Seasonal-spatial Variations Of Surface Ozone Over China And The Influence Of Long Range Transport

Due to the key influence of troposphere ozone on health,ecology,and even climate change,much effort focused on the reduction of surface levels of ozone,but recent efforts to achieve reductions in exposure at a country scale have proved difficult to achieve owing to increases in background ozone at the zonal hemispheric scale.It is necessary to treat ozone across the range of scales,a trans-boundary issue with an emphasis on the city-regional-hemispheric scales.In recent decades,high concentration ozone over China has attracted broad attention and obtained many meaningful achievements.However,seasonal-spatial variations of tropospheric ozone over China and its impact factors,especially the impact of Asia monsoon transport need further evaluations.The background concentrations of ozone over China and its impact factors also need further discussions.The discoveries on regional representativeness of Chinese Global Atmosphere Watch（GAW）stations are distinctive and innovative.Therefore,under the increasingly serious ozone pollution over China in recent years,conducting research on seasonal-spatial variations of surface ozone over China and the influence of long range transport to disclose the transport and chemical conversion mechanisms are of significant meanings on ozone pollution control over China and the research on the impact of Tibet Plateau on the environment and climate change over China and northern hemisphere.Considering the year 2010-2012,this doctoral thesis use troposphere and stratosphere coupled,high resolution global atmospheric chemistry general circulation models,EMAC,and the ground and satellite remote observation data at the six Chinese GAW stations,i.e.,Waliguan,Xianggelila,Akedala,Lin’an,Shangdianzi,Longfengshan,to reveal the seasonal-spatial variation characteristics of ozone and its precursors over China.By means of ozone source-tagging method in EMAC,contributions of ozone originated from fourteen different latitude zones to the six GAW stations are evaluated.Furthermore,using the global CO source-tagging method,influence of pollution air masses long range transporting from various land regions on ozone variations over different areas of China is evaluated as well.The tropospheric column densities of NO₂ and CO are high in winter,while low in summer over China,and the tropospheric column density of ozone reaches its maximum and minimum in summer and winter,respectively.NO_x-involved photochemical cycles produce ozone actively in summer,while ozone photochemical cycles are depressed in winter over most regions of China.Severely polluted air masses transporting to downwind directions benefits ozone photochemical productions.Seasonal-spatial ozone variations at the six Chinese GAW stations are rather different,particularly between the western locations,i.e.,Waliguan,Xianggelila,Akedala,and eastern locations i.e.,Lin’an,Shangdianzi,Longfeng.Asian summer monsoon can transport photochemical pollutants from the southern to the northern areas in eastern China,leading to a northward gradual enhancement of background ozone levels at the eastern stations.The highest levels of ozone most appear in spring to the south of 30oN,and in summer to the north of 30oN.Seasonal variation characteristics of ozone revealed by the Chinese GAW stations are typical,with each station having a considerable large surrounding area with the ozone maximum occurring at the same month,i.e.Waliguan in June,Xianggelila in May,Akedala in April,Lin’an in May,Shangdianzi in June,Longfengshan in July,respectively.EMAC has well simulates the concentrations and variation trends of the surface ozone over China,reflecting the seasonal-spatial ozone patterns of higher in the east,whereas lower in the west,and higher in summer,while lower in winter.Besides,the areas of high and low center in various regions,especially the high center in eastern China and low center in Tibet,respectively,have been precisely reappeared by EMAC.Surface ozone over China dominantly originates from mid-high latitude troposphere in northern hemisphere（NHTS）,especially over the north part of eastern China in spring and summer with the values as high as 30-35 DU in the summer.Ozone from tropical troposphere（TRTS）contributes to southern China with the values over 27DU,but seldom varies with seasons.Ozone from other latitude zones contributes far less than NHTS and TRTS.The dominant elevated regions,seasons,contribute values of tropospheric CO over China from various global land pollution sources due to the long range transport of CO differ significantly with origins.CO from East Asia elevates CO levels by 0 to8 DU over the Northeast in summer,and its significant positive correlations with O_x only appear in the north part of eastern China,where the polluted air masses from East Asia increase the local ozone.The correlations appear significant negative over the Tibet Plateau in summer,showing that the influence of stratospheric transport to the ozone over plateau.The significant negative over South and Southwest China is due to the titration depletion to ozone by the severe pollution air masses.CO from South Asia elevates CO by 10 to 40 DU over the Southeast in spring,and its significant positive correlations with O_x dominantly appear in very local scales over the south of Tibet Plateau.CO from Southeast Asia elevates CO by 10 to 40 DU over South China in spring and winter,and its significant positive correlations with O_x dominantly appear in part of South China in spring.CO from Middle East elevates CO by 4 to 8 DU over Xinjiang throughout the year,and its correlations with O_x only appear in Xinjiang with positive correlations during the whole year,which results from stratospheric transport.CO from Europe elevates CO by more than 30 DU over Northeast China in autumn,nearly without correlations with O_x in China.Generally,with the influence of mid-latitude west circulation and Asia summer monsoon,polluted air masses originated from various global land regions impact the variations of O_x over China rarely in the CO significantly elevated regions,but in its downwind directions.