The differential impact of 2015–2020 El Niño and El Niño Modoki on warm- and cold-season PM2.5 concentration and distribution across China

Using hourly observations of fine particulate matter (PM2.5) from a nationwide air-quality monitoring network of nearly 1500 stations, augmented with gridded atmospheric reanalysis data, we examined the differential influences of the eastern Pacific El Niño and central Pacific El Niño also known as El Niño Modoki on surface PM2.5 concentrations in different regions of China during different seasons. We show that the influence can be opposite during warm season, with PM2.5 anomalies overwhelmingly positive (negative) across China under El Niño (El Niño Modoki), although in both cases the largest anomalies appear in North China and the adjacent areas and smallest in the Qinghai-Tibet Plateau. The cold-season PM2.5 concentration is less sensitive to the El Niño type, with a general pattern of positive anomalies in the North, Northeast, Inner Mongolia, northern parts of the Central and East and along the southern and southeastern coasts and negative anomalies in most other regions. Nevertheless, negative cold-season anomalies are stronger and more widespread during El Niño Modoki compared to El Niño, and they could be opposite in some areas of the Northwest and Qinghai-Tibet Plateau where positive (negative) anomalies may prevail during El Niño (Niño Modoki). Much of these differences can be attributed to the differences in the anomalous atmospheric conditions associated with El Niño and El Niño Modoki, particularly the low-level anomalous wind direction that controls regional PM2.5 transport and precipitation leading to PM2.5 removal. These findings could assist in developing a nuanced approach to seasonal PM2.5 forecasting for different regions in China.