Comparison of the turbulence structure during light and heavy haze pollution episodes

Abstract

To obtain the differences in the turbulence structures between the light and heavy haze pollution process, a light pollution episode (LPE) from November 3 to 72,017, and a heavy pollution episode (HPE) from December 15 to 232,016, were investigated. The weather conditions, mean meteorological fields, and turbulence structures of the two episodes were analysed. The results show that the two cases have similar weather backgrounds, and the differences in mean meteorological field are mainly related to the difference in the degree of solar radiation weakening. The turbulence during the HPE was suppressed to a greater extent in both thermal and dynamic effects. The structure of turbulence is weak and typically characterized by intermittent turbulence in the stable boundary layer. The parameter IS (Intermittent Strength) is introduced to indicate the strength of intermittency. Because of the complexity of atmospheric motion, we use the change in IS, i.e., ∆IS, to indicate the change in the intensity of turbulent intermittency during each pollution process. In the accumulation stage, the weakening of turbulent exchange caused by the influence of sub-mesoscale motion may result in poor turbulent diffusion conditions, which should be considered because they can lead to heavy pollution events when ∆IS > 0.1. The same conclusions were verified in an analysis of Tianjin for these two cases. Further analysis of the vertical structure of turbulence indicated that during the pollution stage of the LPE, the turbulent energy comes from the surface and is consistent with the traditional surface layer turbulent transport mechanism. Moreover, the upper layer intermittent strength is stronger than that in the lower layer, and ∆IS < 0.1 at different heights. During the pollution stage of the HPE, the boundary layer is almost decoupled from the surface, and ∆IS > 0.1 in all layers, which suggests that the weakening of turbulent exchange affected the vertical turbulence structure and resulted in pollutant accumulation. During the dissipation stage of both the HPE and the LPE, the low-level jets provide a turbulence source in the upper layer. The effect of sub-mesoscale motion is weak, and the turbulence component diffuses pollutants. In this stage, ∆IS < 0.1 in all layers. The upside-down structure of the atmospheric boundary layer leads to full coupling with the ground and promotes the dissipation of pollutants during the HPE. And, the analysis of the pollution processes, from December 16, 2016 to January 8, 2017, also indicate that ∆IS > 0.1 is a feature of severe pollution. The results for these cases indicate that ∆IS = 0.1 can be used as an indicator to characterize the turbulent exchange conditions and different levels of contamination.