Atmos. Chem. Phys., 21, 9253–9268, 2021 https://doi.org/10.5194/acp-21-9253-2021 © Author(s) 2021. This work is distributed under the Creative Commons Attribution 4.0 License. Elevated 3D structures of PM2:5 and impact of complex terrain-forcing circulations on heavy haze pollution over Sichuan Basin, China Zhuozhi Shu1,4, Yubao Liu1,4, Tianliang Zhao1,4, Junrong Xia1, Chenggang Wang1, Le Cao1, Haoliang Wang1, Lei Zhang2, Yu Zheng2, Lijuan Shen1, Lei Luo3, and Yueqing Li3 1Collaborative Innovation Center on Forecast and Evaluation of Meteorological Disasters, Key Laboratory for Aerosol-Cloud-Precipitation of China Meteorological Administration, Nanjing University of Information Science and Technology, Nanjing, 210044, China 2State Key Laboratory of Severe Weather & Key Laboratory of Atmospheric Chemistry of CMA, Chinese Academy of Meteorological Sciences, Beijing, 100081, China 3Institute of Plateau Meteorology, China Meteorological Administration, Chengdu, 610072, China 4Precision Regional Earth Modeling and Information Center, Nanjing University of Information Science and Technology, Nanjing, 210044, China Correspondence: Tianliang Zhao ([email protected]) Received: 6 November 2020 – Discussion started: 23 November 2020 Revised: 18 April 2021 – Accepted: 5 May 2021 – Published: 16 June 2021 Abstract. Deep basins create uniquely favorable conditions regional emissions from the SCB contributed 75.4 %–94.6 % for causing air pollution, and the Sichuan Basin (SCB) in to the surface PM2:5 concentrations in the SCB. The SCB Southwest China is such a basin featuring frequent heavy emissions were the major source of PM2:5 over the eastern pollution. A wintertime heavy haze pollution event in the regions of the TP and the northern regions of the YGP, with SCB was studied with conventional and intensive observa- contribution rates of 72.7 % and 70.5 %, respectively, during tion data and the WRF-Chem model to explore the 3D dis- the dissipation stage of heavy air pollution over the SCB, tribution of PM2:5 to understand the impact of regional pol- which was regarded as the major pollutant source affecting lutant emissions, basin circulations associated with plateaus, atmospheric environment changes in Southwest China. and downwind transport to the adjacent areas. It was found that the vertical structure of PM2:5 over the SCB was charac- terized by a remarkable hollow sandwiched by high PM2:5 layers at heights of 1.5–3 km and a highly polluted near- 1 Introduction surface layer. The southwesterlies over the Tibetan Plateau (TP) and Yunnan-Guizhou Plateau (YGP) resulted in a lee Haze pollution has caused serious environmental problems, vortex over the SCB, which helped form and maintain heavy especially in the densely populated and economically devel- PM2:5 pollution. The basin PM2:5 was lifted into the free tro- oped regions in China, which have high levels of fine partic- posphere and transported outside of the SCB. At the bottom ulate matter (PM2:5) (particulate matter with an aerodynamic of the SCB, high PM2:5 concentrations were mostly located diameter equal to or less than 2.5 µm) (Guo et al., 2014; P. Li in the northwestern and southern regions. Due to the blocking et al., 2015; Gu and Yim, 2016; Lin et al., 2018). Owing to effect of the plateau terrain on the northeasterly winds, PM2:5 the significant adverse effects on human health and climate gradually increased from northeast to southwest in the basin. change (Dawson et al., 2007; Langrish et al., 2012; Megaritis In the lower free troposphere, the high PM2:5 centers were et al., 2014; Guo et al., 2016), understanding PM2:5 pollution distributed over the northwestern and southwestern SCB ar- distributions and mechanisms is of high interest in environ- eas, as well as the central SCB region. For this event, the mental and climate studies. Published by Copernicus Publications on behalf of the European Geosciences Union. 9254 Z. Shu et al.: Elevated 3D structures of PM2:5 Anthropogenic pollutant emissions and stagnant meteoro- mate the roles of local emissions and exchanges with outside logical conditions are commonly regarded as two key factors regions more accurately. influencing haze pollution with excessive concentrations of In this study, observation data analysis and numerical ex- PM2:5 (Yim et al., 2014; Zhang et al., 2015; Cai et al., 2017). periments were conducted to analyze the 3D distribution of With strong anthropogenic emissions and favorable meteo- PM2:5 concentrations in the SCB during a heavy haze pollu- rological conditions, four main regions with frequent heavy tion episode in January 2017. The contributions of the SCB haze pollution have been identified, centered over the North pollutant emissions and PM2:5 transport to the surrounding China Plain (NCP) (Tao et al., 2012; Ye et al., 2016; Zhang plateaus and mountains were estimated. Section 2 introduces et al., 2016; Huang