Regional transport characteristics of PM_2.5 pollution events in Beijing during 2018-2021

Although air pollutant emissions have sharply reduced in recent years, the occurrence of PM_2.5 pollution events remains an intractable environmental problem in Beijing, and regional transport is the key influence factor. However, it has been difficult to identify regional transport characteristics and the main contributors to pollution events in recent years. In this study, the relative contribution of regional transport was quantified (61.3%) in PM_2.5 pollution events during 2018-2021 by the Community Multiscale Air Quality model embedded with the Integrated Source Apportionment Model (CMAQ-ISAM). The four regions with the largest fractional contributions to Beijing for all events were Shandong (7.7%), South Hebei (7.3%), Baoding (6.2%), and Langfang (5.8%). Pollution events were classified into the following types based on regional transport directions: local, southwest (SW), southeast (SE), south-mixed (SM), and others. Based on the transport distance, the SW, SE, and SM types can be subdivided into SW-short, SW-long, SE-short, SE-long, SM-short, SM-long distance from southwest, SM-long distance from southeast, and SM-long distance from southwest and southeast. SE-long was regarded as the most important type, with the highest relative frequency (20%). The transport directions were related to the southwest wind at 925 hPa and southeast wind at 1000 hPa in the south of the Beijing-Tianjin-Hebei (BTH) region, and the distance was mainly controlled by wind strength. The wind-field difference can be attributed to the low-pressure and high-pressure systems that control the BTH region. The results suggest that regional joint pollution control should be optimized based on the transport type.