Transport of air pollutants emitted from urban valleys can be strongly restricted by interactions between static and dynamic factors including topographic forcing, low-level atmospheric stability related to temperature inversions, and urban heat island-induced circulations. Interplay between these processes has a complex and dynamic nature, and is determinant for the evolution of different ventilation mechanisms and the associated impacts on air quality. Here we investigate these transport mechanisms through large eddy simulations using EULAG, an established model for multiscale flows, to simulate an idealized atmospheric environment in narrow versus wide urban valleys during critical conditions for air quality (high atmospheric stability). Our results show how the ventilation of valleys depends on a dynamic (variable during the daytime) balance between interacting and sometimes competing processes related to thermally-driven slope flows, urban heat island-induced flows, and the trapping effect of atmospheric stability; and how valley width affects this balance. Particularly important is that the time-space distribution of pollutants (a passive tracer) varies greatly between both valleys despite having the same urban area and emission rates. These variations lead to pollutants being mostly concentrated in different areas of the narrow and wide valleys. We discuss the mechanisms behind these results and their potential implications for real urban valleys. Further understanding of these mechanisms is crucial for explaining the occurrence of severe air pollution episodes and informing related decision-making processes in urban valleys.

中文翻译：

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在狭窄和较宽的城市山谷中，稳定大气中的白天空气污染传输机制

静态和动态因素之间的相互作用（包括地形强迫，与温度反转相关的低层大气稳定性以及城市热岛引起的环流）之间的相互作用会严重限制从城市山谷排放的空气污染物的运输。这些过程之间的相互作用具有复杂而动态的性质，并决定了不同通风机制的发展以及对空气质量的相关影响。在这里，我们通过使用EULAG（一种建立的多尺度流量模型）进行的大型涡流模拟研究这些传输机制，以模拟在空气质量临界条件（高大气稳定性）下狭窄与宽阔的城市山谷中理想的大气环境。我们的结果表明，山谷的通风如何取决于热驱动的坡面流，城市热岛诱导的流以及大气稳定性的诱集作用之间相互作用的过程之间的动态平衡（白天变化），有时甚至相互竞争。以及谷宽如何影响这种平衡。尤其重要的是，尽管市区和排放量相同，但两个山谷之间污染物的时空分布（被动示踪剂）差异很大。这些变化导致污染物主要集中在狭窄和宽阔山谷的不同区域。我们讨论了这些结果背后的机制，以及它们对真实城市山谷的潜在影响。