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首页> 外文期刊>Atmospheric environment >Modelling photochemical pollutants in a deep urban street canyon: Application of a coupled two-box model approximation
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Modelling photochemical pollutants in a deep urban street canyon: Application of a coupled two-box model approximation

机译：在深城市街道峡谷中对光化学污染物进行建模：两箱耦合模型近似的应用

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Air pollution associated with road transport is a major environmental issue in urban areas. Buildings in urban areas are the artificial obstacles to atmospheric flow and cause reduced ventilation for street canyons. For a deep street canyon, there is evidence of the formation of multiple segregated vortices, which generate flow regimes such that pollutants exhibit a significant contrast between these vortices. This results in poor air ventilation conditions at pedestrian level, thereby leading to elevated pollutant levels and potential breaches of air quality limits. The hypothesis of a well-mixed deep street canyon in the practical one-box model approach is shown to be inappropriate. This study implements a simplified simulation of the canyon volume: a coupled two-box model with a reduced chemical scheme to represent the key photochemical processes with timescales similar to and smaller than the turbulent mixing timescale. The two-box model captures the significant pollutant contrast between the lower and upper parts of a deep street canyon, particularly for NO2. Core important parameters (i.e. heterogeneity coefficient, exchange velocity and box height ratio) in the two-box model approach were investigated through sensitivity tests. The two-box model results identify the emission regimes and the meteorological conditions under which NO2 in the lower canyon (i.e. the region of interest for the assessment of human health effects) is in breach of air quality standards. Higher NO2 levels were observed for the cases with higher heterogeneity coefficients (the two boxes are more segregated), with lower exchange velocities (worse ventilation conditions), or with smaller box height ratios (reduced dilution possibly due to secondary smaller eddies in the lower canyon). The performance of a one-box model using the same chemical scheme is also evaluated against the two-box model. The one-box model was found to systematically underestimate NO2 levels compared with those in the lower box of the two-box model for all test scenarios. This underestimation generally tends to worsen for higher heterogeneity coefficients, lower exchange velocities or smaller box height ratios. This study highlights the limitation of the assumption of homogeneity in single box models for street canyon simulation, and the inherent uncertainties that must be borne in mind to appropriately interpret such model output (in particular, that a single-box treatment will systematically underestimate NO2 as experienced at street level). (C) 2016 Elsevier Ltd. All rights reserved.
机译：与道路运输相关的空气污染是城市地区的主要环境问题。城市地区的建筑物是阻碍大气流动的人为障碍，会导致街道峡谷的通风减少。对于较深的街道峡谷，有证据表明形成了多个分离的涡流，这些涡流产生了流动状态，因此污染物在这些涡流之间表现出明显的反差。这导致行人水平的空气流通条件差，从而导致污染物水平升高，并可能违反空气质量限制。在实际的一箱模型方法中，充分混合深街峡谷的假设被证明是不合适的。这项研究对峡谷体积进行了简化的模拟：一个耦合的两盒模型，该模型具有简化的化学方案，以类似于和小于湍流混合时间尺度的时间尺度表示关键的光化学过程。两箱模型捕获了深街道峡谷的上部和下部之间明显的污染物对比，特别是对于NO2。通过敏感性测试研究了两盒模型方法中的核心重要参数（即异质性系数，交换速度和盒高比）。两箱模型的结果确定了下峡谷（即评估人类健康影响的关注区域）中的NO2违反空气质量标准的排放制度和气象条件。对于异质性系数较高（两个盒子更隔离），交换速度较低（通风条件更差）或盒子高度比较小（降低的稀释度，可能是由于较低的峡谷中次生的较小的涡流）而言，观察到的NO2含量较高）。还针对两箱模型评估了使用相同化学方案的一箱模型的性能。在所有测试场景中，发现一箱模型与两箱模型下框相比，系统地低估了NO2含量。对于较高的非均质系数，较低的交换速度或较小的盒高比，这种低估通常会恶化。这项研究突显了单一峡谷模型用于街道峡谷模拟的均质性假设的局限性，以及为适当地解释此类模型输出而必须牢记的固有不确定性（特别是，单一处理会系统性地低估NO2为街头经验丰富）。 （C）2016 Elsevier Ltd.保留所有权利。

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