Pollution Characteristics And Sources Of Atmospheric Ozone And Its Precursors(VOCs) In Wuhan,Central China

In recent years,the atmospheric ozone pollution has been exacerbated in China,and it has become the primary pollutant in summer ambient air quality.In 2018,the average concentrations of PM_2.5,PM₁₀,SO₂,NO₂,and CO in 338 prefecture-level cities and the proportion of excess days decreased compared to 2017,while only the proportion of O₃ concentration and excess days increased.The near-surface ozone is seriously harmful to human health,animals,plants,and the ecological environment.The ozone problem has been become the core and key link of China’s air pollution problem.Wuhan,the capital city of Hubei province,is one of the most important centers for industry,education,and economy in central China.Since 2013,a series of measures have been taken to prevent and control air pollution and achieved certain results.The concentration of particulate matter has decreased,but ozone pollution has increased year by year.This article analyzes the pollution characteristics of O₃ and its precursors in Wuhan based on O₃ and its precursors,meteorological factors online and offline and other multi-source datas.The datas were analyzed using the Observational Based Model（OBM）,Weather Research and Forecasting-Comprehensive Air Quality Model Extensions（WRF-CAMx）and Positive Matrix Factorization Model（PMF）.This study was to research the pollution characteristics,sources and contribution to ozone of point VOCs in different functional areas in Wuhan.The ozone compliance schemes under different emission reduction scenarios were simulated,and the causes and sources of typical ozone pollution processes were clarified.Through the above research,it is helpful to deeply understand the pollution characteristics and causes of O₃ and precursors in Wuhan,so as to scientifically and effectively formulate pollution prevention and control measures for O₃and precursors.Seasonal variations of O₃,NO_X and CO were investigated using the data observed from 2016 to 2019 in Wuhan.It was found that the ozone concentration was highest in late spring（May to June）and early autumn（August to September）.CO and NO₂present low pollution in summer and high pollution in winter.The concentration of ozone was high outside the urban area,while high concentration area of NO₂ was in the central urban area.Ox in Wuhan increases year by year,and the atmospheric oxidizability in the central city is higher than that in the suburbs.The high concentration of PAN appeared from May to June,with an average concentration of0.77ppbv.The concentration of NOx and CO on exceeding days and the non-exceeding days were low in the afternoon and high in the morning and evening peak hours.The concentration of NO₂ on exceeding days was significantly higher than that on non-exceeding days.There are six main types of ground weather systems that exceed the ozone standard:pressure equalization field,high pressure ridge,high pressure center,low pressure trough,low pressure center,and typhoon periphery.The pressure equalization field accounts for 59%of the summer ground weather situation in Wuhan,followed by the high-pressure system,which accounts for 23%.The main circulation situation is divided into five categories:high-pressure control type,no obvious large-scale circulation system control type,two grooves and one ridge type,typhoon peripheral circulation type and low pressure control type.The key meteorological factors for O₃ generation are temperature difference no less than 8℃and maximum temperature no less than 27℃,daily minimum relative humidity less than or equal to 50%,average wind speed less than 1.5m/s,and ozone exceeding standard rate of 67%.The chemical causes of O₃ generation were different in different months.In April,May and July 2018,ozone generation in Wuhan was controlled by VOCs and NOx.In June,August and September,ozone generation was controlled by VOCs.Reducing VOCs and CO helps reduce the production of O₃.A study on the control effect of O₃ at different reduction ratios of NOx and AVOC in different months was found that the reduction of local photochemical generation of O₃ in summer requires the reduction of AVOC emissions.The OBM model and the WRF-CAMx model were used to simulate the two ozone heavy pollution processes.The ozone heavy pollution process in June was caused by the local generation of photochemical reactions.VOCs was the main control factor for ozone generation.The reduction of 17.5%of anthropogenic VOCs can achieve ozone standards.m/p-xylene,1,3-dioxane Ethylbenzene,o-xylene and isoprene were the key VOC in ozone generation.The main cause of ozone pollution in September was the local generation of superimposed regional transport.Out-of-province Anhui,Henan and other ozone generators have made great contributions.In addition to Wuhan,Huanggang,Ezhou and Huangshi have made certain contributions.When the concentration of ozone was higher than 200μg/m³,it was mainly due to the contribution of Hubei Province and Wuhan City.Based on the offline sampling of VOCs from 9 sampling points（urban area,industrial park point,logistics area point and background point）from April 2018 to March 2019,combined with the PMF source analysis model,The concentration level,chemical composition,spatiotemporal distribution and source of VOCs were studied in different functional areas.The average concentration of VOCs was28.31±12.21ppbv.The highest volume concentration of a single species was acetone（4.37ppbv）,followed by ethane（3.52ppbv）and propane（3.15ppbv）.The concentration of VOCs was highest in winter and lowest in summer and autumn.Alkanes,alkenes,alkynes,aromatic hydrocarbons and halogenated hydrocarbons were highest in winter and lowest in summer.Affected by meteorological conditions and secondary conversion of photochemical reactions,OVOCs were highest in summer and lowest in winter.High-value areas of alkanes,olefins,OVOCs,and halogenated hydrocarbons are mainly concentrated in the central urban area affected by traffic sources and industrial production.Due to emissions from upstream industrial areas,high-value areas of olefins were located in the east of Wuhan.The propane concentration range at different points was 2.1ppbv～5.21ppbv.The central urban area was more affected by catering and transportation sources.The propane concentration was higher than that in the far urban area and background points.Through the analysis of the sources of VOCs in different functional areas in different seasons,it was found that the main sources of VOCs in Wuhan included six categories,which were motor vehicle exhaust（27.3%）,industrial emissions（19.7%）,combustion sources（17.0%）,and solvents use（16.3%）,gasoline volatilization（11.5%）and plant emissions（8.2%）.The relative contribution of motor vehicle exhaust to VOCs was between 15%and 41%.The contribution of plant emissions and gasoline volatilization in summer was significantly higher than other seasons.The contribution of plant emissions in summer was up to 25%,and the combustion source was higher in spring.The source of solvent use was relatively high in autumn and winter.Among the VOCs,ethylene,m/p-xylene,toluene,propylene,o-xylene,isoprene,acetone,acrolein,isopentane and n-butane were the key species that affect the local production of ozone.The focus of ozone prevention and control in Wuhan was to give priority to the control of key species such as ethylene,m/p-xylene,toluene,propylene and other highly active olefins and aromatic hydrocarbons.The main sources of pollution controlled were vehicle exhaust,solvent use sources and industrial emissions Source.