Large–scale seasonal control of air quality in Taiwan

By investigating data during 2004–2015, this study explored the dynamic mechanisms of seasonal control of air quality over southern and northern Taiwan. Overall, higher (lower) pollution frequencies were observed in dry (wet) stages. Analysis of seasonality further suggested that pollution over southern Taiwan exhibited a symmetric characteristic, and that tropical modulation was a driving factor, whereas pollution over northern Taiwan displayed an asymmetric seasonality and likely followed the asymmetric East Asian seasonality. With the retreat of summer monsoon, air quality in southern Taiwan entered a poor phase rapidly; by contrast, good air quality was observed over northern Taiwan, which was due to the gradual southward migration of the western North pacific (WNP) high. Detailed dynamic processes contributing to atmospheric conditions and large-scale circulation were further compared between pollution and nonpollution cases. During dry stages, the midlatitude perturbations observed as wavy patterns had downstream effects on the atmospheric condition in Taiwan. The downstream effect of the midlatitude wave activity on air pollution was obviously found in the Dry Northerly stage in northern Taiwan and the Dry Southerly stage in southern Taiwan. In contrast to poor air quality during dry stages, air pollution was rarely observed during wet stages. The strength and location of the WNP high, which can be involved in the formation of nearby tropical cyclones, may mainly influence air quality in Taiwan. •Higher/lower pollution frequencies were observed in dry/wet stages of Taiwan.•The WNP high/midlatitude perturbation influenced air quality in wet/dry stages.•Pollution in southern/northern Taiwan showed a symmetric/asymmetric seasonality.