Earth observation (EO) offers a promising and necessary approach to addressing environmental issues and sustainable development from the city to the global scale. The full potential is currently untapped, as global harmonization is necessary for comparability and scalability, as well as streamlining EO integration into informed and efficient decision-making. Focusing specifically on city needs, local and national authorities regulate urban air pollution, while the UN SDG 11 indicator 11.6.2 explicitly targets air pollution accounting for population in “cities” and aggregating to the national level. EO brings forth novel monitoring methods to achieve this alongside more traditional ones. However, how a city is spatially defined is an ongoing area of research and policy activity where the definitions differ, which can greatly impact the estimation of exposure to air pollution. To address the varying definitions and move toward a harmonized global approach, the H2020 SMURBS/ERA-PLANET project has created a workflow and tool that adopts two well established definitions of cities to assess population-weighted particulate matter (PM) pollution for approximately 800 European cities. The workflow utilizes the Copernicus Atmospheric Monitoring Service (CAMS – regional ensemble reanalysis) data for PM2.5, an open and free European-wide source of air pollution data, overlaid with two European Commission acknowledged city boundary definitions: the Functional Urban Area and the JRC’s Degree of Urbanisation Urban Centre. These two approaches yield different results allowing stakeholders to comprehend the city boundary sensitivity. Statistical analysis on the results will highlight cases throughout Europe to showcase how important and potentially policy relevant the differences can be based on a city’s definition, especially if there is a divergence when aggregated to the national level. The GKH will disseminate the knowledge-based workflow and give prominence to the its global relevance, providing a much needed resource for developing countries and giving decision-makers an EO-based consistent tool to help meet societal challenges that are intertwined with urban growth.