A comprehensive investigation of PM2.5 in the Huaihe River Basin, China: Separating the contributions from meteorology and emission reductions

Due to anthropogenic emission reductions, the mass concentration of fine particulate matter (PM2.5) in China has markedly decreased in recent years. In this study, we selected the Huaihe River Basin (HRB), which is located in the middle of the North-South climatic transition zone of China, to investigate the reasons for the decrease in the PM2.5 concentration. Based on the observed PM2.5 concentration and meteorological data for 2015-2020, the Kolmogorov-Zurbenko (KZ) filter method was employed to decompose the original time series of the PM2.5 concentration. The results demonstrate that the short-term (PM2.5ST), seasonal (PM2.5SN), and long-term (PM2.5LT) components of PM2.5 variations over the HRB accounted for 55.6%, 34.7%, and 4.4% of the total variance, respectively. PM2.5 variations in coastal cities and cities with relatively high latitudes and longitudes were more affected by the short-term component. It was identified that the PM2.5 concentration in the HRB declined at a rate of 2.58-8.12 mu g/m3/year. The meteorological conditions and emission reductions all positively influenced the PM2.5 decrease, which contributed 30.09% and 69.91%, respectively, to the PM2.5LT decrease in the HRB. It is noteworthy that with the PM2.5 decrease, the conversion efficiency of SO2 to sulfate and NO2 to nitrate might be enhanced. The unbalanced emission reductions in SO2 and NO2 are not conducive to the further decline in PM2.5. This study suggests that more efforts should be made to control NO2 emissions in the HRB.