Impacts of Boundary-Layer Structure and Turbulence on the Variations of PM2.5 During Fog–Haze Episodes.

The precise cause of PM_2.5 (fine particular matter with a diameter smaller than 2.5 μm) explosive growth and the contribution of intermittent turbulence to the dispersion of PM_2.5 are uncertain. Thus, the impact of boundary-layer structure and turbulence on the variations of surface PM_2.5 during fog–haze episodes, especially during explosive growth and dispersion episodes, are investigated using turbulence data collected at a 255-m high meteorological tower in Tianjin from 2016 to 2018. Results suggest that the explosive growth of surface PM_2.5 during fog–haze episodes is closely related to weak turbulent mixing, nocturnal inversions, or anomalous inversions, and the barrier effect of strong turbulent intermittency. Turbulent intermittency acts as a lid for hindering pollutant dispersion and is favourable for the fast accumulation of surface PM_2.5. Apart from the potential causes mentioned above, the persistent moderate south-westerly flow is also a contributing factor for the explosive growth of surface PM_2.5 during fog–haze episodes associated with regional transport. In addition, we demonstrate a possible mechanism of how intermittent turbulence affects the dispersion of PM_2.5. Results verify that intermittent turbulence induced by the nocturnal low-level jet (LLJ) indeed plays an important role in the dispersion of PM_2.5. However, the contribution of intermittent turbulence generated by the nocturnal LLJ to the dispersion of PM_2.5 strongly relies on the intensity of the nocturnal LLJ. [ABSTRACT FROM AUTHOR]