Influence of Intermittent Turbulence on Air Pollution and Its Dispersion in Winter 2016/2017 over Beijing, China

With rapid urbanization in recent years, severe air pollution has emerged as a major issue for many regions of China, especially in some metropolises. A persistent pollution case during 6 December 2016–8 January 2017 was selected to investigate the relations between turbulent intermittency and frequent PM_2.5 (particulate matters with diameter less than 2.5 μm) pollution events over the metropolitan region of Beijing, China. The accumulation of PM_2.5 near the surface frequently occurred as a combined result of strong inversion layers, stagnant winds, high ambient humidity levels, and stable stratification during this case. Arbitrary-order Hilbert spectral analysis indicated that steep decreases in the PM_2.5 concentration were simultaneous with the occurrence of intermittent turbulence and strong vertical mixing. A wind profiler observation revealed existence of low-level jets (LLJs) at the end of the polluted periods, suggesting that the upper-level turbulent mixing accompanied by the wind shear of LLJ was transported downward and enhanced the vertical mixing near the surface, which might have caused an abrupt reduction in PM_2.5 and improvement in air conditions.