et al., 2017), the Yangtze River Delta the observation data and the modeling methods used in this (YRD) in East China (Wang et al., 2012; B. Li et al., 2015; study. Section 3 characterizes the horizontal and vertical dis- Tang et al., 2015; Ming et al., 2017), the Pearl River Delta tributions of PM2:5, during the formation, maintenance, and (PRD) in South China (Wu et al., 2013; Zhang et al., 2013; dissipation stages of the heavy haze pollution episode. We Guo et al., 2016), and the Sichuan Basin (SCB) in Southwest also assessed the contribution of local emissions to the heavy China (Tao et al., 2013; Chen et al., 2014; Zhou et al., 2019). PM2:5 pollution within the SCB and the impact of external Haze pollution over the NCP, YRD, and PRD, the main eco- transport of the PM2:5 in the SCB on the surrounding areas nomic centers with large flatlands, has been extensively stud- in Southwest China. The summary and conclusions are pro- ied. However, air pollution in the SCB region with highly vided in Sect. 4. frequent heavy PM2:5 pollution is not completely understood owing to the complex deep basin terrain, particularly the ef- fect of the immediately adjoining Tibetan Plateau (TP). 2 Data and model The TP’s “harbor effect” on the tropospheric westerlies fa- 2.1 Observation data vors a stable atmospheric stratification and low wind speeds in the boundary layer over the downstream SCB (Xu et al., The surface air pollutant concentrations and meteorologi- 2015, 2016), which is conducive to air pollutant accumula- cal elements observed in 18 cities (Fig. 1; Table 1) over tion in the SCB (Yim et al., 2014; Xu et al., 2016; H. Wang the SCB were used to investigate the distribution of PM2:5, et al., 2018a). The downslope flows at the lee side of the weather circulations, and modeling performance. The hourly plateau can induce a special stagnation meteorological condi- meteorological observational data, including surface air tem- tion in the lower troposphere (Wang et al., 2014; Ning et al., perature, relative humidity, wind speed, and wind direc- 2018a). Air stagnation days account for 76.6 % of the total tion, were obtained from the Chinese meteorological mon- days in winter over the SCB (Liao et al., 2018), where near- itoring network, and the hourly observational PM2:5 con- surface weakened wind, strong vertical air temperature in- centrations were obtained from the China National Envi- version, and shallow boundary layer significantly restrain the ronmental Monitoring Center (http://www.cnemc.cn, last ac- atmospheric diffusion capacity (Ning et al., 2018a; X. Wang cess: 1 June 2021). et al., 2018; Tian et al., 2019), resulting in the occurrence of In addition to the above-mentioned conventional observa- heavy air pollution in the SCB. tions, sounding observations were conducted every 3 h us- 2 The SCB, covering 260 000 km of the Sichuan- ing a kite balloon with the sounding system TT12 Digi- Chongqing plain with a dense population of more than 100 CORA (Vaisala, Finland) at the Meteorological Observatory million people, is a deep basin in Southwest China sur- of Chengdu (Site 1 in Fig. 1) during 1–20 January 2017. The rounded by plateaus and mountains. It lies immediately to vertical sounding data of air temperature, wind speed, wind the east of the TP, with a large elevation drop exceeding direction, and relative humidity were observed at time in- 3000 m over a short horizontal distance. The unique terrain tervals of 1 s. In addition, a micropulse lidar type 4 system effect generates the asymmetries of meteorological and air (MPL-4B-IDS, Sigma Space, America) was operated at the pollutant distribution (Zhang et al., 2019), with a remarkable observational site, Ya’an (Site 15 in Fig. 1), in the western difference in PM2:5 concentrations between the eastern and SCB edge to retrieve the vertical PM2:5 structures at 532 nm western regions over the SCB (Chen and Xie, 2012; Ning et (laser emission wavelength), 2500 Hz (laser repetition rate), al., 2018b). The weak vertical diffusion in the atmospheric and 6–8 µJ (optimal laser output range). boundary layer is one of the main causes of air pollution in winter (Hu et al., 2014; Tian et al., 2017; Zhao et al., 2018). 2.2 Model configuration and simulation experiments Many studies have suggested that air pollution over the SCB is mostly caused by the accumulation of air pollutants orig- The Weather Research and Forecasting with Chemistry inating from local emissions (Chen et al., 2014; Liao et al., (WRF-Chem, version 3.8.1) model was employed to 2017; H. Wang et al., 2018a; Qiao et al., 2019). However, simulate severe haze pollution events during 2–7 Jan- because of the complex flows in the SCB, it is important to uary 2017 in the SCB (Fig. 2). The spin-up time study how PM2:5 is circulated in three dimensions to esti- of modeling for the first 24 h, starting on 31 De- cember 2016, was dropped.