Seasonal Evolution of Thermal, Dynamic, and Material Boundary Layers and their impacts on air pollution in the Megacity Beijing

Abstract. Urban planetary boundary layer height (PBLH) regulates the exchange of heat, momentum, moisture, aerosols, and pollutants between the surface and free troposphere, yet a single PBLH estimate cannot represent the multiple physical interfaces controlling turbulent mixing in complex megacities. This study investigates the seasonal co-evolution of thermal, dynamic, and material PBLHs in Beijing using the parcel, Richardson number (Ri), and gradient methods. Dynamic PBLH dominated during winter, with a mean of 828 m exceeding thermal (666 m) and material (625 m) PBLHs. In spring, material (918 m) and dynamic (871) PBLHs also exceeded the thermal PBLH (657 m). Summer exhibited the deepest daytime PBLH growth, with median peak thermal, dynamic and material PBLHs of 1397 m, 1402 m, and 1464 m, respectively. Autumn showed a transition regime, with mean thermal (685 m), dynamic (591 m) and material (709 m) PBLHs. In winter, pollutant concentrations were negatively correlated with dynamic PBLH (ρ= -0.47 to -0.74). Weak thermal buoyancy suppressed vertical mixing and restricted the upward transport of pollutants into the free atmosphere, favouring local pollution accumulation except during strong cold-air advection. In summer and autumn, thermal and material PBLHs exceeded the dynamic PBLH. Strong thermal buoyancy lifted particulate matter and other primary pollutants above the dynamic PBL top, facilitating long range dispersion. In contrast, ozone exhibited positive correlations with all PBLH metrics, consistent with active photochemical production and vigorous convective mixing. Collectively, these results demonstrate that adopting multiple definitions of BLH provides a comprehensive understanding of the processes controlling atmospheric pollution.