Abstract
Abstract. Ozone and PM2.5 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 projection 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 developed 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 PM2.5 is underestimated by 20% over the urban area mainly due to a large wet bias in wintertime precipitation. A significant increase of maximum ozone relative to present-day levels over Paris is modeled under the "business-as-usual" scenario (+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 under 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 NOx-sensitive conditions. This explains why the sharp NOx reductions implemented in the future scenarios have a different effect on ozone projections at different scales. In rural areas, projections at both scales yield similar results showing that the longer timescale processes of emission transport and ozone formation are less sensitive to model resolution. PM2.5 concentrations 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 reductions of road transport and residential emissions resulting in the smoothing of the large urban increment modeled in the control simulation.
Highlights
Climate change can affect air quality through a number of mechanisms related to meteorological variables such as temperature, humidity, precipitation, solar radiation, wind speed and the planetary boundary layer height
An “offline” modeling approach is used to assess air quality in the study region as a response to climate and emission changes. For both climate and air-quality simulations we use a dynamical downscaling approach consisting of two oneway nesting steps: from the global to regional scale over Europe and from the regional to local scale over the Ile-de-France region (IdF) region
The model is able to represent the main features of the ozone photochemical cycle over the Ile-de-France region but fails to reproduce high ozone episodes
Summary
Climate change can affect air quality through a number of mechanisms related to meteorological variables such as temperature, humidity, precipitation, solar radiation, wind speed and the planetary boundary layer height. If the effects of atmospheric pollutants (greenhouse gasses, other gaseous species and aerosols) on climate change have been extensively investigated over the last decades (IPCC, 2001, 2007) the impact of these emissions and changed climate on air quality has only raised interest during the last few years and many issues remain open Most of these studies use global chemistry transport models (CTMs) to study the impact of changing climate on tropospheric ozone at either the global (Brasseur et al, 1998; Liao et al, 2006; Prather et al, 2003; Szopa and Hauglustaine, 2007) or regional scale The climate–chemistry models used in such large-scale studies suffer from a simplistic representation of regional-scale chemistry
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