Meteorological Causes Of Air Pollution In The Northwest Urban Agglomeration Of Sichuan Basin In Winter And Their Numerical Simulation

With the rapidly developing of economy,the accelerating of urbanization and the increasing of energy consumption,Sichuan Basin has become one of the four heavily air polluted regions in China.The basin topography is complex and belongs to deep mountain-basin topography.Under the effects of its complex topography and unique meteorological conditions,the characteristics and the formation mechanism of air pollution over there are distinctive from other places of China,and urgently need to be studied in depth.Firstly,Sichuan Basin had been categorized into three zones?including the bottom,slope,and edge of the basin?in terms of the altitudes of air quality monitoring stations across the Basin.The spatial-temporal distribution characteristics of air pollution in these three zones were investigated,respectively.Secondly,combined with the meteorological data from national ground meteorological observation stations in recent ten years,the differences of air pollution and meteorological filed between Sichuan Basin and the other three heavily air polluted regions in China had been analyzed emphatically.Finally,a rapidly developing urban agglomeration in the northwest Sichuan Basin which is affected by low-pressure systems moving east from the Tibetan Plateau,was selected as the study area in this paper.The meteorological causes of air pollution over the urban agglomeration were explored systematically and deeply by using the methods of statistical analysis,diagnostic analysis and numerical simulation.In particular,the influencing mechanisms of the low-pressure weather system's movement?dry trough and dry low vortex?from the Tibetan Plateau on the formation and dissipation process of winter heavy air pollution over this area,were analyzed emphatically.In order to evaluate the contributions of the air pollutants'emissions within the Basin and outside of the Basin on the winter air quality in Chengdu respectively,two sensitive tests were also conducted by WRF-Chem model.The main results of this thesis are as follows:?1?Heavily air pollution was observed in the bottom zone,medium in the slope zone,and light air pollution in the edge zone of Sichuan Basin.In the bottom and slope of the basin,PM_2.5 as the primary pollutant occurred most frequently.However,in the edge zone of the basin,NO₂ was mostly observed as the primary pollutant.Among the six air pollutants,concentrations of PM_2.5and PM₁₀ decrease dramatically with increasing altitude from the bottom to the edge of the basin,which was fitted by a nonlinear relationship y=?a+b/x?².This relationship was validated by extinction coefficient profiles from CALIPSO observations and EV-lidar data,and hence used to reflect vertical distribution of air PM concentrations.However,gaseous pollutants seemed not to show clear relationships between their concentrations and altitudes in the basin.Their horizontal homogeneities were less significant compared to PM.?2?The differences of air pollution and meteorological filed between Sichuan Basin and the other three heavily air polluted regions in China had been compared and analyzed.We found that the concentrations of PM in Sichuan Basin are lower than that in North China Plain,but higher than that in Yangtze River Delta and Pearl River Delta.The gaseous pollutants'concentrations in the basin are lower than that in North China Plain and Yangtze River Delta.In addition,Sichuan Basin belongs a low wind speed and high relative humidity zone in China due to its deep mountain-basin topography.The average wind speed near surface of four seasons are less than 1.5m s^-1,and the relative humidity in autumn and winter is higher than that in the other three heavily air polluted regions in China.The unique meteorological conditions and complex terrain around the basin lead the characteristics of air pollution to be distinctive from other places of China.The most outstanding performance is that the mean duration of winter heavy air pollution in Chengdu is 14.86 days,which is significantly longer than that in Beijing?4.13 days?and Shanghai?3 days?.Moreover,the formation and dissipation processes of winter heavy air pollution event in Chengdu belong to slow process,while in Beijing and Shanghai,they belong to rapid process.?3?The meteorological causes of eight winter heavy air pollution events in the Northwest urban agglomeration of Sichuan Basin in recent ten years were explored and one typical air pollution event was simulated by WRF-Chem.Results revealed that the?pot effects?of strong inversion layer above the atmospheric boundary layer played a key role in the formation of these heavy air pollution events.This strong inversion layer was caused by sinking warming which was related to the movement of dry low-pressure weather system from the Tibetan Plateau.When the urban agglomeration was in front of the dry low-pressure system and the weather conditions were controlled by a warm southerly air flow,a strong temperature inversion appeared above the atmospheric boundary layer acting as a lid due to sinking warming.Forced by this strong inversion layer,the local secondary circulation was confined within the atmospheric boundary layer and the horizontal wind speed in the lower troposphere was low.As a result,vertical mixing and horizontal dispersion in the atmosphere were poor,favoring the formation of heavy air pollution events.After the dry low-pressure system had transited over the urban agglomeration,the weather conditions over there were controlled by a dry and cold air flow from the northwest at700 hPa.Cold air invaded over the basin,and strong cooling occurred above the atmospheric boundary layer under the effects of cold advection.While in the atmospheric boundary layer,weak warming were found due to the sinking warming.Consequently,the strong inversion layer above the atmospheric boundary layer gradually dissipated,the secondary circulation enhanced and uplifted,and the horizontal wind speed in the lower troposphere also increased,resulting in a sharp decrease in the concentration of air pollutants.?4?The full sample of daily weather patterns for air pollution in the Northwest Sichuan Basin during the winter months from Dec 2013 to Feb 2017,was analyzed by using obliquely rotated T-model principal component analysis?PCA?.Results revealed that the winter atmospheric circulation at 700 hPa over Chengdu could be classificated into 9 synoptic types.According to the characteristics of meteorological filed and air pollution,the above 9 synoptic types could be combined into three types including low-trough type with heavy air pollution,high-pressure front type with light air pollution and wet low-vortex type with lightest air pollution.The influence mechanisms of synoptic types on winter air quality in Chengdu had also been explored.We found that the strong stable layer above the atmospheric boundary layer was formatted under the effects of sinking warming caused by the low-trough type.Forced by this strong stable layer,the local secondary circulation was confined within the atmospheric boundary layer,the horizontal wind speed in the lower troposphere was low and the atmospheric boundary layer was shallow.As a result,vertical mixing and horizontal dispersion in the atmosphere were poor,favoring the deteriorating of local air quality.Moreover,these stagnant weather conditions are consistent with the formation mechanism of the above eight heavy air pollution events.While under the effects of high-pressure front type,the weather conditions were controlled by dry and cold air flow from the northwest at 700 hPa.Cold air invaded over the basin.As a result,the atmospheric stability in the lower troposphere was weakened significantly,local secondary circulation enhanced and uplifted,wind speed increased,and the atmospheric boundary layer developed and deepened,resulting in light air pollution.Under the effects of wet low-vortex type,cold air invaded over the basin and met with the local warm air,often causing precipitation.Consequently,the wet removal effects was remarkable and the air pollution was the lightest among the three synoptic types.?5?Two sensitive tests were also conducted by WRF-Chem model to evaluate the contributions of the air pollutants'emissions within the Basin and outside of the Sichuan Basin on the winter air quality in Chengdu,respectively.Results revealed that the contribution of the air pollutants'emissions within the Basin to the concentrations of particulate matter in Chengdu was more than 90%.While the contribution of the air pollutants'emissions outside of the Basin was weak due to the blocking effect of towering mountains.The above results revealed the characteristics of air pollution in different zones of Sichuan Basin,and exhibited the meteorological causes and physical mechanisms of the formation and dissipation process of winter heavy air pollution in the northwest Sichuan Basin.This research makes up for the shortcoming of the previous studies about air pollution in the Sichuan Basin,expands the theoretical connotations of the air pollution meteorology in the deep mountain-basin topography,and provides new scientific insights for the prevention and control of air pollution in this region of China.