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International audienceOzone and PM 2.5 concentrations over the city of Paris are modeled with the CHIMERE air-quality model at 4 km × 4 km horizontal resolution for two future emission scenarios. A high-resolution (1 km × 1 km) emission projec-tion until 2020 for the greater Paris region is developed by local experts (AIRPARIF) and is further extended to year 2050 based on regional-scale emission projections devel-oped by the Global Energy Assessment. Model evaluation is performed based on a 10-year control simulation. Ozone is in very good agreement with measurements while PM 2.5 is underestimated by 20 % over the urban area mainly due to a large wet bias in wintertime precipitation. A signifi-cant increase of maximum ozone relative to present-day lev-els over Paris is modeled under the "business-as-usual" sce-nario (+7 ppb) while a more optimistic "mitigation" scenario leads to a moderate ozone decrease (−3.5 ppb) in year 2050. These results are substantially different to previous regional-scale projections where 2050 ozone is found to decrease un-der both future scenarios. A sensitivity analysis showed that this difference is due to the fact that ozone formation over Paris at the current urban-scale study is driven by volatile organic compound (VOC)-limited chemistry, whereas at the regional-scale ozone formation occurs under NO x -sensitive conditions. This explains why the sharp NO x reductions im-plemented in the future scenarios have a different effect on ozone projections at different scales. In rural areas, projec-tions at both scales yield similar results showing that the longer timescale processes of emission transport and ozone formation are less sensitive to model resolution. PM 2.5 con-centrations decrease by 78 % and 89 % under business-as-usual and mitigation scenarios, respectively, compared to the present-day period. The reduction is much more prominent over the urban part of the domain due to the effective reduc-tions of road transport and residential emissions resulting in the smoothing of the large urban increment modeled in the control simulation

Air quality in the mid-21st century for the city of Paris under two climate scenarios; from the regional to local scale