The Seesaw Pattern of PM2.5 Interannual Anomalies Between Beijing-Tianjin-Hebei and Yangtze River Delta Across Eastern China in Winter

Since 2013, the winter mean fine particulate matter (PM2.5) had been decreased significantly due to stringent emission controls in most of China. Nevertheless, we found a seesaw pattern of PM2.5 interannual anomalies between Beijing-Tianjin-Hebei (BTH) and Yangtze River Delta (YRD). Using the multiple linear regression method, meteorology-driven PM2.5 interannual anomalies show that the low (high) PM2.5 relative difference between BTH and YRD (RDB&Y) was associated with the strong (weak) East Asian winter monsoon (EAWM). The strong EAWM transported more air pollutants from BTH to YRD. During the COVID-19 lockdown period, due to the weak EAWM, air pollution still occurred in northern BTH, while the PM2.5 was relatively low in YRD, causing high RDB&Y values. Our results imply that the activity of EAWM and characteristics of regional transport have obvious interannual variations, which is indispensable in evaluating the achievements of PM2.5 quality management between up and downstream regions.