The "Pilot Application in Urban Landscapes - Towards integrated city observatories for greenhouse gases" (PAUL) project supports the European Green Deal by creating capabilities to observe and verify greenhouse gas emissions from densely populated urban areas across Europe. Cities are recognized as important anthropogenic greenhouse gas emission hotspots and therefore play a significant role in any emission reduction efforts. The PAUL project aims to increase our understanding of specific needs of greenhouse gas emission assessment in urban environments; it compares available and novel observational approaches and implements an integrated concept for a city observatory, providing unique data sets that feed diverse modelling approaches, scientific studies and will be the base of services towards the city administrations. A specifically innovative approach is the co-design of services, models and observations between city administrators and scientists from multiple disciplines including social and governmental sciences.The PAUL co-design approach will explore the needs of the cities and combine these with the scientific outcomes. This allows to introduce smart services to the cities, supporting evidence-based decisions on climate action and strategic investments. Overarching goals of PAUL are to: 1) implement elements of a pilot city observatory in a large (Paris), a medium (Munich) and a small (Zurich) European city, 2) collaborate with city stakeholders and engage citizens in co-designing services that are required for GHG monitoring in order to validate the implementation of Paris Agreement, and 3) increase our understanding of specific needs of GHG assessment in urban environments and create a service portfolio for setting up an urban greenhouse gas observatory.