Zhuorui Wei1, Hongsheng Zhang1*, Wei Wei2, 3*, Xiaoye Zhang3, Xuhui Cai4, Yu Song4, Tong Zhu4
1. Laboratory for Climate and Ocean-Atmosphere Studies, Department of Atmospheric and Oceanic Sciences, School of Physics, Peking University, Beijing 100871, P.R. China
2. CMA Earth System Modeling and Prediction Centre, Beijing 100081, P. R. China
3. Chinese Academy of Meteorological Sciences, China Meteorological Administration (CMA), Beijing 100081, P.R. China
4. State Key Joint Laboratory of Environmental Simulation and Pollution Control, Department of Environmental Science, Peking University, Beijing 100871, P.R. China
* Correspondence to: Hongsheng Zhang (hsdq@pku.edu.cn), Wei Wei (weiw@cma.gov.cn)
Abstract
During heavy haze episodes, especially the cumulative stage (CS) of pollutants, strong turbulence intermittency and the resulting turbulence barrier effect (i.e., a phenomenon that turbulence at certain heights may disappear forming a laminar flow as if there is a barrier layer impeding the vertical turbulent exchange) suppress the vertical diffusion of pollutants, leading to high PM2.5 concentrations. However, there are still some short-time removal processes of pollutants occurring at different heights in the CS, accompanied with interesting non-simultaneous drop or opposite variation of PM2.5 concentrations at different heights. The ubiquitous internal gravity waves (IGWs) in the stable boundary layer (SBL) may play a critical role in the above situation, as they are closely related to the intermittent turbulence bursts appearing in the persistent weak turbulent motions. In this study, two representative heavy haze pollution cases were chosen to demonstrate the above speculation using five layers of turbulence data, two layers of pressure fluctuations and three layers of PM2.5 concentrations. Results showed that the non-simultaneous drop or opposite variation of PM2.5 concentrations was associated with the destruction of turbulence barrier by the vertical propagation of IGWs. IGWs generated by some certain mechanisms, such as nonhomogeneous terrain and wind shear around the low-level jet (LLJ), can propagate upward or downward with the upward or downward development of the temperature inversion layer. The vertically propagating IGWs then triggered intermittent increasing turbulence layer by layer. Turbulence between layers reconnected sequentially and turbulence barriers were broken in turn. The enhanced turbulent exchange expedited the pollutant diffusion, thus the PM2.5 concentrations at different heights varied non-simultaneously even inversely. This study provides a good explanation for the positive effects of sub-mesoscale motions such as IGWs on triggering intermittent increasing turbulence and facilitating the diffusion of pollutants during heavy haze pollution events.
Keywords
Internal gravity waves; Turbulence barrier; Intermittent turbulence; Pressure fluctuations; Pollutant diffusion; Haze
Fig. 1 Schematic of the destruction of turbulence barrier by vertically propagating IGWs and the resultant diffusion of pollutants during this process.
Acknowledgments
This work was supported by the National Natural Science Foundation of China (42090031, 42175092, 92044301, 91544216).
Citation
Wei, Z., Zhang, H., Wei, W., Zhang, X., Cai, X., Song, Y., & Zhu, T. (2022). Mechanism of the effect of vertically propagating internal gravity waves on turbulence barrier and pollutant diffusion during heavy haze episodes. Science of The Total Environment, 845, 157349. https://doi.org/10.1016/j.scitotenv.2022.157349.
