We investigate the understudied role of diurnally variable urban surface heating in transport phenomena within an idealized urban environment. We also explore whether heating from different roof materials (asphalt, reflective, and green roofs) at different times of the day affects pollution dispersion and ventilation mechanisms. Results show that the ventilation capacity of urban canyons varies diurnally and is influenced by the buoyancy forces from differentially heated urban surfaces, indicating the highest values in the afternoon and the lowest in the evening. The mechanism of canyon ventilation also varies diurnally. In the morning, the pollutant outflux at the roof level is the principal route of ventilation, while in the evening, lateral outfluxes are dominant and pollutant escape at the roof level is damped because of a local stable air layer. In the afternoon, both vertical and horizontal pollutant outflows contribute similarly to the canyon ventilation. In general, lateral turbulent (rather than mean) fluxes from the side canyon boundaries are the main contributors to the canyon pollutant ventilation throughout the day for all roof-type cases, but their significance slightly decreases from morning to evening. Results reveal that different roof types influence the canyon ventilation mechanism and capacity based on their diurnally-varying surface temperatures and their temperature gradients with respect to other urban surfaces. The existence of the green roof type mainly leads to the generation of a local stable layer and suppression of pollutant escape at the roof level.

我们调查了理想化城市环境中城市地表日变化在交通现象中的未充分研究作用。我们还探讨了一天中不同时间使用不同屋顶材料（沥青、反光和绿色屋顶）加热是否会影响污染扩散和通风机制。结果表明，城市峡谷的通风量随昼夜变化，并受不同加热城市表面浮力的影响，下午最高，晚上最低。峡谷通风的机制也随昼夜而变化。早上，屋顶的污染物流出是主要的通风途径，而在晚上，由于局部稳定的空气层，横向流出占主导地位，屋顶水平的污染物逸出受到抑制。下午，垂直和水平污染物流出对峡谷通风的贡献相似。一般来说，对于所有屋顶类型的情况，来自侧峡谷边界的横向湍流（而不是平均）通量是全天峡谷污染物通风的主要贡献者，但它们的重要性从早到晚略有下降。结果表明，不同屋顶类型会根据其每天变化的表面温度及其相对于其他城市表面的温度梯度影响峡谷通风机制和容量。绿色屋顶类型的存在主要导致了局部稳定层的产生和屋顶水平污染物逃逸的抑制。