European air pollution in 2050, a regional air quality and climate perspective under CMIP5 scenarios

Air pollution and climate change are closely related. They share both driving geophysical processes and mitigation strategies. Increased temperature, changes in weather regimes and precipitation patterns will alter the formation of pollution episodes. At the same time curbing greenhouse gases emission will also induce indirect co-benefits for air pollutant emissions. As a consequence, understanding the long-term efficiency of air pollution mitigation strategies requires the integrated implementation of comprehensive geophysical and economical models. Coupling air pollution and climate models for long term projections raise a number of scientific and technical issues. Global scale circulation outputs must be downscaled in order to provide high resolution three dimensional meteorological fields at high temporal frequency to the chemistry transport model. The computational cost of the air quality model is comparable to the cost of the regional climate model. So that the computing demand and storage call for an efficient design of a complex modelling suite. Moreover the cost of the project prohibits the implementation of large ensemble of model, thereby raising concerns on the treatment of uncertainty analyses of the projections. We present an integrated assessment of future air quality that relies on up-to-date emission scenarios and full-frame geophysical models of climate and atmospheric chemistry which are themselves embedded in monetised economical models to propose a cost-benefit assessment. Emissions: For long lived trace species, we use the Representative Concentrations Pathways (RCP) produced for the Fifth Assessment Report (AR5) of IPCC whereas regional air quality modelling is based on the updated emissions scenarios produced in the framework of the Global Energy Assessment (GEA) that offer an explicit representation of air quality policies. Climate and chemistry models: We use the latest sources of recent coordinated model intercomparison projects, each time selecting the IPSL or INERIS member of the ensemble: CMIP5 for global climate, ACCMIP for global atmospheric chemistry, EURO-CORDEX for regional climate. Cost-Benefit analysis: The health impacts of air pollution are evaluated and monetised using the Alpha-RiskPoll tool in order to compare the sanitary benefits to the economical cost of climate mitigation extracted from the GEA scenarios. This first assessment of projected air quality and climate at the regional scale based on CMIP5 is in line with the existing literature using CMIP3. The sensitivity to the climate model selected in the ensemble is pointed out. Sensitivity simulations show that the main factor driving future air quality projections is air pollutant emissions, rather than climate change or long range transport. Whereas the well documented 'climate penalty' bearing upon ozone over Europe is confirmed, other features appear less robust compared to the literature such as the impact of climate on PM2.5. The cost-benefit analysis allows demonstrating that the expected sanitary benefits induced by air pollutant reductions in Europe exceed the economical cost of climate mitigation