Guidelines to develop and evaluate such models are discussed in the FAIRMODE initiative (http://fairmode.jrc.ec.europa.eu/) led by the JRC to support use of modelling in the implementation of the EU directives. What can be done in a city depends as said on the sources, but also resources of the city. Possibilities also depend on whether there is willingness at the political level to not only reduce emission via technological improvements but also by decreasing activity rates or implementing structural and societal changes.