Abstract
Abstract. Air pollution is the number one environmental cause of premature deaths in Europe. Despite extensive regulations, air pollution remains a challenge, especially in urban areas. For studying summertime air quality in the Berlin–Brandenburg region of Germany, the Weather Research and Forecasting Model with Chemistry (WRF-Chem) is set up and evaluated against meteorological and air quality observations from monitoring stations as well as from a field campaign conducted in 2014. The objective is to assess which resolution and level of detail in the input data is needed for simulating urban background air pollutant concentrations and their spatial distribution in the Berlin–Brandenburg area. The model setup includes three nested domains with horizontal resolutions of 15, 3 and 1 km and anthropogenic emissions from the TNO-MACC III inventory. We use RADM2 chemistry and the MADE/SORGAM aerosol scheme. Three sensitivity simulations are conducted updating input parameters to the single-layer urban canopy model based on structural data for Berlin, specifying land use classes on a sub-grid scale (mosaic option) and downscaling the original emissions to a resolution of ca. 1 km × 1 km for Berlin based on proxy data including traffic density and population density. The results show that the model simulates meteorology well, though urban 2 m temperature and urban wind speeds are biased high and nighttime mixing layer height is biased low in the base run with the settings described above. We show that the simulation of urban meteorology can be improved when specifying the input parameters to the urban model, and to a lesser extent when using the mosaic option. On average, ozone is simulated reasonably well, but maximum daily 8 h mean concentrations are underestimated, which is consistent with the results from previous modelling studies using the RADM2 chemical mechanism. Particulate matter is underestimated, which is partly due to an underestimation of secondary organic aerosols. NOx (NO + NO2) concentrations are simulated reasonably well on average, but nighttime concentrations are overestimated due to the model's underestimation of the mixing layer height, and urban daytime concentrations are underestimated. The daytime underestimation is improved when using downscaled, and thus locally higher emissions, suggesting that part of this bias is due to deficiencies in the emission input data and their resolution. The results further demonstrate that a horizontal resolution of 3 km improves the results and spatial representativeness of the model compared to a horizontal resolution of 15 km. With the input data (land use classes, emissions) at the level of detail of the base run of this study, we find that a horizontal resolution of 1 km does not improve the results compared to a resolution of 3 km. However, our results suggest that a 1 km horizontal model resolution could enable a detailed simulation of local pollution patterns in the Berlin–Brandenburg region if the urban land use classes, together with the respective input parameters to the urban canopy model, are specified with a higher level of detail and if urban emissions of higher spatial resolution are used.
Highlights
Despite extensive regulations, air pollution in Europe remains a challenging issue: causing up to 400 000 premature deaths per year in Europe (EEA, 2015), air pollution is the number one environmental cause of premature deaths (OECD, 2012)
The results underline that the underestimation of mixing in the boundary layer is likely to have a strong influence on simulated nighttime NOx concentrations in urban areas, which is not corrected using the mosaic option or specifying the input parameters to the urban scheme
Since the simulated mixing layer height (MLH) is sensitive to the change in urban parameters for high intensity residential and commercial/industry/transport urban areas, it shows that this could potentially have an impact on simulated NOx concentrations
Summary
Air pollution in Europe remains a challenging issue: causing up to 400 000 premature deaths per year in Europe (EEA, 2015), air pollution is the number one environmental cause of premature deaths (OECD, 2012). In urban areas, air pollution is a problem, with 97–98 % of the urban European population (EU-28) exposed to ozone levels higher than 8 h average concentrations of 100 μg m−3, which the World Health Organisation (WHO) recommends not to be exceeded for the protection of human health, and ca. 90 % of the urban European population (EU-28) exposed to PM2.5 (particulate matter with a diameter smaller than 2.5 μm) levels higher than the WHO-recommended annual mean of 10 μg m−3 in 2011– 2013 (EEA, 2016). In 2013, the European limit value of 40 μg m−3 was exceeded at 13 % of all stations, all of them situated at traffic or urban sites (EEA, 2016). In Berlin, measured NO2 annual means exceeded the European limit value of the annual mean at all but three measurement sites close to traffic in 2014 (Berlin Senate Department for Urban Development and the Environment, 2015a). Current controversies on NO2 emissions from cars have triggered additional discussions on NO2 in urban areas
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