Numerical evaluation of urban green space scenarios effects on gaseous air pollutants in Tehran Metropolis based on WRF-Chem model

Although positive environmental aspects of increasing urban vegetative spaces are undeniable, using predefined or unmanaged long-term green programs may cause undesirable and unexpected impacts on the air quality in megacities. In this study, the mitigation potential of three green spaces development scenarios (expansion of surface vegetative area, Green Roof development and combination of surface and roof greenery) on three gaseous air pollutants (O-3, SO2 and NO2) concentrations in Tehran Metropolis in an early summertime period is analyzed using coupled Weather Research and Forecasting model with chemistry (WRF/Chem), single-layer urban canopy model (SLUCM) and biogenic emission from Model of Emissions of Gases and Aerosols from Nature (MEGAN). Current results indicate that general patterns of diurnal distribution of SO2 and NO2 are totally controlled by the location of anthropogenic sources of pollutants in the south and the city center, beside the prevailing wind field over the city of Tehran. Ozone distribution follows the diurnal advection from the northern and eastern sub-urban areas and distribution of primary pollutants such as NO2. By conducting green scenarios, reduction in the wind speed, variations in pollutants dry deposition velocities which are directly affected by the boundary layer and surface properties, and pollutants deposition fluxes are observed. Alterations in deposition fluxes are not uniform across the city area and clearly conform to changes in pollutants concentrations. In addition, the shallower (deeper) boundary layer height and the buoyancy forcing due to the reduction (increase) in near-surface air temperature cause less (more) vertical mass transport during the day (night), which in both situations could have negative impacts on the air quality since under lower wind speed condition and less convective upward transfer, polluted air near the surface can be trapped. The defined scenarios have shown different impacts over the city of Tehran with the complex urban morphology so, suggestion of the most efficient summertime green policy with the minimum negative feedbacks is not conceivable.