| Tropospheric ozone（O3）is not only an important greenhouse gas and oxidant,but also a major air pollutant,which is detrimental to human health,crop yields and the ecological environment.Developed countries,such as Europe,America,and Japan,have paid more attention to O3pollution,as a result of a more complete O3control policy and stricter O3environmental air quality concentration standard.In recent years,China suffers from serious O3pollution problem,which has gradually attracted people’s attention.Beijing,the capital of China,also suffered from severe O3pollution.Therefore,this study aims to provide a scientific basis for prevention and control of O3pollution in Beijing through study the characteristics,sources and formation mechanism of O3pollution in Beijing.In this study,in order to understand the spatial and temporal variation characteristics of O3pollution in Beijing in recent years,characteristics of temporal variation and spatial distribution of O3in Beijing from 2015 to 2017 will be first studied.Furthermore,to explore the sources and formation mechanism of O3pollution in summertime of Beijing,continuous measurements of meteorological parameters,O3,nitrogen oxide（NOX）and carbon monoxide（CO）were conducted at an urban site in Beijing from July 15 to August 10,2017.In addition,offline one-hour whole air samples were collected for analysis of volatile organic compounds（VOCs）and oxygenated VOCs（OVOCs）on 7 selected days（i.e.20 and 30 July,and 1 and 4-7August 2017）.The main conclusions were obtained as follows:（1）From 2015 to 2017,the days of up to the National Standard in Beijing accounted for 50.7%,52.7%and 61.1%of the total monitoring days,respectively.Therefore,the ambient air quality of Beijing has been improved.However,the days of the mean daily8-hour maximum concentration of ozone accounted as the primary pollutants account for 31.7%,31.8%and 42,3%of total pollution days,respectively.The problem of O3pollution was increasingly prominent.And photochemical smog pollution was the main type of air pollution.O3pollution mainly occurred between May and September,with the highest value in summer,followed by spring,autumn and winter in Beijing at present,which showed seasonal variation under the influence of temperature and solar radiation.Hourly variation of O3showed a clear unimodal pattern with its peak around16 o’clock.The daily average O3concentration exceeded/approached 160μg/m3between 14:00 and 18:00 in summer.（2）The traffic pollution sites showed the lowest O3average concentration（78.5μg/m3）in summer,which is 18%lower than the average O3concentration in Beijing（95.6μg/m3）at the same time.Urban control sites,suburban sites and urban sites are characterized by heavy pollution sites in summer with the average O3concentration of100.45μg/m3,99.54μg/m3 and 96.83μg/m3,respectively.The“weekend effect”appeared at the Olympic Sports Center site,Miyun Reservoir site and Yongdingmen site,while Changping site and Dingling site showed an anti-“weekend effect”.（3）The results of field observation experiment between July 15 and August 10 in2017 with the O3episodes accounting for 51.8%indicated that O3episode days and exceeding days were dominated by southeast wind,associating with high temperature,low relative humidity,low pressure,strong solar radiation and high levels of precursors（i.e.VOCs,CO and NOX）.O3pollution of sampling site was mainly influenced by air masses original from southeast,southerly and northerly/northwest and the average overbid O3concentration in cluster analysis with 24h backward trajectories is（113.64±18.02）ppbv,（92.22±14.31）ppbv and（84.54±3.13）ppbv,respectively.The average concentration of O3exceeding the standard for 48h backward trajectories clustering was similar to 24h’s.There was no excessive trajectory from the northeast,which characterized as clear masses.On the VOCs sampling days,high O3episodes were dominantly controlled by the Western Pacific Subtropical High or uniform pressure field（weak high pressure）,as well as air masses regional transport from northwest and southerly.（4）Alkenes+Alkynes showed the largest contribution to ozone formation potential（OFP）and propene-equivalent concentrations（PE）,accounting for 59.3%and 50.5%of total PE（9.07±0.61 ppbv）and total OFP（107.02±10.86 ppbv）,respectively.The key active species of VOCs were propene,isoprene,formaldehyde,ethene and acetaldehyde,accounting 24.3%,16.1%,11.6%,6.8%,5.5%of total PE and 24.1%,2.8%,26.2%,16.0%,5.4%of total OFP,respectively.（5）The simulation results of PBM-MCM model was reasonably（the index of agreement is 0.88）.The results indicated that variation of O3concentration（in volume）on July 20 and August 1 and 6 were mainly from local formation.Other four O3episode days（July 30 and August 4,5,and 7）was affected by both local formation and regional transport.The maximum regional contribution to O3for these 4 days was 40%,50%,53%,and 46%,respectively.The sensitivity analysis revealed that O3formation in Beijing was mainly sensitive to VOCs,especially formaldehyde,propene,m/p-xylene,ethene and cis-2-butene from anthropogenic VOCs（AVOC）emission.（6）Compared with normal days,O3 episode days were more sensitive to O3formation,which showed higher OH radical concentration,reflecting higher photochemical reactivity on episode days.（7）O3 productions with different levels of NOX and VOCs were simulated using the PBM-MCM model.The results showed that the effectively control O3 pollution with the reduction ratio between VOCs and NOX should be higher than 1.02.The above results indicated that effective control of AVOCs emissions（especially formaldehyde,propene,m/p-xylene,ethene and cis-2-butene）in Beijing and the surrounding cities may help to reduce O3levels in Beijing.In addition,the reduction ratio of VOCs to NOX concentration should not be low than 1.02 would be effectively for control O3 pollution. |
