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Formation and Evolution Mechanisms for Two Extreme Haze Episodes in the Yangtze River Delta Region of China During Winter 2016


Published:2019-04-08


Recently, the research group of Prof. Tijian Wang made progress in the formation and evolution mechanisms for the wintertime haze in eastern China. The paper titled 'Formation and evolution mechanisms for two extreme haze episodes in the Yangtze River Delta region of China during winter 2016' was published on (Impact Factor=3.5). The first author is Dr. Mengmeng Li, the corresponding author is Prof. Tijian Wang, and the corresponding affiliation is Nanjing University.

The causes for wintertime haze involved complex physical and chemical processes, but the formation and evolution mechanisms for such high PM2.5 episodes in eastern China were still poorly understood. Secondary inorganic aerosols (SIA) is the main reason for PM2.5 increase during haze episodes, but the current atmospheric chemical models could not accurately simulate the key characteristics of SIA, e.g., underestimating the sulfate concentrations by 2 times or more, and overestimating the HNO3/NOx ratio by 5-10 times, implying that some processes related to the production of SIA are ignored in the model.

The paper characterized two extreme haze episodes in the winter of 2016 (1-9 December and 19-24 December) in the Yangtze River Delta (YRD) region using comprehensive measurements and model analyses. The heterogeneous uptakes and reactions of gaseous SO2, H2SO4, NO2, NO3, N2O5 and HNO3 on particle surfaces were included in the WRF-Chem model to better represent the formation of SIA. The optimized mechanisms substantially improved the simulations of PM2.5 composition. The heterogeneous conversion from SO2 to sulfate is the main reason for sulfate increase, accounting for more than 50% of sulfate production. The two episodes show different synoptic conditions and evolution stages, with gradual PM2.5 increase under stagnant weather conditions in Episode 1 (Stage I: Slow Increase Stage, Stage II: Rapid Formation Stage, and Stage III: Dissipation Stage), and with explosive PM2.5 increase mostly associated with cross-border transport from North China in Episode II (Stage I': Clean Stage, Stage II': Fast Transport Stage, and Stage III': Clear Stage). The simulation also confirmed that the fast SIA increase in Episode II is presumably caused by the cross-city cluster transport from North China, contributing 33.2% to the PM2.5 maximums. The study improved the model bias in traditional atmospheric chemical models, and revealed the formation and evolution mechanisms for wintertime haze in eastern China. It was meaningful for further understanding of the causes for wintertime haze in eastern China.

This study is supported by the National Natural Science Foundation of China (91544230, 41605115 and 91644106) and the National Key Basic Research Development Program of China (2016YFC0203303).

Figure 1. (Left panels) Observed gaseous and particulate pollutant concentrations and meteorological elements at the Shanghai supersite in December 2016; (Right panels) Simulated local and regional contributions to the PM2.5 concentrations in the YRD regions (black shades: local production; red shades: transport from the BTH region; blue shades: transport from the other regions) 


Reference:

Li M., T. Wang* and M. Xie et al., Formation and evolution mechanisms for two extreme haze episodes in the Yangtze River Delta region of China during winter 2016, Journal of Geophysical Research, 2019, doi: 10.1029/2019JD030535.


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