Investigation On The Impact Of Extreme Meteorological Events On Ozone Pollution In Eastern China

Eastern China has experienced rapid urbanization,motorization and industrialization since the 1990s,resulting in a significant increase in the emission of anthropogenic ozone?O₃?precursors.Eastern China has been facing a severe problem of photochemical pollution with high concentration of O₃ as its main feature in recent years,which has aroused great concern in society and academia.Extreme meteorological events?EMEs?,including heat wave?HW?,atmospheric stagnation?AS?,and temperature inversion?TI?,exert significant impacts on O₃.Therefore,it is particularly important to carry out research on the impact of EMEs on O₃pollution in eastern China.Based on O₃ monitoring data with high spatial coverage in eastern China,long time span in the Pearl River Delta?PRD?region and the meteorological re-analysis data of the same region in the same period.Spatiotemporal characteristics of O₃ and EMEs in eastern China are first described.Secondly,the spatiotemporal characteristics of the effects of EMEs on O₃ are analyzed,and then the sources of O₃ precursors are qualitatively identified.Furthermore,induction and maintenance of O₃ pollution events caused by EMEs are analyzed,and the impacts of EMEs on O₃ are further examined by its dependence on surface temperature.Finally,effects of EMEs on O₃ under different synoptic-scale circulation patterns are determined.Above all,effects of EMEs on O₃ in eastern China are comprehensively analyzed.The results show that the mean values of O₃ concentration and exceedance rate,the occurrence frequencies of HW and AS in eastern China during 2013²017 are 91.9?g?m^-3 and9.4%,6.5%and 13.4%,respectively.From 2013 to 2017,the O₃ concentration and exceedance rate in eastern China both show an obvious upward trend,HW shows a trend of first decreasing and then increase while AS tends to rise first and then fall.O₃ concentration and exceedance rate,occurrence frequencies of HW and AS are all showing a unimodal pattern during January to December.HW in eastern China increases O₃ concentration by 52.5?g?m^-3 and the exceedance rate by19.9%compared with non-HW,respectively,which are higher than AS increases the O₃concentration by 38.2?g?m^-3 and exceedance rate by 14.3%.HW has the most significant impact on O₃ in north China,Chengdu-Chongqing region,PRD and Yangtze River Delta region,while the spatial difference of the impact of AS on O₃ is relatively weak.The impact of EMEs on O₃ in eastern China shows an increasing trend in 2013²017,which is mainly affected by the emission of O₃ precursors.O₃ precursors in northern and northwestern China are mainly derived from local emissions,while O₃ precursors in eastern,central,southwestern and southern China are more affected by regional transport,especially in southwest and central China.The effect of EMEs on O₃ in different parts of eastern China under the control of different synoptic-scale circulation patterns is different,the effects of HW and AS on O₃ in eastern China are mainly controlled by two circulation patterns:Siberian high and subtropical high in the northwest Pacific.Base on observational O₃ data and meteorological reanalysis data,we further analyze the impact of EMEs on O₃ during O₃ season of April-October in 2006²017 over the PRD.Statistical analysis indicates significant but spatially heterogeneous sensitivities of O₃ to EMEs.AS poses the largest impact on O₃ concentration over the PRD,resulting in 58%increase compared with normal days,while the increases by HW and TI are 28%and 14%,respectively.O₃ pollution events are largely initiated by HW and AS which favor formation and build-up of O₃,while O₃ pollution events are maintained mostly by persistent AS and TI.HW poses higher impacts on northern and eastern PRD,while AS impacts more on central and western PRD.The effect of AS on O₃ concentration is similar as 10 K temperature increase during non-AS days,while the effect of AS and TI on O₃ exceedance is comparable with 6 K temperature increase during non-EMEs condition.O₃ concentrations under different synoptic patterns are largely associated with the occurrence of AS,and Siberian high and the approaching of a tropical cyclone are the dominant synoptic patterns for EMEs impact on O₃ and largely determines the long-term increasing trend of O₃ concentration over the PRD.This study comprehensively and systematically analyzes the effects of EMEs on O₃pollution in eastern China,highlights the importance of establishing a location-specific O₃control strategy targeting on normal conditions and O₃ pollution events separately.This study also provides scientific support to use EMEs forecast as an indicator to implement contingency O₃ control in advance so as to maximize peak O₃ reduction over the PRD.