Assessment of Air Quality and Meteorological Changes Induced by Future Vegetation in Madrid

Nature-based solutions and green urban infrastructures are becoming common measuresin local air quality and climate strategies. However, there is a lack of analytical frameworks toanticipate the effect of such interventions on urban meteorology and air quality at a city scale. Wepresent a modelling methodology that relies on the weather research and forecasting model (WRF)with the building effect parameterization (BEP) and the community multiscale air quality (CMAQ)model and apply it to assess envisaged plans involving vegetation in the Madrid (Spain) region.The study, developed within the VEGGAP Life project, includes the development of two detailedvegetation scenarios making use of Madrid's municipality tree inventory (current situation) andfuture vegetation-related interventions. An annual simulation was performed for both scenarios(considering constant anthropogenic emissions) to identify (i) variations in surface temperature andthe reasons for such changes, and (ii) implications on air-quality standards according to EU legislationfor the main pollutants (PM10, PM2.5, NO2 and O3). Our results suggest that vegetation may havesignificant effects on urban meteorology due to changes induced in relevant surface properties suchas albedo, roughness length or emissivity. We found a net-heating effect of around +0.18 C whentrees are introduced in dry, scarcely vegetated surfaces in the city outskirts. In turn, this enhances theplanetary boundary layer height (PBLH), which brings about reductions in ambient concentrations ofrelevant pollutants such as NO2 (in the range of 0.5-0.8 g m?3 for the annual mean, and 2-4 g m?3for the 19th highest 1 h value). Conversely, planting new trees in consolidated urban areas causesa cooling effect (up to ?0.15 C as an annual mean) that may slightly increase concentration levelsdue to less-effective vertical mixing and wind-speed reduction caused by increased roughness. Thishighlights the need to combine nature-based solutions with emission-reduction measures in Madrid.