Observational and Modeling Tools for Monitoring Urban Greenhouse Gas Emissions: Results of the ICOS Pilot City Munich

Cities are hotspots for anthropogenic greenhouse gas (GHG) emissions and, therefore, play an essential role in mitigating climate change. It is crucial to accurately monitor urban GHG emissions to track the reduction targets that many cities have set. The ICOS Cities project aims to pioneer GHG measurement methodologies. Munich, along with Zurich and Paris, serves as one of the pilot cities for this project. Here, we present key results of the ICOS Cities project for Munich.

We developed a multi-scale sensor network in Munich, including five solar-tracking spectrometers (MUCCnet) for total column CO₂, CH₄, and CO measurements, 20 mid-cost CO₂ sensor systems, 99 low-cost CO₂ sensors, and 30 low-cost air quality sensor systems on rooftops and streetlamps. A mobile measurement unit, consisting of an e-bike carrying a high-precision instrument, is used to find emission hotspots in the city. In addition, CO₂ and CH₄ satellites specifically target Munich to enable a comparative analysis with MUCCnet.

To achieve a high-resolution and accurate emission estimate for Munich, we also developed (i) an emission inventory with high spatial and temporal resolution (100m, hourly) for the road traffic, heating, public power, and human respiration sectors; (ii) a microscale transport model (10m, hourly) based on GRAMM/GRAL simulations; and (iii) a high-resolution biosphere model (10m, hourly) based on VPRM using a self-developed vegetation land cover (1m) and Sentinel-2 satellite vegetation indices.

Combining these fundamental components, we performed inverse modeling to assess the emissions in Munich for more than five years. We compared the performances of inverse modeling algorithms using different background approaches, transport models, and prior emissions. Changes due to interventions and policies, e.g., COVID lockdown and energy transition, can be observed in the emission assessments. We present measurement-based emission trends determined by our observations and modeling tools, and provide recommendations on monitoring strategies for other cities.