| Near-ground ozone（O3）is an important secondary pollutant,mainly generated through atmospheric photochemical reactions of volatile organic compounds（VOCs）with nitrogen oxides（NOx）.High concentrations of O3 pose a potential hazard to human health and ecosystems.Due to the complex non-linear relationship between the formation of O3 and its precursors,it is hard to control O3 pollution and make effective control measures.Thus the scientific guidance for controlling O3 is particularly important.With the implementation of a series of emission reduction measures in recent years,the emissions of VOCs and NOx from anthropogenic sources in Beijing have started to decrease,but the O3 pollution in Beijing is still severe.And O3 has become a major pollutant affecting the ambient air quality in Beijing,thus it is of great significance to study the causes and sources of O3 for mitigating O3 pollution in Beijing.In this study,based on real-time data from the Beijing air quality monitoring network and the enhanced observations of O3 and its precursors in different urban areas of Beijing during typical ozone pollution processes,we investigated the characteristics of O3 in Beijing from 2018 to 2020 and the causes and sources of O3 pollution in different urban areas of Beijing during typical ozone pollution processes by using an observation-based model（OBM）and combining the Texas Commission on Environmental Quality（TCEQ）method,ozone formation potential（OFP）,OH radical loss rate（LOH）,relative incremental reactivity（RIR）and positive matrix factorization（PMF）.The scientific issues related to the sources of HONO in O3 formation,its contribution to O3 formation,the O3 formation budget and atmospheric oxidation capacity also have been discussed.Based on the real-time data of the Beijing air quality monitoring network,the results indicate that the annual value of O3 was highest in 2019（97 ppb）and was the lowest in 2020（88 ppb）in 2018-2020 in Beijing.The annual average concentrations of O3 at the regional background sites were always been greater than 80 ppb,indicating that regional transportation may contribute significantly to the O3 concentration in Beijing.The trend of monthly mean value variation showed that the highest monthly mean value of O3 at all sites in 2018-2020 occured in June.The daily variations of O3 showed the highest O3 concentration at the Dingling site at night,indicating a high background concentration of O3 in Beijing.The statistics results of O3 pollution in 2018-2020 revealed that O3 pollution occured from April to September each year,and the most serious O3 pollution occured in June.From 2018 to 2020,the maximum daily 8 h average O3（MDA O3）and maximum O3 hourly concentrations have increased year by year.The total oxidant Ox（Ox=O3+NO2）concentrations of regional background and local formation in Beijing were calculated using the TCEQ method.And the contribution of regional background to Ox was significantly higher than that of local formation,but the contribution of regional background decreased in summer due to the enhanced photochemical activities.The analysis of the enhanced observation data of O3 and its precursors during the typical ozone pollution processes in Beijing from May to September 2019 and from June to September 2020 showed that the meteorological conditions of high temperature,low humidity,low wind speed and occasional rainfall were observed at the northern urban site（CRAES）during the observation period.O3 pollution were severe during the typical pollution processes,with the maximum hourly concentrations of 170.0 ppb and 121.4 ppb for O3 and 92.1 ppb and 54.0 ppb for NO2 in 2019 and 2020,respectively.The concentrations of O3 and NO2 were lower during the pollution processes in 2020 compared to those in 2019.Comparison of VOCs concentrations at the CRAES site and another central site（CGZ）site in Beijing showed that the concentrations of alkanes were the highest among VOCs in Beijing,followed by alkenes or aromatic hydrocarbons.The monthly average concentrations of VOCs at the CRAES site were lower than those at the CGZ site in 2019,but consistently higher than those at the CGZ site in 2020.Formaldehyde,acetone,and acetaldehyde were the top three species in OVOCs in Beijing,with a total contribution of more than 80%.Similar to the changes in O3 concentrations,OVOCs concentrations decreased in 2020 compared to those in 2019.Concentrations of the maj or OVOCs species increased sequentially from north to south.In order to study the sources of HONO during O3 pollution period and the influence on the formation of O3 pollution,a comparative study on the ambient levels,diurnal