Urban Vegetation Effects on Meteorology and Air Quality: A Comparison of Three European Cities

Nature based solutions (NBS) have been put forward as an effective approach for improving human well-being in the cities during the last decade and many of them make use of vegetation (trees, grass, etc.). The role of vegetation in regulating air temperature and moisture availability is recognized together with its capacity to remove pollutants. Yet, cities lack of quantitative evaluation of these effects that may set the basis for the assessment of the impacts of new NBS. This study conducted within the framework of Life VEG-GAP project shows the impact of vegetation on temperature and, further, on air concentrations and deposition of ozone (O3), particulate matter (PM10) and nitrogen dioxide (NO2) in three European cities: Bologna, Madrid and Milan. The simulations were carried out with two state-of-the-art air quality modelling systems using the same meteorological model WRF with building effect parametrization (BEP) to account for the differences in urban morphology, but different chemical transport models (CTM): CMAQ for Madrid and FARM for Bologna and Milan. The emissions of biogenic volatile organic compounds (BVOC) were produced with the same species-specific model, called PSEM. The simulations were carried out over several nested domains from European to city level where the spatial resolution was 1 km2. The results show that vegetation effects on air temperature, pollutant concentrations and depositions depend on the characteristics of the city (morphology, geographic location and size), of the vegetation (extension and species) and on local chemical conditions (the cocktail of anthropogenic emissions, dispersion conditions, etc.). By applying this one-atmosphere approach for vegetation-meteorology-chemistry, the combined effect of pollutants removal, BVOC emissions distribution as well as changes of wind patterns, temperature, etc., leads to contrasting time-dependent variability patterns of vegetation effects across the cities.