Relationship between fine particulate matter concentration and winter monsoon in the tropics under the influence of climate change and topographic effects: A case study of Kaoping area, Taiwan, since 1959

The negative trend of PM_2.5 in southwestern Taiwan could be offset in the future because of the stabilisation of the atmospheric layer with the decreasing strength of the East Asian winter monsoons. However, phenomena such as changes in the moisture source and the washout effect can help reduce PM_2.5 levels. To assess these phenomena, hourly PM_2.5 levels, weather parameters, and surface weather maps were obtained from the relevant government authorities in Taiwan. Trend, multiregression, and basic statistical methods were used in this study. The study period was divided into two separate parts. P1 (1959–1976) represents the averaged period, and P2 represents the warming period (1977–2021). The rainy-day weather patterns suggested that the percentage of moisture in the weak continental high area south of the Yangtze River basin in China and the southern region outside Taiwan Island was more substantial during P2 (28.3%) than during P1 (7.3%) because the frequency and intensity of rainfall increased in P2. The rainfall pattern in the Kaoping area from 2005 to 2021 changed from low rainfall and decreasing rainfall frequency to high rainfall and increasing rainfall frequency. Multiregression models showed that the negative contribution of the washout effect to the variation in the annual mean PM_2.5 was more significant than the positive contribution of air dispersion. This suggests that in tropical regions with poor air dispersion and winter monsoons, the frequency and type of rainfall affect the variations in PM_2.5 under climate change.