Air pollutant dispersion altered by urban-rural breeze and urban sprawl

Air pollution becomes a serious issue due to the population growing up and residential area sprawl in decades. Residential area is not only a major source of air pollutants but also an impact to generate an urban-rural thermal wind and to alter the dispersion of air pollutants. However, the urban-rural breeze caused by a metropolitan is not the only impact on the dispersion of air pollutants. Generally, a synoptic weather condition is the major impact to dominate how the air pollutant exactly diffuses. The most metropolitans are located in the coastal regions. Therefore, a naturally thermal wind, sea-land-breeze, plays also commonly an essential role to transfer the air pollutant. Additionally, topography and natural obstacle are unable to be ignored as an impact to obstruct flow streaming which brings the air pollutant away.

Overall, synoptic weather conditions, local sea-land-breeze patterns, and natural obstacles are three major natural impactors to influence air pollutant dispersion. The urban-rural-breeze pattern and the roughness of the urban area are regarded as the two anthropogenic factors to alter the large breeze system and thereby affecting the spreading pathway of the air pollutant. To analysis the interaction of above mentioned five impactors could be regarded as a comprehensive approach to consider how the air pollutant transfer from a metropolitan to air pollution suffering areas.

In this study, we apply either computational or measurement tools to consider the effects of metropolitan, Taichung, which is located in middle Taiwan, in the heat island effect and modification of the roughness to alter the natural breeze and also the dispersion of the air pollutant. Several intensive observation periods of 3-dimensional wind field network in boundary layer have been proposed as the evidences to discuss the impacts of urban sprawl on the breeze circulation in Taichung. Otherwise, a large-eddy-simulation model, Parallel-Large-Eddy-Simulation Model (PALM) is applied in the study initially to realize the influence of synoptic weather conditions and topography on air pollutant dispersion. Thereafter, the impacts of the heat fluxes and the roughness changing due to the urban sprawl are proposed in the study. Overall, the altering of metropolitan on the natural breeze is a slight but significant impact and could change the air pollutant dispersion.

 

Key Words: Boundary Layer, Wind field, Large-Eddy-Simulation, PALM, Urban Sprawl, Heat Island Effect, Thermal Wind