Pedestrian-level air quality in urban areas is largely affected by the pollutants emitted from street canyons to the atmospheric boundary layer (ABL). Most vehicular exhausts are chemically reactive that evolve to their secondary counterparts. Besides, building roughness modifies the flows, which, in turn, influences the chemical reactions in the ABL. The ABL pollutant transport is affected by advection, diffusion, and chemical reactions. The roles of individual terms and their collective effect on the overall ABL pollutant transport are not yet clear. In this study, turbulent dispersion of reactive pollutants in the ABL over hypothetical urban area in the form of an array of idealised street canyons is investigated using large-eddy simulation. Nitric oxide (NO) is emitted from the ground level of the first street canyon into the urban ABL doped with ozone (O3). Budget analysis of the transport processes is conducted. It is found that the contributions from advection, diffusion, and chemistry vary in the streamwise direction and they couple closely with each other. For inert pollutants, streamwise advection and vertical diffusion mainly counterbalance each other. For chemically reactive pollutants, on the other hand, chemistry plays a key role in the far field where the mixing of pollutant species is rather uniform. In view of the elevated shear stress at the roof level, advection, diffusion, and chemistry show abrupt changes, complicating the pollutant dispersion processes.

中文翻译：

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理想城市地区反应性羽流运输的预算分析

从街道峡谷到大气边界层（ABL）排放的污染物在很大程度上影响着城市地区的行人空气质量。大多数车辆排气具有化学反应性，会演变成其次要的对应物。此外，建筑物的粗糙度会改变流量，进而影响ABL中的化学反应。ABL污染物的运输受到对流，扩散和化学反应的影响。单个术语的作用及其对整个ABL污染物运输的集体影响尚不清楚。在这项研究中，使用大涡模拟研究了以理想街道峡谷阵列形式在假设城市区域内ABL中反应性污染物的湍流扩散。一氧化氮（NO）从第一条街道峡谷的地面排放到掺有臭氧（O3）的城市ABL中。进行运输过程的预算分析。发现平流，扩散和化学作用的贡献沿流向变化，并且彼此紧密耦合。对于惰性污染物，流向平流和垂直扩散主要相互抵消。另一方面，对于具有化学反应性的污染物，在污染物种类混合非常均匀的远场中，化学起着关键作用。鉴于屋顶水平的剪切应力升高，对流，扩散和化学性质会发生突然变化，使污染物扩散过程变得复杂。发现平流，扩散和化学作用的贡献沿流向变化，并且彼此紧密耦合。对于惰性污染物，流向平流和垂直扩散主要相互抵消。另一方面，对于具有化学反应性的污染物，在污染物种类混合非常均匀的远场中，化学起着关键作用。鉴于屋顶水平的剪切应力升高，对流，扩散和化学性质会发生突然变化，使污染物扩散过程变得复杂。发现平流，扩散和化学作用的贡献沿流向变化，并且彼此紧密耦合。对于惰性污染物，流向平流和垂直扩散主要相互抵消。另一方面，对于具有化学反应性的污染物，在污染物种类混合非常均匀的远场中，化学起着关键作用。鉴于屋顶水平的剪切应力升高，对流，扩散和化学性质会发生突然变化，使污染物扩散过程变得复杂。在遥远的领域中，化学物质起着关键作用，污染物种类的混合相当均匀。鉴于屋顶水平的剪切应力升高，对流，扩散和化学性质会发生突然变化，使污染物扩散过程变得复杂。在遥远的领域中，化学物质起着关键作用，污染物种类的混合相当均匀。鉴于屋顶水平的剪切应力升高，对流，扩散和化学性质会发生突然变化，使污染物扩散过程变得复杂。