[Regional Air Pollution Process in Winter over the Yangtze River Delta and Its Influence on Typical Northern Cities]

In this study, we analyzed several pollution episodes that occurred in the autumn and winter of 2018-2019 using multiple methods including the hierarchical clustering analysis, backward trajectory, and potential source contribution analysis based on monitored air quality and meteorological data. Bengbu, being a representative city to the north of the Yangtze River Delta (YRD) region and located in a heavily polluted area during these two pollution processes, is the focus of this work. The results indicated that the northern part of the YRD region is affected because of unfavorable meteorological conditions such as weak ground pressure, high humidity, low temperature, low wind speeds, and regional transport. The regional pollution processes over the YRD region in the autumn and winter seasons exhibit characteristics of wide influence and long duration with mainly two types of pollution:regional transport and intra-regional accumulation. During the two selected pollution episodes, the average PM_2.5 concentration in the northern YRD region reached 131.6 μg·m^-3 and 115.4 μg·m^-3, respectively. The former type had a shorter duration but exhibited rapid accumulation of pollutants in a short period of time with greater pollution intensity, wider pollution range, and deeper pollution intrusion. Qualitative and quantitative analysis of the potential sources of PM_2.5 based on PSCF and CWT showed that the PM_2.5 concentration during EP1 was due to transport from cities such as Linyi, Xuzhou, Suqian, and Lianyungang to the pollution trajectory. The CWT value generally exceeded 80 with the highest value near 200. In contrast, EP2 was affected by the neighboring cities such as Suqian, Suzhou, and Xuzhou, and the CWT value was over 60 with the highest approaching 160, indicating that the interaction among cities in the study area is significant. This study shows that cross-regional air pollution control strategies are particularly important for alleviating the pollution situation in the northern part of the YRD region.