variation,the sources in daytime and nighttime,and the formation mechanisms were investigated for a haze pollution period and an ozone pollution period using HONO observation data from the CRAES site in summer 2019.Compared with that of during the haze pollution period,a lower HONO concentration level was found during the ozone pollution period.The emission from vehicle exhaust was an important source of nocturnal HONO formation.The homogeneous production rate of NO and OH and the heterogeneous conversion from NO2 were lower during the ozone pollution period than those during the haze pollution period.However,based on the analysis of HONO budget,it was found that the daytime unknown source during the ozone pollution period was higher than that during the haze pollution period.Correlation analysis implied that the photo-enhanced NO2 conversion on the aerosol surface might be a potential source for daytime HONO.Without HONO constraint,the OBM model would largely underestimate the averaged daytime atmospheric oxidative capacity（24%）,OH production rate（57%）,and net O3 production rate（20%）.The study results further demonstrated the necessity for clarifying the formation mechanism of HONO to improve the understanding of the influence of HONO to atmospheric chemical processes.The study on the causes of O3 pollution in different urban areas during the typical pollution processes in Beijing found that when the meteorological conditions exhibited a temperature greater than 25℃,humidity less than 60%,wind direction was south and wind speed less than 2 m/s,the hourly concentration of O3 was prone to exceed the standard.The sensitivity analysis of O3 formation by the RIR method showed that the two urban sites were basically under the VOCs control during the observation period.Reduction of anthropogenic VOCs（alkenes and OVOCs）was an effective way to reduce O3 concentrations.The in-situ O3 photochemical budget analysis showed that HO2+NO reaction dominated O3 formation,followed by RO2+NO reaction.Without HONO constraint in the model The photolysis of O3 and photolysis of formaldehyde were the main sources of OH radicals and HO2 radicals,respectively.The O3 formation rate,atmospheric oxidation capacity and HOx radical formation rate at the CRAES site were consistently higher than those at the CGZ site,indicating that different urban areas of Beijing have different atmospheric photochemical activities.By the analysis of OFP and LOH,OVOCs and alkenes make significant contributions to O3 formation.Among them,isoprene,ethylene,propylene,formaldehyde,acetaldehyde and butenal were the key O3 precursors.In order to control O3 pollution effectively,the sources of O3 pollution in different urban sites during typical pollution processes in Beijing were further analyzed.The results of the OBM model analysis showed that the O3 at both CRAES and CGZ sites was dominated by local photochemical formation.The sources of VOCs including vehicle exhaust,diesel vehicle exhaust,oil and gas volatilization,solvent use,industrial process and biogenic emission were identified at both sites by using PMF.Among them,vehicle exhaust,industrial process and oil and gas volatilization were the main VOCs sources at both sites.Further analysis with OFP,LOH and RIR showed that anthropogenic sources such as vehicle exhaust,solvent use and oil vapor volatilization had more significant effects on O3 generation.Among them,vehicle exhaust and solvent use was the primary source controlled at CRAES and CGZ sites,respectively.Overall,the O3 pollution in Beijing is still severe.In order to effectively reduce ambient O3 concentrations,continuous NOx reduction and differentiated VOCs pollution control measures should be adopted in different urban area of Beijing based on the analysis of their own O3 pollution causes.Beijing should focuse on controlling VOCs emission sources such as vehicle exhaust,solvent use and oil and gas volatilization,reducing the concentration of reactive VOCs（mainly alkenes and OVOCs）,thus reducing the atmospheric oxidation capacity and local O3 generation capacity,and reducing the impact of O3 local formation on O3 pollution.At the same time,Beijing should also actively establish a regional joint prevention and control mechanism for O3 pollution to minimize the contribution of regional transport to O3.The results of this thesis will help to improve the understanding of the formation mechanism of O3 pollution in the urban areas of Beijing in the past two years,and provide a scientific basis for the Beijing government to develop more effective O3 pollution control measures. |
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