Sensitivity And Evolution Of Atmospheric Ozone Formation To The Ozone Precursors In The Petrochemical Industrialized Lanzhou City

Heavy ozone（O₃）pollution has been often observed in the petrochemical industrial city of Lanzhou,located in the semi-arid and mountainous northwest China.The mountain-valley topography,special meteorological conditions,and high precursor emissions from the petrochemical industry across the city caused considerable high surface ozone concentrations in recent years.From April 25 to June 15,2019,Lanzhou Petrochemical Company,the largest VOCs emission source in western suburban Xigu District and downtown Lanzhou,launched an instrument overhaul project that shut down many instruments,resulting a significant reduction of VOCs concentration.We referred this period to be the‘overhaul period’,and the period of June 15 through August 31,2019,to be‘post overhaul period’in this period.We then examined the sensitivity of ozone production to its precursor emission,during the post-overhaul period and overhaul period.In this thesis,based on online monitoring data and online weather monitoring data from April 25 to August 31 in 2018 and 2019,the ozone iso-concentration curve model（Ozone Isopleth Plotting Package Research,OZIPR）is implemented to stimulate the ozone formation.The Empirical Kinetic Modeling Approach（EKMA）was employed to estimate the sensitivity of ozone formation to its precursor emission and atmospheric oxidation.We also used a photochemical-age-basement patermater approach to estimate the consumed concentration of VOCs and Maximum Incremental Reactivity（MIR）method and the Prop-Equiv method to calculate the Ozone Formation Potential（OFP）.Foremore and the Regional Atmospheric Chemical Mechanism Version2（RACM2）Model to examine the chemical processes of atmospheric free radicals.Finally the coupled Weather Research and Forecasting model and the Community Multiscare Air Quality（WRF-CMMAQ）model was used to examine the ozone atmospheric transport process and explore the intrinsic dynamics of ozne and its precursor（NOx and VOCs）concentration and major atmosphere free radicals（OH,HO₂,and RO₂ free radicals）concentration.The main conclusions are outlined below:（1）The impact of large overhaul on ozone and its precusors concentrations in Xigu Industrial District,Lanzhou City:During the overhaul period in 2019,averaging O₃＿8h concentration increased from 139.21μg/m~3 to 157.75μg/m~3 after the overhaul period.Compared with the same period in 2018,O₃＿8h averaging concentration was reduced by 13.72%during the overhaul period,and also decreased 1.49%after overhaul period,respectively.The NO₂ concentration was 38.48μg/m~3 during the overhaul period,10.20%lower than that during the same time in 2018.The TVOC（total VOC）concentration was 70.11 ppb during the overhaul period in 2019,48.26%higher than that the same period of 2018,but felling to 30.69 ppb after the overhaul period（about 56.23%）.（2）The transition from a NOx-limited regime to a VOCs-limited regime in Xigu Industrial District:In summer 2018,the VOCs/NOx ratio defined in the ridgeline of the EKMA curve was 13.8:1,indicating that ozone was formed in the NOx-limited regime,whereas in summer 2019,this ratio in the ridgeline curve was 15:1,indicating that ozone was formed in the VOC-limited regime.These results differ from a previous study,which reported that O₃ was formed in the NOx-sensitive regime in XG.（3）The character of TVOC consumed concentration in Xigu Industrial District:The mean TVOC concentration in the summer of 2018 was 79.16 ppb,the concentration of alkanes,alkenes,alkynes,and aromatics were 47.26 ppb,25.11 ppb,0.83 ppb,and5.96 ppb,respectively.The consumed TVOC concentration was 21 ppb,and the consumed concentration of alkanes,alkenes,and aromatics were 3.46 ppb,17.30 ppb,and 0.24 ppb,but the consumed concentration of alkyne was almost zero.Observed mean TVOC concentration in the summer of 2019 was 41.40 ppb,the concentration of alkanes,alkenes,alkynes,and aromatics were 22.89 ppb,12.82 ppb,1.38 ppb,and 4.31ppb,respectively.The consumed concentration of TVOC was 6.12 ppb,whereas the atmospheric levels of alkanes,alkenes,and aromatic hydrocarbons were 0.69 ppb,5.06ppb and 0.37 ppb,respectively,but the alkyne hydrine was almost zero.Using the MIR method,the VOCs species with the highest OFP were alkenes,accounting for 88%and75%in the summer of 2018 and 2019,respectively.Using the Prop-Equiv method,the VOCs species with the highest OFP were alkenes,accounting for 78.31%and 84%in the summer of 2018 and 2019,respectively.Although the observed mixing ratio of alkenes was lower than alkenes,the chemical reaction of alkenes was the strongest among the four VOCs species.Relatively higher alkenes mixing ratio was attributed to fast reactions between OH radical and active alkenes.In summer 2019,observed TVOC oncentration and photochemical consumed concentration were,in general,low than2018.（4）The character of major atmospheric free radicals concentrations in Xigu Industrial District:The concentration of OH free radicals reached 12.4×10~6 molecule/cm~3 and 10.3×10~6 molecule/cm~3 in summer 2018 and 2019,perspectively,markly higher than that in summer 2013 estimated using the MCM model simulation（5×10~6molecule/cm~3）.This result indicates that the atmospheric oxidation capacity of Lanzhou City continues to increase over time.Compared to the summer of 2018,the concentration of OH free radicals in 2019 increased by 24.7%.（5）The atmospheric chemical process of ozone and its precusors,and major free radicals in Xigu Industrial District:Our WRF-CMAQ simulations revealed that NOx peak took place at 9:00 am and 18:00 pm（LST）than other times of a day,which could be attributed,to a large extent,to traffic rush hours and active stationary combustion sources.The higher TVOC concentrations at 09:00 am in doentown Lanzhou were not present in western suburban XG but to the southwest of this suburban region.Higher OH concentrations also occurred in the afternoon,and higher HO₂ and RO₂concentrations were identified in the morning from 09:00 to 12:00 am.