Compact city forms are associated with minimal consumption of land and energy, hence, they are often promoted as being the more sustainable thus preferred mode of urban development. In this context, numerical simulations were performed to evaluate the effect of urban sprawl on air quality and associated human exposure. Working on a highly urbanised area in the German Ruhrgebiet, models dealing with satellite data processing, traffic flows, pollutant emission and atmospheric dispersion were applied in an integrated fashion, under conditions representative of the urbanised area as it is today. A fair agreement was obtained between simulated and observed meteorological variables, as well as between simulated and observed concentrations of ozone and particulate matter. Simulated atmospheric pollution fields were found to closely reflect urbanisation patterns. In a companion paper [De Ridder, K., Lefebre, F., Adriaensen, S., Arnold, U., Beckroege, W., Bronner, C., Damsgaard, O., Dostal, I., Dufek, J., Hirsch, J., IntPanis, L., Kotek, Z., Ramadier, T., Thierry, A., Vermoote, S., Wania, A., Weber, C., 2008. Simulating the impact of urban sprawl on air quality and population exposure in the German Ruhr area. Part II: Development and evaluation of an urban growth scenario], the results of this base case simulation will be compared with those of a scenario simulation, designed to mimic urban sprawl, so as to allow the evaluation of the latter on air quality and associated human exposure.

The impact of uncontrolled urban growth (‘sprawl’) on air pollution and associated population exposure is investigated. This is done for the Ruhr area in Germany, by means of a coupled modelling system dealing with land use changes, traffic, meteorology, and atmospheric dispersion and chemistry. In a companion paper [De Ridder, K., Lefebre F., Adriaensen S., Arnold U., Beckroege W., Bronner C., Damsgaard O., Dostal I., Dufek J., Hirsch J., Int Panis L., Kotek Z., Ramadier T., Thierry A., Vermoote S., Wania A., Weber C., 2008. Simulating the impact of urban sprawl on air quality and population exposure in the German Ruhr area. Part I: reproducing the base state.], a description was given of the coupling of these models and of the validation of simulation results. In the present paper, a land use change scenario was implemented to mimic urban sprawl, relocating 12% of the urban population in the study domain to the green periphery. The resulting updated land use, population and employment density patterns were then used as input for traffic simulations, yielding an increase of total traffic volume by almost 17%. As a consequence, the domain-average simulated pollutant concentrations of ozone and particulate matter increased, though by a smaller amount, of approximately 4%. In a final step, population exposure to air pollution was calculated, both for the base case and the scenario simulations. A very slight domain-average exposure increase was found, of the order of a half percent. A compensating mechanism was identified, explaining this small figure. However, when stratifying the population into groups of individuals that were relocated to the urban periphery and those that were not, much larger exposure changes following urban sprawl emerged. Indeed, it was found that the relatively small proportion of relocated individuals benefited of a decrease of exposure to particulate matter by almost 13%, mainly because of their moving out of the most polluted areas; and that this came at the expense of an increase of exposure of 1.2% by the individuals not having moved.