Uncertainties in future air quality: a scientific workflow tool

Future air quality in large cities will not only depend on how sustainable the city administration will be in terms of how they introduce clean technologies and organize the energy, heating and transport systems. Urban air quality will also be affected by global, hemispheric and regional contributions of long- range transported pollutants, for which the future evolution is depending on international emission protocols and undertakings. An additional factor is climate change, which will modify the atmospheric response to a certain trace gas emission scenario. All these factors - local urban emissions, long range regional contributions to air pollution and finally the climate change effect - should be considered in long term urban planning, so that appropriate actions can be taken to avoid negative health impacts due to air pollution exposure. The FP7 project SUDPLAN (www.sudplan.eu) facilitates a platform for assessing the importance of all the factors mentioned above, separately and all together. This allows quantitative assessments in a particular European city - e.g. in Stockholm, the example given here - of how much climate change affects future air pollution levels, how much the EU undertakings in terms of emission reductions will contribute to long range transported pollutants and how much the city itself can influence future air pollution by mitigating local emissions. Future emissions and climate are uncertain, thus they are given as possible scenarios. By comparing different scenario simulations, based on different global climate models and different scenarios for how European emissions will evolve, an indication is given of how "robust" an individual model result is (the ensemble approach). Another uncertainty originates from the assumptions made while preparing the city's future emission inventory, used as input data to the model. This latter uncertainty can be assessed separately from the climate and European emission uncertainties. The SUDPLAN tool includes distributed services both for accessing scenarios of global/regional climate and environmental information and - with use of those at the boundaries - for executing downscaling simulations of intense rainfall (stormwater flooding), hydrological conditions (river discharge, soil moisture) and urban air quality. The present paper describes an application of SUDPLAN to assess air quality in Stockholm, how future ozone and nitrogen dioxide levels will be affected by climate change and varying European trace gas emissions. For ozone, which is a secondary pollutant generated by precursors emitted outside Stockholm city centre, there is a rather clear trend that air coming into Stockholm will have a lower ozone level in the future. The simulation is based on the assumption that Europe can follow the emission reductions required for achieving no more than a 4.5 Wm -2 radiative forcing at year 2100. The SUDPLAN downscaling of ozone shows that up to ~2030 average ozone levels in the Stockholm center will be reduced by about 5 µgm -3 (~10%). The rather large inter-annual variability will however permit high extreme values to occur also in the future. For NO2, which is to a high degree locally generated, there is no clear trend in long range transport levels. The dominating impact of local sources makes the effect of long-range transport and climate change of little importance, so that local actions on NOx emissions will be decisive for accomplishing reduced NO2 levels. Measured inter-annual differences in measured NO2 levels, larger than the differences found between simulated present and ~2030 levels, point towards the importance of meteorological variability.