Analysis Of Coordinated Relationship Between Nitrate And Ozone And Its Affecting Factors In The Two Major Chinese Regions

The Northern China Plain（NCP）and the Yangtze River Delta（YRD）regions have recently been suffering from complex air pollution,which is characterized with higher concentrations of fine particulate matter(PM_2.5)in autumn and winter,higher ground-level ozone（O₃）concentrations during spring and summer,as well as double high PM_2.5 and O₃（P-O）during the transition between cold and warm seasons.Due to the recent emissions control policies in China,nitrate（NO₃^-）has been found as the least reduced specie among the major components of PM_2.5,making NO₃^-as the dominant chemical component of PM_2.5.NO₃^-and O₃（N-O）are the core of the coordinated prevention and control of air pollution in the NCP and YRD regions.In this paper,the N-O coupling relationship and its influencing factors in the two regions were comprehensively analyzed by using the observation data and the Weather Research and Forecasting-Community Multiscale Air Quality（WRF-CMAQ）model.The analyses include the characteristics of N-O concentrations trend change,the correlation relationship,and the influence of internal chemical generation.The main conclusions are as follows:（1）The trend of O3 and NO3^-concentrations based on long-term observation data.The results revealed that during 2015～2019,the changes in the concentration of major criteria pollutants in the NCP and YRD were:PM_2.5（–26.1 and 20.7%）<NO₂（–4.6 and 1.8%）<O₃（27.2 and 27.3%）.The annual average and growth trend of O₃ were the highest;At the same time,the changes in NO₃^-trends of Beijing and Shanghai in winter were～4.6 and 0.5%yr^-1（p<0.01）,respectively,making NO₃^-a key component that could influence the further decrease of PM_2.5 concentrations.During 2013～2019,the trend of NO₃^-was:autumn<summer<winter<spring.The concentration of O₃ at nighttime and the nitric acid free radicals（NO₃）showed an upward trend in Beijing.In Shanghai,a significant downward trend shown at nighttime,especially in autumn(–6.0%yr^-1（p<0.01）).The correlation between N-O in the two municipal cities increased year by year,and the N-O synergy relationship between spring and autumn has turned from negative to positive in recent years,reflecting the intrinsic chemical titration effect of O₃ on NO₃^-production.The N-O concentration relationship was initially inversely correlated,then positively correlated relationship,and the transition concentration of O₃ was 100μgm^-3.It is speculated that N-O synergistic growth mechanism may exist at high O₃ concentration level.（2）The synergistic relationship between P-O and affecting factors.The correlation analysis was used to comprehensively identify the P-O synergistic relationship on timescales,and analyze the influence of NO₃^-components and meteorological conditions.During warm season in the YRD,the P-O correlation coefficients of the two timescales increased by 54 and70%,respectively from 2015 to 2019,reflecting that O₃ enhanced the formation of secondary PM_2.5.On the daily timescale,the P-O correlation was positive in seasons except autumn and winter in the NCP.The positive relationship in summer was influenced by the chemical components,due to the chemical reaction.In other seasons,NO₃^-dominated P-O relationship in the YRD.On the hourly timescale,the negative correlation between N-O led to the synergistic relationship in spring and autumn.The meteorological effects weaken the positive synergistic relationship（or enhance the negative synergistic relationship）,which was due to the adverse effects of relative humidity（RH）.Wind speed and planet boundary layer（PBL）height contributed to the meteorological effects,which enhanced the negative relationship in winter.（3）Modeling analysis of nitrate formation mechanisms and chemical processes with ozone.The CMAQ model was applied to investigate the contributions of regional transport,as well as the major pathways of total nitrate（TNO₃,i.e.,HNO₃+NO₃^-）production during four seasons in 2017.Process analysis revealed that aerosols（AERO）and total transport（TRAN）processes were the dominant sources of NO₃^-in Beijing and Shanghai within the PBL.OH+NO₂pathway dominated the TNO₃ production rates（59–84%）.The N₂O₅ heterogeneous pathway（HET N₂O₅）became more important（12–36%）,especially up to 68%in winter nighttime.Local emissions dominated YRD-regional NO₃^-concentration（50-62%）,while indirect transport contributed 24-37%.The regional transport impact indirectly produced TNO₃（transported O₃ reacts with local NO_x emissions）through the HET N₂O₅ pathway at nighttime.In the causes of NO₃^-and O₃ concentrations growth processes of cold and warm seasons,growth processes had specific meteorological characteristics.AERO and horizontal transport（HTRA）dominated the growth of NO₃^-（over 20 times）in winter.The TNO₃ was dominated by OH+NO₂pathway in Beijing,while HET N₂O₅ pathway dominated in Shanghai.In summer,the superposition of gas-phase chemistry（CHEM）and TRAN processes enhanced the rapid growth of O₃ and HNO₃ concentrations（over 40 times）,and the dominant pathway was OH+NO₂.（4）Evolutionary characteristics of NO3^-trend and driving factors.The simulation results showed that the wintertime NO₃^-trend from 2015 to 2019 in the NCP and YRD regions were 2.6 and 1.6%yr^-1,respectively,while the trend of NO₃^-/PM_2.5 were 6.9 and 4.2%yr^-1.In addition,the growth rates of wintertime the atmospheric oxidant capacity（AOC）in the two regions were 24 and 31%,respectively.AOC dominated the coordinated relationship of N-O.The wintertime change trend of NO₃ in two cities were opposite(–2.6 and 3.4%yr^-1).In the winter of 2015 and 2019,the main sources of NO₃^-（AERO）decreased by 14 and 10%in Beijing and Shanghai,respectively.Due to the rapid decline of Sa value,the chemical production of TNO₃ rates decreased significantly,and the contribution of HET N₂O₅ pathway also decreased significantly（up to–50%in Beijing）.The decrease in chemical production was the dominant driving factor for the decreasing trend of NO₃ concentration in Beijing.Though,the TNO₃chemical production also decreased in Shanghai,the meteorological parameters（such as WS and PBL）decreased significantly,and the weakened removal process of NO₃^-（TRAN）（such as a 52%decrease at nighttime）,leading to a rise trend of NO₃^-in Shanghai(3.4%yr^-1).Furthermore,this study systematically investigated the coupling relationship between NO₃^-and O₃,and its influencing factors in the two regions,including N-O through chemical formation and regional transport.The driving factors of NO₃^-trend in Beijing and Shanghai were chemical production and meteorological conditions.Overall,the simultaneous controlling of O₃ and NO₃^-in the larger scale region is also important for NO₃^-reduction in the two regions.