Are cities responsible for their air pollution?

While the burden caused by air pollution in urban areas is well documented, the origin of this pollution and therefore the responsibility of the urban areas in generating this pollution is still a subject of scientific discussion. Source Apportionment represents a useful technique to quantify the city responsibility but the approaches and applications are not harmonized, therefore not comparable, resulting in confusing and sometimes contradicting interpretations. In this work, we analyze how different source apportionment approaches apply to the urban scale and how their building elements and parameters are defined and set. We discuss in particular the options available in terms of indicator, receptor, source and methodology. We show that different choices for these options lead to very large differences in terms of outcome. In average over the 150 EU large cities selected in our study, the choices made for the indicator, the receptor and the source each lead to an average factor 2 difference. We also show that temporal and spatial averaging processes applied to the air quality indicator, especially when diverging source apportionments are aggregated into a single number lead to favor strategies that target background sources while occulting actions that would be efficient at the city center. We stress that methodological choices and assumptions most often lead to a systematic and important underestimation of the city responsibility, with important implications. Indeed, if cities are seen as a minor actor, plans will target in priority the background at the expense of potentially effective local actions.