Frequent haze events associated with transport and stagnation over the corridor between North China Plain and Yangtze River Delta

Abstract. PM_2.5 pollution is a major air quality issue deteriorating human health, and numerous studies focus on PM_2.5 pollution in major regions such as North China Plain (NCP) and Yangtze River Delta (YRD). However, the characteristics of PM_2.5 concentrations and the associated formation mechanism in the transport corridor (referred to as SWLY) between NCP and YRD are largely ignored. Based on observational data, we find the number of PM_2.5 pollution events in SWLY is comparable to that in NCP, far exceeding those in YRD, indicative of the severity of air pollution over this area. Utilizing a regional climate and air quality model, we isolate the effect of seesaw transport events, e.g., transport between NCP and YRD, as well as the atmospheric stagnation on the accumulation of PM_2.5 over SWLY. Specifically, seesaw events and stagnation, comparable to each other, collectively account for an average of 67 % pollution days with PM_2.5 exceeding 75 µg/m³, and this fraction (85 %) is even larger for severe haze events with PM_2.5 exceeding 150 µg/m³. Furthermore, the connection between seesaw transport and large-scale circulation is examined. The trans-regional transport of pollutants from NCP to YRD (YRD to NCP) is likely stimulated by positive (negative) to negative (positive) geopotential height anomaly at 500 hPa located in northern China. The health effect due to short-term PM_2.5 exposure induced by the trans-regional transport and stagnation is investigated, yielding a total of 8,634 (95 % CI: 6,023–11,223) and 9,496 (95 % CI: 6,552–12,413) premature deaths respectively in SWLY during winter 2014–2019, as high as 9 % of the total premature deaths in China although the area coverage of SWLY is within 1 %. While atmospheric stagnation is in general projected to occur more frequently under a warming climate, this study indicates the importance of regional emission control to alleviate PM_2.5 pollution from seesaw transport and stagnation.