Source apportionment and regional transport of PM2.5 during haze episodes in Beijing combined with multiple models

Atmospheric fine-particle (PM2.5) pollution is a prominent regional issue that is affected by local emissions, regional transport, and secondary formation and impacts human health, air quality, and climate. Identifying the origin and types of pollution sources that are transported to the receptor site is essential for the development of joint prevention and control measures. This study developed a combined method to explore the source-receptor relationship during haze episodes of the heating season in Beijing, by combining positive matrix factorization (PMF) model with Lagrangian particle model (FLEXPART-WRF) and chemical transport model (CAMx). Six source types identified by PMF with high-time-resolution online measurements were counted in different transport pathways including local, south, east, and north. The results showed that secondary source contributed the most to PM2.5, which was affected by regional transport of Baoding, Langfang, and Tangshan. Among major primary sources, vehicle source was concentrated locally. Coal combustion contributed a higher proportion locally (15.8%) and in the south (15.0%), and Baoding, Langfang, and Xingtai along the Taihang Mountains were predominant potential source regions. Biomass burning contributed a higher proportion in the south (19.0%) and northeast (22.6%), and adjacent southern Hebei and Northeastern areas were the predominant potential source regions. Moreover, the variations in PM2.5 sources, footprints, and source regions of two haze cases from pre-pollution to dissipation were analyzed. EP1, with a more southern transport, was caused by high secondary source and coal combustion, while EP2 was characterized by secondary source and biomass burning from Hebei and Liaoning Province transport. In particular, the emission distribution of surrounding cities along transport pathways had an important effect on PM2.5 sources in Beijing. This study demonstrated that the combined method can provide guidance for implementing effective emission mitigation measures and improving the regional air quality.