| With the implementation of emission reduction policies of air pollutants,surface ozone（O3）concentration presents a trend of rising instead of decreasing in China.At present,O3 has become a critical pollutant for air quality improvement in the Yangtze River Delta（YRD）region.It is difficult to prevent and control O3 pollution due to the complex non-linear response relationship between O3 and its precursors（volatile organic compounds（VOCs）and nitrogen oxides（NOx））.Hence,it is particularly important to clarify the causes of O3 pollution.The observation-based model（OBM）and the air quality model（CMAQ）are adopted in this study to systematically analyze the non-linear response relationship between O3 and its precursors,source contributions to O3,the determination for O3 pollution process,and the impact of anthropogenic and non-anthropogenic factors on the change of O3 concentration during summertime in YRD,aiming to provide scientific and efficient policy reference for local ozone pollution control.The main conclusions are as follows:（1）A source-oriented CMAQ model was used to quantify the contributions of different sources to ground-level O3 over the YRD region during the EXPeriment on the e Lucidation of the atmospheric Oxidation capacity and aerosol fo Rmation,and their Effects in the Yangtze River Delta（EXPLORE-YRD）campaign.O3 formation attributed to VOCs（O3＿VOCs）plays a more important role than that attributed to NOx（O3＿NOx）in the core areas of YRD（including Shanghai,southern Jiangsu province,Hangzhou Bay area and Hefei）.O3＿VOCs contributed81.1%,78.5%,60.2%and 55.1%in Shanghai,Nanjing,Hefei and Hangzhou,respectively.The contribution of O3＿NOx in other YRD areas is generally higher than that of O3＿VOCs,indicating that O3 formation is more affected by NOx in these regions.The contribution of NOxand VOCs emissions from industry to O3 is 41%-50%and 60%-83%respectively,higher than that from transportation,accounting for 24%-40%and 12%-18%,respectively.Power generation and biogenic source are the third important source for O3＿NOx and O3＿VOCs,respectively.The anthropogenic emissions dominate the non-background O3 in the YRD（more than 90%）.For the five major cities in YRD,the industrial sector is the largest emission source for O3 production（52%-75%）,followed by transportation（13%-25%）.Compared to other types of pollution episodes(fine particulate matter(PM2.5)pollution during this campaign,both O3-PM2.5 pollution),the contributions of biogenic emissions and open burning to O3produced are relatively higher during high O3 episodes.（2）The OBM,which coupled with the Master Chemical Mechanism version 3.3.1（MCMv3.3.1）,was applied to conduct the phototchemical simulations to explore the O3formation as well as the relationship between O3 and its precursors in the Nanjing metropolitan area of China in summer 2015.The results based on observations of VOCs concentrations indicate that,alkanes are the most abundant VOCs,accounting for 46.8%among the total VOCs.Aromatics contribute the most to the ozone formation potential（OFP）（61.6%）although the concentrations of aromatics are relatively low,and m/p-xylene has the highest OFP（over 30μg/m3）comparing to other VOCs.The relative incremental reactivity（RIR）results of precursors during summertime at urban site in Nanjing demonstrate that,O3 formation is in VOCs-limited regime.O3 formation is more sensitive to anthropogenic VOCs（AVOCs）,and O3 pollution can be aggravated if the emission reduction of NOx alone occurred.The RIR value of m/p-xylene is still the highest among all observed VOCs.Combined with the results of OFP,it shows that m/p-xylene is a key precursor for O3 formation.The results of the variation of O3concentration with the emission reduction of precursors show that,a cutting ratio of anthropogrnic AVOCs to NOx more than 0.46 is proposed to implement under the condition that local O3 concentrations remain unchanged.（3）The OBM and CMAQ were used to diagnose the causes of nearly one-week regional O3pollution event occurred in the YRD region during 2020 summertime.The RIR analysis concludes that O3 formation in Nanjing is co-limited by VOCs and NOx and is more sensistive to NOx.For O3 formation,trans-2-butene and m/p-xylene are the critical alkenes and aromatics,respectively.The O3 source apportionment in Nanjing suggests that the transportation contributes the most to the O3 production（52%）,followed by industry（24.7%）.The contribution of O3＿NOx（～70%）to O3 production is significantly higher than that of O3＿VOCs（approximately 30%）.The process analysis reveals that the process of vertical mixing increases the surface O3 concentrations in the early morning,and photochemical reactions promote O3formation and accumulation during the daytime within the planetary boundary layer.The contributions of inter-city transport during the O3 pollution period indicate that the predicted O3 concentration is largely recorded from long-distance regions,reaching 46%,followed by local sources（38%）and surrounding cities（16%）.（4）Compared with 2015,the observed concentration of O3 in Nanjing decreased by about19.1μg/m3during summertime in 2020.The changes in meteorological conditions contribute8.4μg/m3（accounting for 44%）to the decrease of O3,and the O3 concentration decreases by10.7μg/m3（accounting for 56%）due to the change of anthropogenic emissions.Temperature,relative humidity（RH）and wind speed are key meteorological factors affecting the O3formation.On the one hand,compared with 2015,the low temperature and high RH in early August as well as the clean air mass brought by the strong wind during mid-August in 2020 are not conducive to the formation and accumulation of O3,resulting in a drop of O3by 17.4μg/m3.On the other hand,the weather conditions of the high temperature and low RH in the medium term of August 2020 maybe promoted the natural emissions of VOCs and NOx（up by 27.6%and 28.8%,respectively）in Nanjing,causing the O3 concentration increase by 9μg/m3.Compared with 2015,the anthropogenic emissions of VOCs and NOx in Nanjing in August2020 decrease by 7.8%and 11.7%,respectively.The contribution of O3＿NOx is greater than that of O3＿VOCs,reaching 59.3%abd 40.7%,respectively.The longer hydroxyl radical（OH）chain length and higher ozone production efficiency（OPE）indicates that the reduction of anthropogenic emissions accelerates the NOxcycle and makes O3 more sensitive to NOx.O3formation has shifted from VOCs-limited in 2015 to a transition regime jointly controlled by VOCs and NOxin 2020,hence,the concentration of O3 decreases with the reduction of precursor emissions. |
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