Evaluation and intercomparison of Ozone and PM10 simulations by several chemistry transport models over four European cities within the CityDelta project

The CityDelta project Cuvelier et al. [2006. CityDelta: a model intercomparison study to explore the impact of emission reductions in European cities in 2010. Atmospheric Environment] is designed to evaluate the air quality response of several emission abatement scenarios for 2010 at the scale of the European continent, and specifically in the areas where most people live: the cities. Before evaluating this response, the model simulations in a control case must be evaluated against observations in order to understand their main strengths and weaknesses. In this article six different models are used to simulate a full year (1999) of air quality pollutant concentrations over domains encompassing a large area around four major European cities: Berlin, Milan, Paris and Prague. Three models are used both at large-scale (typically 50 km) and small-scale resolution (5 km). The intercomparison of the simulation results for ozone and particles smaller than 10 microns (PM10) leads to the following conclusions: (i) Models capture fairly well the mean, daily maxima and variability of ozone concentrations, as well as the time and intercity variability. However, a significant overestimation of ozone in city centres is found especially for large-scale models. (ii) PM10 simulation skill is generally poor, and large-scale models underestimate their mass. The difference between Milan (highly polluted) and the other cities is not reproduced. All models have difficulties in capturing the observed seasonal variations. (iii) The fine scale models show higher PM10 and lower ozone concentrations in urban areas, which are closer to the observations than are the large-scale models.