Differences in particulate matter concentrations between urban and rural regions under current and changing climate conditions

Abstract

Pollution levels in urban areas and their surrounding rural regions differ due to different sources and density of emissions, different composition of pollutants as well as specific meteorological effects. These concentration differences for PM10 are investigated and compared in this study for three different north-west European urban agglomerations: The German Ruhr area, the Dutch Randstad and the German city of Berlin. Measurement data for PM10 for the years 2003–2008 at urban and rural background stations are selected from the AirBase database to specify the PM10 concentration difference between these urban areas and their surrounding rural regions, here defined as the urban increment. Whereas the absolute and relative measured urban increment averaged over the years 2003–2008 for the Ruhr area (7.4 μg m−3, 35%) and Berlin (8.5 μg m−3, 46%) are comparable in magnitude, a significantly smaller value is found for the Randstad (3.1 μg m−3, 12%). To analyze whether the regional chemistry transport model LOTOS-EUROS is able to reproduce the measured urban increment simulation runs were performed for 2003–2008 on a 0.5° × 0.25° lon-lat grid covering Europe and for the year 2008 on a finer grid of 0.125° × 0.0625° covering the Netherlands and Germany, both with ECMWF meteorology as input. Although the model underestimates the absolute PM10 urban increment averaged over the years 2003–2008 for the Ruhr area (3.3 μg m−3, 33%), the Randstad (1.5 μg m−3, 12%) and Berlin (1.7 μg m−3, 27%), the relative urban increment for the Ruhr area and the Randstad is in general agreement with the measurements. The tested increase of the horizontal resolution gives no systematic improvement of the simulated urban increment. However, an even higher resolution than used here seems to be more appropriate to capture the urban increment (especially for Berlin).The variability of the PM10 urban increment with weather is tested by means of the summer 2003, such an extreme synoptic situation is expected to occur more often in future. Measured and simulated PM10 concentrations in summer 2003 were compared to the summer average of 2003–2008. The response of the observed urban increment was found to depend on the urban area. In general the model reproduces the main features for the Randstad and Berlin.In order to investigate the impact of a changing climate on the PM10 urban increment, simulations were performed with the off-line coupled model system RACMO2 (regional climate model) – LOTOS-EUROS (air quality model) over Europe. Different sets of simulations were carried out using RACMO2 meteorology with ECHAM5 A1B and with MIROC3.2-hires A1B boundary conditions for the time period 1970–2060, as well as with ERA-interim boundary conditions for the time period 1989–2009. Anthropogenic emissions were kept constant in the LOTOS-EUROS simulations. Simulated concentrations differ between the runs using ECHAM and MIROC boundary conditions and both runs differ from the present-day simulations with ERA-interim forcing. The impact of climate change on the modeled PM10 concentrations and the urban increment was found to be small in both scenario runs. However the concentration differences between the simulations forced by either ECHAM or MIROC indicate that PM10 concentration levels are sensitive to circulation patterns rather than temperature change alone, and that PM10 concentration levels may thus change when circulation patterns change in the future.