Seasonal Changing Effect on Airflow and Pollutant Dispersion Characteristics in Urban Street Canyons

Jingliang Dong,Jiyuan Tu,Yimin Xiao,Zijing Tan

doi:10.3390/atmos8030043

Jingliang Dong, Jiyuan Tu + Show 2 more

Open Access

https://doi.org/10.3390/atmos8030043

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Abstract

In this study, the effect of seasonal variation on air flow and pollutant dispersion characteristics was numerically investigated. A three-dimensional urban canopy model with unit aspect ratio (H/D = 1) was used to calculate surface temperature distribution in the street canyon. Four representative time events (1000 LST, 1300 LST, 1600 LST and 2000 LST) during typical clear summer and winter days were selected to examine the air flow diurnal variation. The results revealed the seasonal variation significantly altered the street canyon microclimate. Compared with the street canyon surface temperature distribution in summer, the winter case showed a more evenly distributed surface temperature. In addition, the summer case showed greater daily temperature fluctuation than that of the winter case. Consequently, distinct pollutant dispersion patterns were observed between summer and winter scenarios, especially for the afternoon (1600 LST) and night (2000 LST) events. Among all studied time events, the pollutant removal performance of the morning (1000 LST) and the night (2000 LST) events were more sensitive to the seasonal variation. Lastly, limited natural ventilation performance was found during the summer morning and the winter night, which induced relatively high pollutant concentration along the pedestrian height level.

Highlights

According to the latest statistics from the World Health Organization (WHO), the world’s urban population stands at 3.7 billion, implying more than half of the global population resides in cities.This rapid urbanization poses challenges for sustainable development and public health
No apparent pollutant concentration differences were observed in near-wall-zones for the summer condition
The numerical model was first validated against experimental data [41,51], the Through comparing the localized pollutant concentration (Figure 12b,c) with the combined results (Figure 12a), it is found majority of the residual pollutant in winter night (65%) were accumulated in the near-ground-zone due to weak ground ventilation

Summary

Introduction

According to the latest statistics from the World Health Organization (WHO), the world’s urban population stands at 3.7 billion, implying more than half of the global population resides in cities. Nazarian and Kleissl [30] performed unsteady simulations of a street-scale urban environment based on idealized geometries, in which, non-uniform surface heating caused by solar insolation and building shadowing were dynamically coupled with the airflow field. In their following study [31], they found the highest convective heat transfer coefficient occurred at the windward wall throughout the day. Despite the numerous studies previously conducted, researches focusing on the seasonal difference considering the solar radiation and the anthropogenic heating from the building interiors remain limited, and their effects on air flow and pollutant transport are not clear [34]. To reveal the seasonal differences of flow patterns and pollutant dispersion in street canyons between summer and winter, numerical simulations considering exterior solar radiation and interior anthropogenic heating were performed in this paper

Numerical Model

Model Validation

Air Flow Pattern

Pollutant Dispersion Characteristics

Conclusions