Phytoremediation Potential of Urban Trees in Mitigating Air Pollution in Tehran

The rapid urbanization and growing number of factories, human population, and motor vehicles have led to a drastic increase in the concentration of air pollutants. This smog is one of the most important disturbances in city planning. Urban trees play a vital role in the improvement of air quality. The selection of high-potential trees to capture air pollutants provides an attractive route for the mitigation of urban smog. The current study explored the air purification potential of the four most abundant trees, i.e., white mulberry (Morus alba L.), plane tree (Platanus orientalis L.), European ash (Fraxinus excelsior L.), and Tehran pine (Pinus eldarica Ten.)], as phytoremediators grown in three parks located in regions with low, moderate, and high levels of air pollution in Tehran on the mitigation of four urban hazardous gases (O3, NO2, CO, and SO2) and in altering the content of respiratory gases (CO2 and O2). The measurement of gas levels was carried out in September–October, from 1.30 to 1.50 m above the ground. The concentration of gases was measured by an ambient gas assessment device (Aeroqual). Broad-leaf deciduous species had a greater ability to mitigate O3, NO2, CO, CO2, and SO2 concentrations than needle-leaf evergreen species. The lowest levels of O3 and CO were found around P. orientalis (0.035 and 0.044 ppm, respectively), whereas the content of O2 was the highest in the atmosphere of this tree (20.80 ppm). The lowest content of NO2 (0.081 ppm) and SO2 (0.076 ppm) was determined in the vicinity of M. alba and F. excelsior, respectively. Among the studied species, P. orientalis proved to be the best for air phytoremediation, effectively mitigating hazardous gases more than the other species. Conversely, P. eldarica is not recommended for air phytoremediation in urban green spaces. Future research should focus on exploring a wider range of tree species and their potential for air pollution mitigation in diverse urban settings across different seasons and climatic conditions.