An air pollution modeling system for Switzerland using WRF-Chem : development, simulation, evaluation

Air pollution has become one of the world’s most concerning environmental problems. The Swiss cohort study on air pollution and lung diseases in adults (SAPALDIA) examines long-term health effects of air pollution and other vironmental and meteorological conditions in Switzerland. Health surveys were conducted in the years 1991, 2002 and 2010. This work contributes to SAPALDIA by developing, simulating and evaluating a new approach for population based air pollution exposure assessment. The approach is based on a chemical transport model (CTM) with an on-line implementation of the chemical reactions to a numerical weather forecast (NWP) model. The Weather Research and Forecasting (WRF) model with its chemistry extension (WRF-Chem) has been applied to the whole of Switzerland with a horizontal resolution of 2 km. This high resolution domain is nested into a coarser European domain to have the meteorological as well as the chemical initial and boundary conditions. For the initialization of the coarser European domain itself, different chemical and meteorological boundary and initial conditions have been evaluated. Anthropogenic emissions were built on the basis of Federal emissions for PM10, PM2.5 and NOx.
The evaluation of spatial and temporal distribution of O3, NO2, NO, PM10 and PM2.5 revealed systematic bias. Post-processing routines with multi-linear regressions eliminated such biases and finally achieved satisfactory results. Yearly averages of air pollutants were simulated with Pearson correlation coefficients up to 0.8. Seasonal trends and spatial distribution are captured correctly by the modeling system. For example, a strong PM10 advection from northern Italy could be shown. Outliers of measurement stations compared to WRF-Chem are explainable by the geographical parameters of the station. Compared to a dedicated dispersion model (PolluMap), WRF-Chem achieved roughly the same statistical values for PM10 and slightly lower ones for NO2 . However, WRF-Chem is able to produce temporally refined output for more pollutants. Modeled diurnal cycles showed smaller amplitudes as measurements. The necessary daily peak values for violations of air quality standards as set by the Swiss government and the European commission can not be reproduced due to the spatial resolution. Overall, WRF-Chem produced useful yearly averages of air pollutants for exposure assessment.