeline, the results from several months of CO₂ monitoring, and future plans with the other two ICOS Cities sensor network collaborators (Munich and Paris) will be discussed.