A Lagrangian view on severe haze in Beijing: local and long-range sources of trace gases and primary and secondary aerosols

Beijing is particularly prone to frequent wintertime haze episodes. In this study, we introduce the FLEXPART (FLEXible PARTicle dispersion model) and SOSAA (the model to Simulate the concentration of Organic vapors, Sulfuric Acid, and Aerosols) modeling system for air quality analysis and applied the newly developed modeling system to a severe pollution episode during a case study in Beijing in November 2018. FLEXPART-SOSAA uses a Lagrangian approach following backward trajectories, and its novelty includes detailed chemical and aerosol mechanisms along each trajectory, incorporating aerosol size distribution in addition to simple mass concentrations. This is particularly valuable from a health perspective. In this study, we demonstrated FLEXPART-SOSAA's capabilities along 7-day backward trajectories to investigate the origins and processes of haze during the case study. Our findings indicate that approximately 75 % of the particles in Beijing originated from outside the city. During the haze episode, the back-trajectories passed through the high-emission region of southern Hebei province. Some of the particle mass was traced back to the Xinjiang region in far western China. After evaluating the results, we tested reduced-emission scenarios and sensitivity tests. We determined that comprehensive emission control across multiple emission sectors was more effective than addressing only one individual sector, and emission control at a regional or national level would be significantly more effective than implementing restriction only in the city of Beijing. Additionally, we found that postponing agricultural waste burning to a time when haze is not in the forecast could help mitigate pollution in the city.