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A numerical investigation of reactive air pollutant dispersion in urban street canyons with tree planting

机译：植树造林在城市街道峡谷中反应性空气污染物扩散的数值研究

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Vegetation acts as a momentum and thermal sink, affecting the mixing of species and temperature-dependent constants of reaction rates. Numerical simulations were performed to investigate the effects of vegetation on the dispersion of reactive pollutants using a computational fluid dynamic (CFD) model coupled with NO-NO2-O3 photochemistry. Moreover, characteristics of temperature and flow fields were analyzed for different aspect ratios and leaf area densities. The results showed that flow is reversed in the presence of trees, and it enhances as leaf area density (LAD) increases; additionally, vegetation creates downward and vortex flows. The results also revealed that the dispersion of nitrogen oxides is influenced by the flow patterns; nevertheless, chemical reactions are significant for the dispersion of ozone. In addition, the vegetation is observed to weaken ventilation efficiency of NO and NO2; however, ventilation efficiency of O3 improves in LAD??=??0.5 and 1.0. Aspect ratios and leaf area densities are also found to interact with each other; consequently, the optimum LAD is different for each aspect ratio. The larger regions with maximum concentrations of nitrogen oxides at the height of 2??m for aspect ratios of 0.5, 1.0, and 2.0 correspond to LAD??=??2.0, 1.5, and 1.0, respectively. Furthermore, vegetation as compared to tree-free environment, mostly leads to a better chemical equilibrium.
机译：植被充当动量和热沉，影响物种的混合和反应速率的温度相关常数。使用计算流体动力学（CFD）模型与NO-NO2-O3光化学结合，进行了数值模拟，以研究植被对活性污染物扩散的影响。此外，针对不同的纵横比和叶面积密度，分析了温度和流场的特征。结果表明，在有树木的情况下，水流反向，并且随着叶面积密度（LAD）的增加而增加;此外，植被会产生向下的涡流。结果还表明，氮氧化物的分散受到流动模式的影响;但是，化学反应对于臭氧的扩散很重要。此外，观察到植被削弱了NO和NO2的通风效率。但是，在LAD 50 ＝ 0.5和1.0时，O 3的通气效率提高。还发现纵横比和叶面积密度相互影响。因此，每个长宽比的最佳LAD都不相同。对于纵横比为0.5、1.0和2.0，最大的氮氧化物浓度在2θm处的较大区域分别对应于LADδ＝ α2.0、1.5和1.0。此外，与无树环境相比，植被通常会导致更好的化学平衡。

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