| Other Abstract | Air pollution events are usually caused by adverse meteorological conditions and excessive emission of air pollutants. Southwest China (SWC) is affected by complex terrain and plateau weather system. As a result, the atmospheric diffusion conditions in winter over SWC are different from other areas, result in air pollution events occur frequently over SWC, especially in the Sichuan Basin of SWC, which has become the fourth of heavily polluted regions in China following the Beijing-Tianjin-Hebei region, the Yangtze River Delta and the Pearl River Delta. However, little attention has been paid to the meteorological causes of air pollution over SWC, especially the impact of winter drought on local air pollution. Moreover, the impact of the activities of the south branch trough of the Tibetan Plateau on air pollution in the Sichuan Basin and its mechanism are still unclear. Additionally, the effect of bowl-shaped basin on local air pollution is badly in need of research. Therefore, this study was carried out from the following aspects with the methods of diagnostic analysis and numerical simulation. (1) Air pollution characteristics over SWC was analyzed. (2) Air pollution characteristics and their responses to meteorological conditions among the four mostly polluted areas in China were compared. (3) A typical heavy air pollution event induced by the activities of the south branch trough of the Tibetan Plateau in the Sichuan Basin was investigated. (4) Impact of bowl-shaped deep basin on local air pollution was quantified. (5) The association between winter air pollution and atmospheric circulation background of winter droughts over SWC was explored. (6) The response of winter air pollution over SWC to local drought and non-drought events was quantified. Some main conclusions are as follows: (1) Air pollution over SWC is serious in spring and winter, with PM2.5, PM10 and O3 as the primary pollutants in spring, and PM2.5 and PM10 as the primary pollutants in winter. Air pollution in summer and autumn is light, with O3 as the primary pollutant in summer, and PM2.5 as the primary pollutant in autumn. Spatially, air pollution in the Sichuan Basin is the most serious over SWC, followed by Yunnan and Guizhou provinces, and western plateau is the least polluted. The number of non-attainment days in the Sichuan basin, Guizhou Province and western plateau mainly appeared in winter, while that in Yunnan Province mainly occurred in spring. (2) It has been found that the relative humidity was about 70%, the visibility was poor (8 km), the temperature was low (7.2 °C), and the wind speed was small (1~2 m/s) during the period of heavy air pollution in the Sichuan Basin. Moreover, the boundary layer height was low, the atmospheric inversion layer was very significant, the water content in the atmosphere was high, the subsidence dominated in the troposphere, the southerly wind prevailed below 700 hPa. The mechanisms of the activities of the south branch trough of the Tibetan Plateau affecting air pollution of the Sichuan Basin are as follows. When the Sichuan basin is in the front of the southern trough, southwest airflow affected by the Tibetan Plateau and Yunnan-Guizhou Plateau formed a leeward slope vertical secondary circulation above the Sichuan Basin. Meanwhile, warm and humid air flows transported by southwesterly formed inversion layer over the Sichuan Basin, producing “pot lid effect”. Thus, the inversion layer defines the activity height of the low-level secondary circulation, and further restricts the vertical transportation and diffusion range of air pollutants, leading to air pollutant concentrations near the ground increasing rapidly. When the south branch trough moved east out of the Sichuan Basin, the inversion layer over the Sichuan Basin weakened and disappeared, and the activity height of the leeward slope vertical secondary circulation increased significantly, which was conducive to the dilution and diffusion of air pollutants in the vertical direction. The concentrations of air pollutants near the ground decreased. (3) Compared to the PM2.5 levels under simulations of elevated Sichuan Basin topography with altitudes of 2500 m and 4000 m, the PM2.5 levels simulated under the real Sichuan Basin topography were comprehensively enhanced with an average increase reaching up to 82 μg m-3 and 112 μg m-3, indicating that the basin contributed 54% and 76% PM2.5 to the heavy air pollution event, respectively. The bowl-shaped topography was the main cause of high temperature, high humidity, high sea level pressure, low atmospheric boundary layer height, low wind speed and inversion layer over Sichuan Basin. When the amount of the natural and anthropogenic emissions is a constant, the bowl-shaped topography together with adverse meteorological conditions are the main factors inducing heavy air pollution over Sichuan Basin. (4) The number of winter haze days over SWC has two main modes. The first leading mode, accounting for 25.9% of the variance, displayed a northwest-southeast dipole pattern over SWC, with the two anomalous centers locating over the Western Sichuan Plateau and Western Guangxi Province, respectively. The second leading mode, accounting for 10.2% of the variance, primarily reflected the spatially in-phase change pattern, the number of winter haze days over SWC changing synchronously with the strongest signal over the Sichuan Basin. Regression analysis and synthetic analysis were used to explore the association between winter air pollution and atmospheric circulation background of winter drought over SWC. It has been found that atmospheric teleconnections associated with winter air pollution over SWC were La Niña events, conventional Eurasian teleconnection, and negative phases of Polar/Eurasia pattern, Arctic Oscillation and North Atlantic Oscillation. This provides an important scientific basis for the local monthly air pollution prediction, and makes up for the shortage of short- and medium-term air quality prediction. (5) Data analyses and numerical simulations showed that the meteorological conditions over SWC have changed significantly under drought conditions. Precipitation, the number of precipitation days, atmospheric boundary layer height and relative humidity all decreased during the droughts, resulting in unfavorable conditions for the dispersion of air pollutants. WRF-Chem model not only simulated the spatial and temporal characteristics of the meteorological parameters (rainfall, 2 m temperature, sea level pressure, relative humidity, atmospheric boundary layer height) and air pollutant (PM2.5, PM10, CO, NO2, SO2 and O3) concentrations, but also reproduced the responses of winter air pollution over SWC to local drought and non-drought events, reflecting the physical processes and mechanisms of winter droughts affecting air pollution from the perspectives of kinetics and thermodynamics. The results of this study further reveal the meteorological causes and mechanisms of air pollution in the Sichuan Basin. Particularly, the association between winter air pollution and atmospheric circulation background of winter drought over SWC not only provides important scientific basis for the development of monthly-term air pollution potential forecast, but also is helpful to improve the forecast accuracy of local short-term air quality, enriching and developing the theoretical connotation of meteorological causes of air pollution over the region with complex terrain. |
