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. El Niño events differ widely in their patterns and intensities. The regional climate anomalies caused by different types of El Niño eventslikely 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, thisstudy explores the effects of eastern Pacific (EP) and central Pacific (CP) types of El Niño events on the number of winter haze days (WHDs) inChina's Jing-Jin-Ji (JJJ) region and the physical mechanisms underlying WHD changes. The results show statistically significant positive andnegative correlations, respectively, between WHDs in the JJJ region and EP and CP El Niño events. At most sites in the JJJ region, the averageWHD increased in all EP El Niño years, with the maximum change exceeding 2.0 d. Meanwhile, the average WHD decreased at almost allstations over this region in all CP El Niño years, with the largest change being more than −2.0 d. The changes in large-scalecirculations indicate obvious positive surface air temperature (SAT) anomalies and negative sea level pressure (SLP) anomalies over North China, as well assoutherly wind anomalies at the middle to low troposphere over eastern China in the winters of EP El Niño years. These anomalies are conducive toincreases in WHDs in the JJJ region. However, there are significant northerly and northwesterly wind anomalies at the middle to low troposphere overeastern China, as well as stronger and wider precipitation anomalies in the winters of CP El Niño years, which contribute to decreased WHDs over the JJJregion. Changes in local synoptic conditions indicate negative SLP anomalies, positive SAT anomalies, and weakened northerly winds over the JJJregion in the winters of EP El Niño years. The total occurrence frequency of circulation types conducive to the accumulation (diffusion) ofaerosol 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 Niño years. Our study highlights the importance of distinguishing the impacts of these twotypes of El Niño events on winter haze pollution in China's JJJ region.

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