Contrasting impacts of two types of El Nino events on winter haze days in China's Jing-Jin-Ji region

El Nino events differ widely in their patterns and intensities. The regional climate anomalies caused by different types of El Nino events likely lead to various impacts on winter haze pollution in China. Based on long-term site observations of haze days in China from 1961 to 2013, this study explores the effects of eastern Pacific (EP) and central Pacific (CP) types of El Nino events on the number of winter haze days (WHDs) in China's Jing-Jin-Ji (JJJ) region and the physical mechanisms underlying WHD changes. The results show statistically significant positive and negative correlations, respectively, between WHDs in the JJJ region and EP and CP El Nino events. At most sites in the JJJ region, the average WHD increased in all EP El Nino years, with the maximum change exceeding 2.0 d. Meanwhile, the average WHD decreased at almost all stations over this region in all CP El Nino years, with the largest change being more than 2.0 d. The changes in large-scale circulations indicate obvious positive surface air temperature (SAT) anomalies and negative sea level pressure (SLP) anomalies over North China, as well as southerly wind anomalies at the middle to low troposphere over eastern China in the winters of EP El Nino years. These anomalies are conducive to increases in WHDs in the JJJ region. However, there are significant northerly and northwesterly wind anomalies at the middle to low troposphere over eastern China, as well as stronger and wider precipitation anomalies in the winters of CP El Nino years, which contribute to decreased WHDs over the JJJ region. Changes in local synoptic conditions indicate negative SLP anomalies, positive SAT anomalies, and weak-ened northerly winds over the JJJ region in the winters of EP El Nino years. The total occurrence frequency of circulation types conducive to the accumulation (diffusion) of aerosol pollutants is increased (decreased) by 0.4% (0.2 %) in those winters. However, the corresponding frequency is decreased (increased) by 0.5% (0.6 %) in the winters of CP El Nino years. Our study highlights the importance of distinguishing the impacts of these two types of El Nino events on winter haze pollution in China's JJJ region.