Relationship between summertime concurring PM_2.5 and O₃ pollution and boundary layer height differs between Beijing...

The rapid development in the economy during past decades has caused serious air pollution issues in China with high concentrations of PM_2.5 and O₃, particularly in the densely populous cities. To integrate PM_2.5 and O₃ controls, it is necessary to understand the impacts of meteorology on both pollutants. Thereby, the complex linkages between planetary boundary layer (PBL), synoptic forcing, regional transport, and heavy pollution in Beijing and Shanghai during summer were investigated using long-term measurements, simulations, and reanalysis. Influenced by the unfavorable meteorological conditions, PM_2.5 pollution and O₃ pollution often simultaneously occurred. In Beijing, the heavy concurring pollutions usually happened on the days with shallow afternoon PBL and southerly/southwesterly prevailing winds. Within the PBL, the pollutants emitted from the southern plains can be transported to Beijing and accumulated on the windward side of the mountains. At the top of PBL, the synoptic southerly warm advections can strengthen the elevated thermal inversion layer and suppress the development of PBL, leading to worse pollution. Contrarily, the heavy pollutions in Shanghai usually occurred on the days with deep afternoon PBL and southwesterly warm advections within the PBL. Although the warm advections were more favorable to the PBL development than the movements of cool marine air mass, the input of pollutants from the southwest can overweigh this advantage, resulting in poor air quality in Shanghai. The occurrence of heavy pollution or clean condition in Shanghai was primarily determined by the synoptic forcing rather than the local PBL structure. This comparative study indicates that the relationship between PBL height and pollution level is changeable and complicated, which needs to be elucidated from the synoptic perspective.