With emission control efforts, the PM 2.5 concentrations and PM 2.5 exceedance days (daily mean PM 2.5 concentrations >35 µg m −3 ) show an apparent declining trend from 2006–2017. The PM 2.5 concentrations increase from the northern to southern part of western Taiwan, and reductions in the PM 2.5 concentration generally decrease from northern to southern part of western Taiwan. Thus, mitigation of the PM 2.5 problem is less effective in southwestern Taiwan than in other regions in Taiwan. Analysis of a 39-year ERA-interim reanalysis dataset (1979–2017) reveals a weakening of the East Asian winter monsoon, a reduction in northeasterly (NE) monsoonal flow, and a tendency of enhanced stably stratified atmospheric structures in Taiwan and the surrounding area. The observed surface wind speed also presents a long-term decline. We can conclude that the long-term PM 2.5 variations in Taiwan are mainly associated with changes in local anthropogenic emissions and modulated by short-term yearly variations due to strong haze events in China. In southwestern Taiwan, the long-term trend of PM 2.5 reductions is possibly offset by worsening weather conditions, as this region is situated on the leeside of the mountains and often subject to stagnant wind when under the influence of NE monsoonal flow.

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More than 70% of Taiwan, an island located off the southeastern coast of East Asia, is covered by mountainous topography. Thermally driven flows, including mountain-valley winds, are strongly influenced by the underlying surface conditions, such as the land use characteristics and terrain height distribution (Cheng et al., 2013). On cloud-free days, thermally driven flows can develop in mountain valleys and affect the boundary layer structure. These flows are normally formed via the superposition of mountain-valley winds (parallel to the valley axis) and upslope-downslope winds (perpendicular to the valley axis) (Zardi & Whiteman, 2013). The latter winds form due to the difference in heating processes over the surfaces of valleys and slopes and are typically recognized as thermally driven flows, whereas the formation mechanism of the former is more complicated and is affected by both thermal and mechanical forcings. In addition, thermally driven flows can induce subsidence over valleys, trapping air pollutants therein and increasing atmospheric stability, and can partially suppress the convective boundary layer in the early stage of their development (Serafin & Zardi, 2011). Furthermore, the subsidence induced by upslope winds can produce top-down warming and increase the valley temperature. During the nighttime, cold air can be advected by downslope flow, which increases the atmospheric stability over the basin floor. As a result, a surface inversion layer forms and decouples from the synoptic flow (Kondo et al., 1989).The synoptic weather conditions in Taiwan are dominated by the western North Pacific subtropical high-pressure system during summer and the Asian continental anticyclone during winter and spring. The weather conditions are further complicated by the characteristics of the coastline and Central Mountain Range (CMR), which traverses the island from north to south and whose peaks reach almost 4,000 m. Figure 1a displays an elevation

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