Impact of urban trees on ozone concentration in a densely built-up area of Seoul using a CFD model coupled with a chemical mechanism

Urban trees are primarily planted to enhance city aesthetics and provide relaxation spaces for citizens. However, their aerodynamic effects reduce wind speed, and Biogenic Volatile Organic Compounds (BVOCs) emitted from trees contribute to the formation of secondary pollutants, such as ozone (O₃). These factors significantly influence urban air quality and underscore the need for comprehensive research to better understand their impacts. This study aims to analyze the contribution of urban trees to the O₃ concentrations caused by two different pathways which are aerodynamic effects and BVOCs emissions. For this purpose, a CFD model coupled with a chemical mechanism (SAPRC) was used. The model domain covers a densely built-up area of Seoul, Republic of Korea. Air pollutant emission rates were estimated using the Source Object-based Model for Emission (SOME), while BVOCs emission rates from urban trees were calculated using the Model of Emissions of Gases and Aerosols from Nature (MEGAN). Several scenarios were designed to investigate the aerodynamic effects and BVOC emission. The simulation results showed that wind speed changes due to trees' aerodynamic effects significantly influenced the dispersion of air pollutants, altering their spatial distribution. Furthermore, BVOCs emitted from urban trees contributed to the increase in O₃ concentration. The findings underline the needs to careful consideration of urban trees’ effects when planning urban tree planting strategies to mitigate air pollution effectively. This work was supported by a grant from the National Institute of Environmental Research (NIER), funded by the Ministry of Environment (MOE) of the Republic of Korea (NIER-SP2020-305) and in part by the National Research Foundation of Korea(NRF) grant funded by the Korea government(MSIT) (RS-2024-00356913).