Effects of inhomogeneous ground-level pollutant sources under different wind directions

Exposure risk and emergency evacuation modeling of toxic gas leakage in urban areas under the influence of multiple meteorological factors

Frequent toxic gas leakage accidents in urban environments cannot be timely controlled and often cause great harm due to the many factors affecting gas diffusion. In this study, based on the coupling method of the Weather Research and Forecasting Model (WRF) and the open source OpenFOAM software platform, the chlorine gas diffusion process at different temperatures, wind speeds, and wind directions in a chemical laboratory and nearby urban areas in Beijing was studied numerically. A dose-response model was used to calculate chlorine lethality and assess exposure risk at the pedestrian level. To predict the evacuation path, an improved ant colony algorithm, a greedy heuristic search algorithm based on the dose-response model, was applied. The results demonstrated that the combination of WRF and OpenFOAM could consider the effects of various factors such as temperature, wind speed, and wind direction on the diffusion of toxic gases. The direction of chlorine gas diffusion was affected by wind direction, and the range of chlorine gas diffusion was affected by temperature and wind speed. The area of high exposure risk (fatality rate above 40%) at high temperatures was 21.05% larger than that at low temperatures. When the wind direction was opposite the building, the high exposure risk area was 78.95% smaller than that under the building direction. The present work provides a promising approach for exposure risk assessment and evacuation planning for the emergency response to urban toxic gas leakage.

Evaluation of pollutant exposure using virtual walkers and large-eddy simulation: Application to an idealised urban neighbourhood

Urban air quality studies have primarily focused on pollutant dispersion; however, spatial or temporal concentrations collected at discretely distributed grid points (or fixed receptors) do not reflect the actual pollutant exposure of pedestrians. Using large-eddy simulation (LES) with virtual walkers implemented, this study investigates pollutant exposure of walking agents (or moving receptors) in an urban turbulent boundary-layer flow developed over an aligned building array under the influence of different wind directions. The spatial variability of the exposure risks are found to be better captured by the moving receptors than the fixed receptors along the same agent walking tracks. We demonstrate that the actual exposure can differ significantly from results interpreted from data recorded by the fixed receptors (corresponding to Eulerian estimates) and show that large discrepancies occur in avenues near the source, wherein dispersion of the point release has not occurred on larger spatiotemporal scales. In most scenarios, optimal evacuation routes are shown to be ones that deviate as much as possible from the dominant wind direction; however, one needs to decide the priority of moving to further avenues first or immediately adjusting the walking direction. The results should serve as a useful baseline reference for environmental health impact assessment and evacuation route planning against hazardous releases of air pollutants in more complex urban environments.

Influence of wind direction on the ventilation and pollutant dispersion in different 3D street canyon configurations: numerical simulation and wind-tunnel experiment

Configuration of street canyon and the wind environment have a great influence on the self-ventilation capacity of the canyon, but the couple-effect of these two factors could not be considered in the previous study. The purpose of this study is to clarify the couple effect of street canyon configuration and wind environment on the ventilation and pollutant dispersion inside the street canyon. For this purpose, five wind directions of α = 90°, 60°, 45°, 30°, and 0° (α is the angle between the approaching wind and street canyon) and three canyon configurations (flat, step-up, and step-down canyons) were considered with numerical simulation and wind-tunnel experiment. Meanwhile, ACH (air exchange rate) and NEV (net escape velocity) were used to evaluate the ventilation capacity of the canyon. The results reveal that the wind direction has a vital influence on the ventilation in the different canyon configurations. Under the parallel wind direction (α = 0°), the airflow and ventilation capacity inside the three canyons are similar. Relative difference of ACH, named as RDA ((ACH_asymmetric-ACH_symmetric)/ACH_symmetric [Formula: see text] 100%), is 1.82%. However, under the oblique (α = 30°, 45°, and 60°) and perpendicular wind direction (α = 90°), the airflow of the step-down canyon is very different from the step-up and flat canyons. In step-down canyons, reverse flow occurs under the oblique and perpendicular wind direction, and the strength of the reverse flow increases as α increases. Due to this reverse flow, the ventilation capacity of the step-down canyon is lower than that of the step-up and flat canyons. As for the ventilation capacity in the pedestrian respiration domain, the ventilation capacity of the leeward pedestrian domain (leeward NEV) is higher in the step-down canyon than in the step-up canyon and the flat canyon (when α = 90°, leeward NEV of step-down canyon is 2.47 times the flat canyon). Conversely, the ventilation capacity of the windward pedestrian domain is lower in step-down canyons than in step-up or flat canyon (when α = 90°, windward NEV of step-down canyon is 0.1 times that of step-up canyon). The aforementioned findings are helpful to understand the effects of canyon configurations together with wind directions on the airflow as well as pollutant concentration inside the canyon. Although further researches are still required to provide practical guidelines, this study present effective methodologies to quantify the influences of street configurations and wind directions on street canyon ventilation for urban design purpose.