Dynamics of Explosive PM2.5 Growth Events in Beijing

Globally, air pollution is the fourth highest ranking risk factor for death, and is correlated with acute and chronic respiratory illnesses and decreased cardiovascular health. A significant component of air pollution is particulate matter consisting of particles of 2.5μm diameter or less (PM_2.5). PM_2.5 is of increasing concern in China especially in the Beijing-Tianjin-Hebei (Jing-Jin-Ji; J3) region in Northern China. Annual mean PM_2.5 concentrations in the J3 region can be over eight times recommended WHO Air Quality Guidelines. However, while air pollution studies focusing on China are gaining more attention both within China and internationally, there are considerable gaps in our understanding of the dynamics surrounding severe pollution events in the J3 region. In this study, we examine particulate matter pollution Explosive Growth Events (EGEs) where PM_2.5 mass concentrations at least double over a 7-10h period. Based on ground-level observations, 12 such EGEs occurred in Beijing winters between 2013 and 2016. Previous studies have qualitatively demonstrated how high levels of PM_2.5 exacerbate existing pollution events by suppressing planetary boundary layer (PBL) growth via decreasing incoming shortwave radiation. However, few studies have quantified the extent of this feedback in the J3 region, and none have done so at the fine-scale resolutions required for exploring PBL dynamics. Here we use the Weather Research and Forecasting model v3.6.1 coupled with chemistry (WRF-Chem) in high-resolution (50-100m) Large Eddy Simulation (LES) mode to explore and quantify the effects of PM_2.5-PBL feedbacks on EGEs. We center our model on a site in the northern urban area of Beijing, where a LIDAR instrument provides backscatter data to evaluate the performance of LES-derived PBL heights. Our work will contribute toward understanding the dynamics of extreme pollution in Northern China and could aid in forecasting and mitigation of events, thus limiting their considerable health and economic impacts.