Abstract
Traffic-related air pollution in street canyons can cause health problems for pedestrians. In order to clarify the behavior of reactive pollutants, such as NOx and O3, in street canyons, a computational fluid dynamics (CFD) model coupled with a chemistry model and tree canopy model was developed, and then, a set of numerical experiments were performed to investigate the impacts of chemical reactions and aerodynamic effects of trees planted in a canyon. The results were compared with the observation data. Through the results of the numerical experiments designed to simulate a realistic urban street canyon, it was found that chemical reactions have a dominant impact on the NO/NO2 ratio and O3 concentration. While the tree canopy had little impact on the NO/NO2 ratio, it had a moderate impact on the flow field in the canyon and the amount of NOx and O3 in the canyon. In accordance with the aerodynamic effects of tree canopies, the local NOx concentration in the experiments increased and decreased by up to 51% and 11%, respectively. The current findings of this study demonstrate the utility of the proposed model for conducting air quality investigations in urban areas.
Highlights
Traffic-related air pollution is one of the key causes of respiratory and allergic diseases, including asthma, chronic obstructive pulmonary disease, and pneumonia [1]
In an urban street canyon, where tall buildings surround the street on both sides, several pollutants such as NO, NO2, CO, volatile organic compounds (VOCs), and fine particles such as PM2.5 and PM10 are emitted from vehicles and can circulate throughout the air
The results of wind tunnel experiments and computational fluid dynamics (CFD) simulations were compared, and the findings indicated that appropriately designed CFD simulations can reproduce the results of wind tunnel experiments [6,8,9,10]
Summary
Traffic-related air pollution is one of the key causes of respiratory and allergic diseases, including asthma, chronic obstructive pulmonary disease, and pneumonia [1]. There are two types of turbulence model have been used in CFD for evaluation of flow field in a street canyon, namely Reynolds-averaged Navier–Stokes (RANS) and large eddy simulation (LES). CFD simulations are known to be a powerful tool for understanding the dispersion of pollutants in street canyons, few studies have considered the aerodynamic effects and the chemical reactions simultaneously. In order to understand the reactive pollutants concentration dispersion in realistic urban street canyon, it is necessary to simulate chemical reactions and flow fields. The tree canopy model and chemical reaction model were employed simultaneously in the CFD model to evaluate the behavior of the pollutants in a street canyon.
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