An investigation into the effects of green space on air quality of an urban area using CFD modeling

Tehran, the capital of Iran, is one of the most polluted cities in the Middle East. However, there is no clear evidence of the effects of Tehran green spaces on reducing air pollution because there are still challenges to application of vegetation for improving air quality. The current study investigates the effects of a large volume of green space with 275 ha area in the vicinity of major highways on nearby air quality. Similar environments are often seen in many cities around the world. A computational fluid dynamics model containing aerodynamics and deposition effects of vegetation was used to simulate pollutant dispersion for eight wind directions and three different foliage densities. Analysis of results showed that the existence of trees in the green space reduces averaged concentrations of pollutants, including PM10, CO, NOx, and VOC over the studied site at pedestrian level. The best improvement of air quality was achieved on PM10 with 9% and 8% reductions for the scenarios without and with background concentration, respectively. In addition, the aerodynamics effect of vegetation was dominant for all pollutants in the scenarios without background concentration, while the deposition effect of vegetation was more important for PM10 and VOC in the scenarios with background concentration due to effects of deposition velocities. By increasing leaf area density, small variations of aerodynamic effect were observed, but deposition effects for PM10 and VOC were significantly intensified. Wind directions of 135 degrees and 315 degrees, which air easily flows within the domain, maximized the performance of park vegetation. PM10 deposition on tree leaves during dust storm also reduced its concentration by a maximum of 7%. Furthermore, it was found that biogenic VOC emission from trees in the park was not comparable with road emissions at city scale since it increased VOC concentration up to a maximum of 3%.