Reactivity-driven Identification Of Ozone Formation Regimes(OFRs) And Its Impact On Ozone Control Strategies

In recent years,ambient fine particulate matter(PM_2.5)have been curbed and even reversed in China.However,surface ozone(O₃)pollution worsened over the same period resulting in China being a hot spot of surface O₃pollution.At present,the formulation of O₃control strategies is based on EKMA isopleth diagram which illustrates the non-linear relationships between O₃and its precursors.The diagram can essentially identify the O₃formation regime(OFRs)belongs to VOCs-limited or NOx-limited.Thus,according to the response relationship between O₃and the reduction ratio of precursors,the optimal solution for the emission reduction of precursors can be proposed.However,this method is insufficient for the effective O₃control policy as it failed to take some critical issues into consideration,including 1)the EKMA isopleth diagram cannot demonstrate the dynamic fluctuation of ozone formation regimes 2)The identification of OFRs based on precursors is out of touch with the actual O₃control strategies 3)and the VOCs emission based on components cannot cover some undetected species which have high reactivity.Thus,the precursors-based OFRs identification cannot reflect the impact of these undetected reactive components on atmospheric chemical process and the OFRs.Targeting on these three issues which closely relating O₃control,this study focuses on the following four aspects,by using WRF/SMOKE/CMAQ modeling system with the latest Pearl River Delta(PRD)high-resolution emission inventory.Firstly,we propose a novel approach to diagnose OFRs based on the understanding that atmospheric oxidation capability(AOC)is the core driving factor of secondary pollution.By conducting a series of sensitivity experiments,we explore the interrelationship between atmospheric radical and spatio-temporal variations of OFRs during an O₃exceedance event in September 2017 in the PRD.The result shows that the HO₂/OH performs as a good indicator to separate NOx-and VOC-limited regimes,in comparison with other photochemical indicators.The enhancement of HO₂/OH ratio resulting in the OFRs dominated by NOx-limited.Otherwise,the OFR is dominated by VOCs-limited when HO₂/OH ratio decreased.There is a spatial difference in the HO₂/OH threshold which correspond to the OFR shifts.Generally,the HO₂/OH ratio is lower in the central west of PRD which located in NOx-limited OFR,while the HO₂/OH ratio is higher in the east and south of PRD where is dominated by VOC-limited regime.Compared with other photochemical indicators,the ratio of HO₂/OH performed better and can be applied to fast OFRs identification.Rapid OFRs identification is the foundation of identifying OFRs'dynamic change.Given the fluctuations of OFRs,forecasting the dynamic change of OFRs is indispensable for O₃control.We delineated the non-linear relationship between peak O₃levels and precursors in Shanghai during the entire month of July 2017 when O₃pollution was frequent and intense.The OFRs shows a great spatiotemporal variation during the O₃pollution episode.Local OFRs shifted between VOCs-limited,NOx-limited and transitional,indicating a clear connection between large-scale circulation(LSC)patterns.In the process of Western Pacific Subtropical High(WPSH)moving from South China Sea northward,wind field over Shanghai changed from weak westerly to moderate southwesterly and to the one without an obvious direction.Meanwhile,local OFRs shifted from AVOCs-limited to NOx-limited and to transitional.During the episode when spatial heterogeneity of OFRs was relatively larger,western Shanghai was more in a NOx-limited regime while northern,central and eastern Shanghai were in VOCs-limited regime.Current technology can support LSC features to be accurately forecasted 48-72 hours in advance.Hence,the OFRs identification approach associated with the LSC pattern can realize fast OFRs forecasts.This will save substantial time for adjusting O₃control strategies.This innovative approach has been initially applied in the emergency management and control of O₃pollution in Shanghai.Given that there is a gap between O₃precursors control which rely on precursors-based EKMA isopleth diagram and actual emission sources control policy,we for the first time developed source-based O₃isopleth and iso-surface diagrams.Taking the two cities of the Pearl River Delta,Guangzhou and Foshan as examples,the necessity and importance of source-based O₃isopleth and iso-surface diagrams in the formulation of O₃control strategies are illustrated.Although the precursors-based EKMA isopleth shows that the two cities should adopt similar precursor emission reduction strategies,but the result of iso-surface based on the emission sources proposes a completely different O₃control strategies.Guangzhou should give priority to the control of solvent,while Foshan requires a 3:1:2 coordinated emission reduction of mobile,solvent and combustion source to achieve better O₃control effects.Moreover,aiming at the dilemma that some reactive species cannot be undetected which leads to uncertainty of the emission inventory and OFRs,this study takes diesel vehicles as the research subject to quantify the total OH radical reactivity and to estimate the missing reactivity.We propose an approach to incorporate the missing reactivity into the emission inventory of reactive organic species,and built an reactivity-constrained emission inventory for diesel vehicles.We found that diesel vehicle's missing reactivity ranging from 30 to 70%.The simulation which applied the reactivity-constrained inventory shows that the O₃peak value enhanced after the restriction of missing reactivity.Taking Guangzhou and Foshan for example,the simulated O₃concentration of Guangzhou elevated 0?2.5ppb,and Foshan raised between 0?1ppb.Due to the enhancement of the simulated value,the simulation effect of O₃concentration in Guangzhou and Foshan has been improved.The NMB and NME in Guangzhou increased by 0.29 and 0.09 respectively,while the Foshan increased by 0.19 and0.03 respectively.Meanwhile,the OFRs has shifted from VOCs-limited to transition,T values of OFRs in Guangzhou increased from-1.18 to-1.16,and increased from-0.78 to 0.59 in Foshan.Although it is difficult and uncertain to measure emission source reactivity at present,this study believes that the reactivity restriction is the future development direction of the high-resolution emission inventory.The application of measuring reactivity in emission sources is expected to provide new ideas for identifying reactive organic species characteristics and detecting its atmospheric chemical processes.This will provide scientific support for accurate O₃ control strategies.