ICOS pilot observatories to monitor greenhouse gas emissions from three different-size European cities

By continuously monitoring greenhouse gas emissions from densely populated urban areas, we can independently assess and monitor emission reduction efforts at a policy-relevant scale. The EU-funded project PAUL (Pilot Application in Urban Landscapes - Towards integrated city observatories for greenhouse gases) develops, evaluates, and refines innovative greenhouse gas monitoring technologies including observational strategies for urban areas that enhance the capabilities of the Integrated Carbon Observation System at urban scales (ICOS Cities). In 2022, three pilot observatories have been set up to test, refine and optimize approaches for monitoring emissions from a metropolitan area (Paris, France), a large isolated city (Munich, Germany) and a mid-size city (Zurich, Switzerland). The three observatories have been developed in a co-design approach and integrated different observational technologies in support of inverse and inventory modelling. The pilot observatories focus on carbon dioxide (CO₂) emitted from fossil-fuel sources.

The three observatories operate for a pilot phase of two years and collect comparative data across cities with a multitude of instrument networks that serve three main goals: (1) to assess the input for inverse models of CO₂ at city-scale and attribute inferred CO₂ emissions to sub-city scale and emission sectors; (2) to refine spatial, temporal, and sectoral attribution of emissions in emission inventories and parametrize process models that separate urban fossil-fuel and biogenic fluxes; and (3) as independent validation datasets to evaluate estimated emission products.

In all three cities, CO₂ concentrations and selected co-emitted species are continuously sampled on tall towers, on top of high buildings and/or at street level. In the Paris metropolitan area, 10 tall tower sites and 30 roof-top sites continuously measure high-precision CO₂ and co-emitted species in the boundary layer upwind, over and downwind of the city. The Munich and Zurich observatories feature a combination of roof-top and street-level sensor networks placed closer to sources and sinks, with a stronger signal strength that is more forgiving in terms of the sensitivity, hence allowing the deployment of mid- and low-cost sensors. In Paris and Munich, additionally, total column observations of CO₂ are performed upwind, over and downwind of the main urban emission sources using concurrent ground-based FTIR spectrometers. Three new tall-tower eddy-covariance (EC) systems have been established in central Paris, Munich and Zurich. The EC-towers provide total CO₂ fluxes for defined sub-areas of each city and their characteristic diurnal, weekly and seasonal cycles. Further, the three EC-towers provide sector-specific emission ratios and fossil-fuel CO₂ fluxes based on differences of measured CO₂-fluxes, six co-emitted species and radiocarbon fluxes. Finally, all cities have observational systems in place that monitor biogenic fluxes, vegetation dynamics and meteorological conditions, including lidars for wind and mixed layer determination for an improved quantitative description of atmospheric transport and vertical mixing.

We highlight design considerations for the three observatories and exemplarily show how multi-scale systems can efficiently complement and constrain fossil-fuel emissions in urban areas. Knowledge and experience from these observations will feed into the establishment of guidelines for operational greenhouse gas monitoring systems in European cities on their way to climate neutrality.