Influences Of Atmospheric Particle Source-sink Processes On Air Quality Change In China

The atmospheric haze pollution is mainly manifested by a massive accumulation of fine particles(PM_2.5),which has a significant impact on human health and ecological environment system.The formation,maintenance and dissipation processes of PM_2.5 pollution are closely related to atmospheric physicochemical processes such as air pollutant emissions,meteorological conditions and air pollutant depositions.At present,the research on the formation mechanism of PM_2.5 pollution mainly focuses on emission sources,meteorological effects,chemical conversion and topographical effects.However,the sedimentation of air pollutants in China's air quality changes in the source-sink process of PM_2.5 remains to be understood.Therefore,this study develops the atmospheric chemical coupling model WRF-CUACE with improving the parameterization schemes physical and chemical processes such as particle dry deposition,and then conducts a series of simulation experiments of regional air quality changes in China to investigate the uncertainities of atmospheric pollutant emissions and dry deposition schemes with the effects on air quality simulation,assess the seasonal variation of PM_2.5 source-sink relationship,and further explore the influences of seasonal changes in vegetation on the PM_2.5 pollution in winter.The main research conclusions of this thesis are summarized as follows:(1)Coupling WRF/CUACE model with improveing chemical mechanism and particle dry deposition schemeThe chemical model CUACE is coupled online with the meso-scale meteorological model WRF following the coupling weather chemical model GRAPES-CUACE of the China Meteorological Administration and the model WRF-Chem.The particle dry deposition scheme developed by Petroff et al.(2010)is introduced in the coupling model WRF-CUACE,and the distribution of vegetation leaf area index(LAI)was improved by using MODIS data.Nine heterogeneous reaction mechanisms are introduced to update the secondary inorganic Aerosol simulation.The evaluation of WRF-CUACE simulations shows that the PM_2.5,NO₂and O₃ in North China,Yangtze River Delta and Pearl River Delta region are better modelled with the correlation coefficient R rangeing 0.55-0.80.The PM_2.5 simulation is valiated in the the level of good has only been systematically overestimated in summer;the simulated PM_2.5,NO₂ and O₃ in the Sichuan Basin have declined with underestimated NO₂ and overestimated O₃,which could be connneted with the poor meteorological simulation in the basin.(2)Assecements on uncertainties in air pollutant emissions and dry deposition schemes with the impact on atmospheric environment simulationThere are significant uncertainties in the estimation of atmospheric pollutant emissions by two different power plant emission calculation models,UEIPP and MEIC.Based on the simulation analysis of the uncertainty of power plant emissions,it is found that in the VOC control area,NOx emission reduction enhance the atmospheric oxidation(increased O₃ and OH concentrations)and promote the generation of secondary aerosols.Futhermore,two schemes of atmospheric particle sedimentation from Zhang et al.(2001)and Petroff et al.(2010)have the large uncertainties with the estimation of sedimentation rate varying greatly,which has a significant impact on the simulation of PM_2.5concentration,and the large uncertainties even exceed the simulated PM_2.5 concentrations without the dry deposition scheme.In the central and eastern plains of China and the Sichuan Basin,the difference in deposition rate can reach an order of magnitude,and the ground PM_2.5 simulation deviations are 20-30%,and the deviations over the forest-covered areas could even reach 30-50%.(3)Atmospheric particle deposition and regional transport in air quality changes over central-eastern ChinaThe average annual dry deposition of PM_2.5 is higher than wet scavenging in the most areas of central and eastern China.The proportion of dry subsidence in total subsidence is about 50-60%.The PM_2.5 dry depositions are spatially distributed differently from PM_2.5concentrations and anthropogenic emissions over China.In the source areas with high PM_2.5concentrations,there are weak PM_2.5 depositions,especially in the central-eastern region of China with haze pollution occuring frequently in winter.Weak PM_2.5 particulate deposition is also a reason for the formation of high PM_2.5 concentration.The deposition rates of atmospheric particulate matter are low in the main emission source areas in the middle and east of China,resulting in the weak deposition of PM_2.5 and major PM_2.5 export area for regional transport of atmospheric pollutants in China,while the emissions of air pollutants are weak in the central-southern-southwestern region of China(except Sichuan Basin),and the deposition rates of atmospheric particulate matter are high.In autumn and winter,PM_2.5mainly transports to the south through the Central and East China Plains,forming the main sink of PM_2.5 particulate matter in the middle and lower reaches of Yangtze River in China.In addition,the regional transport of PM_2.5 in China has a complex vertical structure,which is mainly attributed to the forcing effect of the third-order terrain in China and the seasonal variations of the East Asian winter and summer monsoon circulations.(4)Impacts of vegetation seasonal cycle altering PM_2.5 dry deposition on haze pollution in ChinaThe effect of dry deposition of atmospheric particulates caused by the seasonal shift of vegetation between winter and summer on PM_2.5 pollution level is studied.The decrease of vegetation in winter could contribute to the PM_2.5 concentrations increasing by about 10%.The increase of PM_2.5 concentration(10.2%)in non-urban areas with dense vegetation coverages and low atmospheric pollutant emissions is significantly greater than that in urban areas with sparse vegetation and high pollutant emissions(6.5%)in China.In the pollution regions of Beijing-Tianjin-Hebei,Yangtze River Delta and Pearl River Delta,the PM_2.5concentration in non-urban(major cities)areas increased by 7.4(5.3)%,10.0(6.7)%,9.5(6.9)%,8.9(6.2)%and 11.4(9.3)%respectively.The seasonal differences of vegetation in winter and summer change the dry deposition of atmospheric particulate matter,which excerts a more significant negative impact on the air quality of clean non-urban areas in winter.Regional transport of air pollutants over China changed the regional source-sink budget of PM_2.5.Seasonal vegetation degradation in winter weakens and strengthens the air quality effect of PM_2.5 dry deposition in the export and reception areas of air pollutants,respectively.