Formation and Evolution Mechanisms of Severe Haze Pollution in the Sichuan Basin, Southwest China

Xianyu Yang,Yaqiong Lu,Qing Jiang,Kai Wu,Shigong Wang,Xinsheng Zhu,Haolin Wang,Xiaoling Zhang,Jianjun He

doi:10.4209/aaqr.2020.04.0173

Abstract

ABSTRACT Severe haze episodes are important environmental issues, and the rapid formation and evolution mechanisms of such episodes over complex terrain remain poorly understood. The Sichuan Basin (SCB) periodically experienced heavy haze pollution during the winter of 2016, with the maximum regional average PM2.5 concentration reaching almost 120 µg m–3. In this study, we characterize a severe haze episode in the SCB from 20 to 30 January 2017 using comprehensive measurements and model analyses. The evolution of this severe episode shows clear stages, with gradual PM2.5 increases under stagnant weather conditions in Stage I (aerosol accumulation stage) and with explosive PM2.5 increases mainly associated with cross-border transport from the southern SCB in Stage III (rapid formation stage). The process analysis results indicated that primary emissions and aerosol processes were the major sources of PM2.5 in these urban regions, whereas vertical transport and dry deposition generally acted as sinks of PM2.5. In the presence of southwesterly synoptic winds, the aerosols emitted from the southern SCB were transported to Chengdu and the surrounding areas through horizontal transport and accounted for 66% of the PM2.5 concentration in Chengdu during Stage III. Our results reveal the detailed formation mechanism of a severe haze episode in the SCB under the effects of regional transport and synoptic forcing patterns to improve the understanding of haze formation in areas with complex terrain.

Highlights

Due to rapid economic development and the acceleration of urbanization in China, regional haze pollution characterized by high fine particle (PM2.5) concentrations has become a serious environmental issue (Chan and Yao, 2008; Lei et al, 2011)
Observational Data The simulated meteorological parameters, including temperature, relative humidity, wind speed (WS) and wind direction, are compared with hourly observation data recorded at four typical national standard basic weather stations (Chengdu, Meishan, Ziyang and Neijiang), which were obtained from the China Meteorological Data Service Center
Meteorological Parameters We evaluated the performance of the WRF model to demonstrate the capability of the modeling system through verification statistics, including the normalized mean bias (NMB), the index of agreement (IOA), and the root mean square error (RMSE), for the hourly 2-m temperature (T2), 2-m relative humidity (RH2), 10-m wind speed (WS10) and 10-m wind direction (WD10) at the Chengdu, Meishan, Ziyang and Neijiang meteorological stations

Summary

Introduction

Due to rapid economic development and the acceleration of urbanization in China, regional haze pollution characterized by high fine particle (PM2.5) concentrations has become a serious environmental issue (Chan and Yao, 2008; Lei et al, 2011). Because of serious haze pollution resulting from high anthropogenic emissions and unfavorable meteorological conditions, the Sichuan Basin (SCB) has become one of the most polluted city clusters in China over the last few years (Zhao et al, 2018). Extensive studies have been conducted to explore the source and formation of haze pollution in the SCB (Tian et al, 2017; Qiao et al, 2019a). Based on the National Centers for Environmental Prediction (NCEP) reanalysis data and clustering analysis method, Sun et al (2020) classified the circulation patterns during periods of continuous pollution in the SCB and found that a weak highpressure system located in the SCB could enhance pollution by suppressing the planetary boundary layer height (PBLH)

Methods

Results

Conclusion