Influences Of Synoptic And Climate Systems On Tropospheric Ozone In East Asia Through Global Transport Of Foreign Pollution And Meteorological Processes

Tropospheric ozone is an important air pollutant detrimental to human health and vegetation growth.It is also a greenhouse gas and an oxidant,which greatly impacts global climate and atmospheric chemistry.In recent years,East Asia has experienced increasingly severe ozone pollution.Exploring the sources and influencing factors of tropospheric ozone in East Asia has implications for atmospheric environment and climate change.Tropospheric ozone in East Asia is influenced by the transport of ozone from foreign regions around the world.However,the magnitudes and variations in such influences remain unclear in various aspects.By tagged ozone and emission perturbation methods implemented in a global chemical transport model,GEOS-Chem,together with the satellite retrievals and surface observations,this study quantifies the foreign influences and further analyzes the possible mechanisms of the transport.Specifically,the intercontinental transport of ozone from Africa to Asia is characterized.Tropospheric ozone in East Asia is influenced by meteorological processes on multiple scales.However,the relationship between climate systems and ozone transport to East Asia is not well understood.This study investigates the impacts of prominent climate systems on the seasonal and interannual variations in the foreign influences.On synoptic scale,a quantitative understanding of the influences of different synoptic weather systems on ozone pollution in China is lacked.The relative importance of different local meteorological variables to ozone pollution in China is still unclear.Using regression analysis and weather classification,this study statistically assesses the impacts of local and synoptic meteorology on daily variability in surface ozone in eastern China in summer during 2013-2018.Carbon monoxide is one of the key precursors of tropospheric ozone and one of the main air pollutants.Therefore,this study further assesses the sensitivity of free tropospheric carbon monoxide over eastern China to synoptic weather,which helps in understanding the influence of meteorology on ozone pollution in China.Main conclusions are drawn as follows.(1)Foreign emissions are important sources for tropospheric ozone in East Asia.Foreign ozone is transported to East Asia mainly through the middle and upper troposphere.In East Asia,the influence of foreign ozone increases rapidly with altitude.In the middle and upper troposphere,the regional mean concentrations of foreign ozone range from 32 to 65 ppbv,being 0.8-4.8 times higher than its native counterpart(11-18ppbv).Annually,60%of foreign ozone in the East Asian middle and upper troposphere comes from North America(5-13 ppbv)and Europe(5-7 ppbv),as well as from foreign oceanic regions(9-21 ppbv).Over the East Asian tropospheric columns,foreign ozone appears most in spring,when ozone concentrations in the foreign regions are high and the westerlies are strong,and least in summer,when the South Asian High blocks eastward foreign ozone from reaching East Asia south of 35°N.At the East Asian surface,the annual mean of foreign ozone concentrations is 22.2ppbv,which is comparable to its native counterpart of 20.4 ppbv.In the meantime,the annual mean of anthropogenic ozone concentrations from foreign regions is 4.7 ppbv,half of which comes from North America(1.3 ppbv)and Europe(1.0 ppbv).Global biomass burning contributes 1-5 ppbv to springtime surface ozone in East Asia,of which,75%comes from foreign regions,while 25%from biomass burning inside East Asia.Of all the foreign contributions,Africa contributes to 23%,and Southeast Asia contributes to 21%.(2)Climate systems including the East Asian monsoon have great impacts on the seasonal and interannual variabilities in the foreign influences.Seasonally,imported foreign ozone at the East Asian surface is highest in winter(27.1 ppbv)and lowest in summer(16.5 ppbv).This strong seasonality is largely modulated by the East Asian monsoon via its influence on vertical motion.The large-scale subsidence prevailing during the East Asian winter monsoon favours the downdraft of foreign ozone to the surface,while widespread convection in the East Asian summer monsoon blocks such transport.Interannually,the variation in imported foreign ozone at the East Asian surface is found to be closely related to the intensity of the East Asian monsoon.Specifically,the stronger the East Asian winter monsoon is in a winter,the more ozone from North America and Europe reaches the East Asian surface because of the stronger subsidence behind the East Asian trough.In summer,ozone from South and Southeast Asia is reduced in strong East Asian summer monsoon years due to weakened southwesterly monsoon winds and downdrafts.(3)The transport of African ozone to Asia is mainly through two pathways and is influenced by different climate systems.The upper branch of the Hadley cell connects with the subtropical westerlies to form a primary pathway in the middle and upper troposphere throughout the year.The Somali jet that runs from eastern Africa near the equator to the Indian subcontinent in the lower troposphere is the second pathway that appears only in summer.The interplay of Hadley circulation,subtropical westerlies,Intertropical Convergence Zone,Saharan High,Arabian High,and South Asian High modulates the seasonality of the ozone transport from Africa to Asia.In winter,the strength of the Intertropical Convergence Zone in Africa is positively correlated with the interannual variation of the transport of African ozone to Asia.In summer,the Somali jet can explain 30%of the interannual variation in the ozone transport from southern hemispheric Africa to Asia.(4)A quantitative linkage between ozone in eastern China and meteorology on multiple scales is established.Through developing a multiple linear regression model driven by local and synoptic weather factors,the influence of meteorology on ozone in eastern China is assessed.The results show that meteorology can explain 43%of the daily variability in summertime surface ozone across eastern China.Among local meteorological factors,relative humidity is the most influential variable in the Yangtze River Delta and the Pearl River Delta regions,while temperature is the most influential variable in the Beijing-Tianjin-Hebei region.Synoptic weather over eastern China in summer during 2013-2018 can be represented by six predominant patterns.The six patterns are formed under the integral influence of the East Asian summer monsoon,the western Pacific subtropical high,the Meiyu front,and the typhoon activities.Two patterns bring about positive ozone anomalies,when eastern China is under a weak cyclone system or under the prevailing southerly wind.The maximum impact of synoptic patterns can reach±16%of the daily mean ozone concentrations over some areas in eastern China.(5)The sensitivity of tropospheric carbon monoxide,an ozone precursor,over eastern China to synoptic weather is quantified.Synoptic weather patterns over eastern China during 2003-2015 are classified into eight predominant patterns.Carbon monoxide concentrations in the free troposphere over eastern China tend to increase under three patterns that are characterized with anticyclonic circulations and downdrafts in all or part of eastern China.Carbon monoxide outflow from eastern China is enhanced under two patterns that prevail in winter and spring.Carbon monoxide concentrations either increase or decrease by 5-15%under the persistent control of a pattern for two to seven days.In most cases,the anomalies of carbon monoxide concentrations and outflow tend to be amplified if a synoptic pattern persists for several days.This study reveals substantial foreign influences on ozone at the surface and in the tropospheric columns over East Asia.It also finds the importance of the East Asian monsoon to the seasonal and interannual variations in the foreign influences.The statistical assessment highlights the importance of meteorology in modulating ozone pollution over China.This study has enhanced our understanding of the connections between weather and air pollution.The findings from this study can be useful for air quality forecast in China and for regional and international collaborations on ozone pollution mitigation in East Asia.