| In recent years,the Twain-Hu(Hunan-Hubei)Basin in the middle Basin of the Yangtze River has become a new regional haze pollution center with the dense rivers and lakes constituting a unique regional"water network"underlying surface.The influence of the underlying surface is a scientific issue of atmospheric environmental change that needs to be deeply understood.Using multi-source data of environmental-meteorological observations and remote sensing from 2015 to 2021,this thesis comprehensively understood the changing characteristics of atmospheric particulate pollution in the Twain-Hu Basin in winter,as well as the unique wind field and underlying surface effects.Using the approximate envelope method separating primary and secondary aerosol contributions,the temporal and spatial changes of secondary PM2.5 in the Twain-Hu Basin in winter from 2015 to 2020 were analyzed,and the contribution of secondary PM2.5 pollution to heavy pollution events and long-term changes in air quality in the Twain-Hu Basin was assessed.Employing the atmospheric chemical coupling model WRF-Chem,the sensitivity simulation of the"water network"underlying surface of a stagnant type heavy pollution event in the Twain-Hu Basin in winter was carried out to reveal the impact and regulating mechanism of PM2.5 under the dense"water network"in the Twain-Hu Basin in winter.The main research contents are summarized as follows:1)Temporal and spatial variation of atmospheric particulate matter in the Twain-Hu Basin and its related influencing factorsIn order to understand the characteristics of atmospheric environmental changes in the Twain-Hu Basin in recent years,where is in the middle reaches of the Yangtze River,this paper uses the near-surface PM2.5 and PM10 observation data of the Twain-Hu Basin in the winter of 2015-2019,combined with wind speed,topography,vegetation index and other data to explore the variation characteristics of atmospheric particulate matter PM2.5 and PM10 in the Twain-Hu Basin in winter and their correlation with wind speed,topography and underlying surface.The results show that:PM2.5 pollution occurred frequently in the winter of2015-2019 in the Twain-Hu Basin,and Xiangyang and Jingmen,which in the western part of the Twain-Hu Basin,had PM2.5 pollution on average for 62 and 61 days in winter(PM2.5>75?g·m-3),even more,heavy pollution in Xiangyang(PM2.5>150?g·m-3)up to 19 days,indicating that the Twain-Hu Basin is an air pollution center in the middle reaches of the Yangtze River Basin.In terms of space,the pollution of the Twain-Hu Basin is heavier in the northwest than in the southeast,and heavier in the urban agglomeration,which is mainly related to the regional transport of air pollutants driven by winter monsoon and the strong emission from urban areas in the Twain-Hu Basin.There is a special nonlinear relationship between near-surface wind speed and PM2.5 and PM10 concentrations.The inflection points of PM2.5 and PM10 concentrations are 153?g·m-3 and 210?g·m-3,respectively,which revealed that the accumulation of local atmospheric particulate matter in the Twain-Hu Basin dominates light/medium pollution,while the regional transport of air pollutants determines heavy pollution.PM2.5 and PM10 in winter are significantly negatively correlated with terrain height and vegetation index,which reflected the atmospheric environmental effects of topography and urbanization changes in the Twain-Hu Basin.2)Contribution of secondary particulate matter to PM2.5 pollution in the Twain-Hu Basin in winterIn order to fully understand the temporal and spatial variations of atmospheric particulate matter in the Twain-Hu Basin,an approximate envelope method for separating primary and secondary aerosol contributions was used to evaluate the temporal and spatial variation of the secondary PM2.5 and its impact on PM2.5 contribution.in the Twain-Hu Basin in winter.The average contribution of secondary PM2.5 to PM2.5 in the Twain-Hu Basin in the winter of2015-2020 was as high as 62.26%,indicating that the secondary particulate matter played an important role in the changes of PM2.5 in the Twain-Hu Basin.Although the primary PM2.5concentration in winter has been decreasing year by year in the past 6 years,the contribution rate of secondary PM2.5 to PM2.5 has been increasing,revealing the increasingly important role of secondary particulate matter on air quality changes in the process of emission reduction in the Twain-Hu Basin.With the intensification of PM2.5 pollution,the secondary PM2.5 contribution continued to increase,reflecting the significant contribution of secondary aerosols to PM2.5 heavy pollution.There were differences in the diurnal variations of primary and secondary PM2.5 concentrations.The primary PM2.5 peaked around 09:00 and 21:00,which would be affected by the morning and evening peak emissions;while the secondary PM2.5 peaked appear at 03-06:00 and 15-17:00,which would be mainly related to the promotion of liquid phase oxidation by warm and high humidity at night and the promotion of photochemical reactions by strong solar radiation in the afternoon,reflecting the effect of climate change on air quality in the Twain-Hu Basin.There were two high centers in the spatial distribution of secondary PM2.5 contribution in the Twain-Hu Basin,which were located in the Chang-Zhu-Tan urban agglomeration in the southeast basin and Xiangyang and Jingmen in the northwest mouth of the basin,which may be connected with the regional environment-meteorological conditions and regional transport of PM2.5 and the gaseous precursors.3)Influence of dense"water network"underlying surface on changes of PM2.5 in the Twain-Hu Basin in winterThe dense rivers and lakes in the Twain-Hu Basin form a unique"water network"underlying surface,which is a unique influencing factor for the change of PM2.5 in the Twain-Hu Basin in winter.The sensitivity simulation analysis of the underlying surface of the"water network"showed that,the underlying surface of the lake mainly affected the local meteorological conditions and the atmospheric boundary layer process by changing the sensible heat flux and latent heat flux,thereby regulating the changes of PM2.5 concentrations.The warm(cold)lake effect near the ground at night(daytime)promotes(suppresses)the development of turbulence,strengthens(weakens)the wind speed,increases(reduces)the height of the boundary layer,leading to the atmospheric structure more unstable(stabilized),which is beneficial to PM2.5 diffusions(accumulates).In winter,the underlying surface of the"water network"generally exhibits a warm lake effect,which increases the average air temperature of the surrounding areas by about 1°C.As air temperature near the ground rises,the atmosphere tends to be unstable,which is conducive to air pollutant diffusions and weaken PM2.5 pollution.During the pollution period,the concentrated big lakes(CBL)can reduce the regional PM2.5 concentration by 6.24%during the day and by 9.6%at night;the vertical impact height is within 1.5 km,which can reduce the PM2.5 by 9%-14%on average from the ground to 500 m.The warm lake effect will also promote the convergence of local circulation(land breeze),forming a concentration center of air pollutants,especially the small discrete lakes distributed in the surrounding areas of Wuhan,where urban pollutants accumulate and converge,thereby increasing local pollution.The thermal driver and water vapor are two critical meteorological factors on the underlying surface of the"water network"in the Twain-Hu Basin,significantly changing the local PM2.5 concentrations. |
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