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Characteristics of the upstream flow patterns during PM2.5 pollution events over a complex island topography
Atmospheric Environment ( IF 5 ) Pub Date : 2020-04-01 , DOI: 10.1016/j.atmosenv.2020.117418 Hsin-Chih Lai , Mei-Chi Lin
Atmospheric Environment ( IF 5 ) Pub Date : 2020-04-01 , DOI: 10.1016/j.atmosenv.2020.117418 Hsin-Chih Lai , Mei-Chi Lin
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Abstract Meteorological and geographic conditions play important roles in the occurrence of air pollution. This is especially true in island countries like Taiwan, where the atmospheric flow plays a key role in determining air quality. The atmospheric flows of 334 air pollution events between 2013 and 2017 were analyzed via radiosonde data and WRF simulations to elucidate how the spatial distribution of PM2.5 concentrations in Taiwan is correlated to its terrain and environmental wind fields. The variables included in this analysis were the wind directions of the environmental wind field, Froude number (Fr), pitching angle (PA) of the surface-level and atmospheric boundary layer (ABL)-level airflows through the terrain, differences in wind speed between surface-level and ABL-level airflows, and the formation of lee-side vortices. Thus, we examined how the areas of air pollutant accumulation are related to the passage of environmental airflows through mountain ranges. Based on a composite analysis, we determined that the air pollution events can be divided into 10 categories, depending on the direction of the environmental airflows and the geographic location of the air pollution events. Fr was less than 0.25 in 87% air pollutant events, which indicates that the environmental airflows had a high stability and tended to split around the terrain. Each category corresponded to different environmental wind directions and PAs. It was found that air pollution events occurred the most frequently when PA was close to the north, and these events were accompanied by obvious differences in wind speed between surface-level and ABL-level. During the analysis of airflow characteristics via the WRF simulation, it was found that lee-side vortices were formed on the orthographic lee-side of terrain in seven synoptic flow categories. The results of this study provide evidence and variables to demonstrate how atmospheric flow, terrain, and pollutant accumulation areas are linked to each other in island.
更新日期：2020-04-01
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