Seasonal Characteristics And Sources Of Composite Air Pollution In Wuhan

In the past ten years,the characteristics of urban and regional air pollution in China have changed significantly,from single soot-type pollution to a combination of coal,motor vehicle exhaust and dust.The composite air pollution problems with PM_2.5 and O₃ as the primary pollutants become increasingly prominent.Based on the monitoring data from March 2017 to February 2018,the spatial and temporal distribution characteristics of air pollution and the causes of composite pollution weather in Wuhan are analyzed.The PMF model,HYSPLIT model and PSCF model are used to explore the local sources,external sources and potential sources of Wuhan.The main results are as follows:Wuhan show obvious seasonal and regional characteristics of air pollution.Summer show optimal air quality（AQI=66）and the pollution characteristic of O₃ is remarkable.The air quality in spring（AQI=92）and autumn（AQI=80）are relatively good,showing the combined pollution characteristics of O₃ and PM_2.5.However,winter show the worst air quality（AQI=114）and the obviously pollution characteristics of PM_2.5.The concentration of major pollutants in Wuhan is higher than the national average,which is equivalent to the levels in Beijing,Tianjin and Chengdu,and significantly higher than Shanghai and Guangzhou.The air quality in Hongshan District and Xinzhou District is relatively good,while in Qingshan District and Qiaokou District are relatively poor.The regional characteristic radar chart shows that the pollution characteristics of the 13 administrative districts in Wuhan can be roughly divided into six types:comprehensive type,secondary type,motor type,coal type,coarse particle type and ozone type.Most areas in Wuhan exhibit a complex pollution characteristic,which is caused by multiple pollution factors.During the monitoring period,there were 9 pollution incidents with PM2.5 as the primary pollutant and 5 pollution incidents with O₃ as the primary pollutant.The pollution degree of PM2.5 was heavier and the pollution degree of O3 was lighter.The high incidence period of PM_2.5 pollution is from November to February of the second year,while the high frequency period of O₃ pollution is from April to September.The results of pollution analysis show that the emission and secondary conversion of precursors are the main internal causes of polluted weather,and the meteorological conditions for unfavorable diffusion are external causes.The low temperature and high humidity environment in the PM2.5 pollution process effectively promoted the conversion of secondary inorganic ions and the formation of secondary organic aerosols.In high temperature and low humidity environments during O₃ pollution,VOCs have a large potential for ozone generation.Wuhan is located in the VOCs sensitive area of ozone-forming.Aromatic hydrocarbons have the greatest contribution to the formation potential,followed by olefins and alkanes,with an average contribution of 57%,26%and 17%,respectively.The source analysis results show that the main sources of PM2.5 in Wuhan are:steel and coal industry（26.4%）,dust（5.4%）,motor vehicle emissions（29.3%）,secondary aerosol（29.3%）and biomass burning（9.6%）.The main sources of VOCs as an important precursor of O₃ in Wuhan are:combustion source（23.4%）,motor vehicle source（31.5%）,solvent usage（10.2%）,industrial emissions（22.5%）,gasoline volatilization（8.7%）and biological sources（3.7%）.The backward trajectory analysis shows that the PM2.5 polluted process is mainly affected by the long-distance transport air mass in the northwest,and the O₃ pollution process is mainly affected by the short-distance transport air mass in the northeast and the south.The main pollution transmission routes in Wuhan include the northwest route,the southern route and the easthern route.The potential source area analysis shows that the potential source areas of PM_2.5 in Wuhan are mainly gathered in western Henan,western Shaanxi,southwestern Shanxi,southwestern Jiangxi and northeastern Hubei.The potential source areas of O₃ are mainly gathered in southern Jiangsu,southeastern Anhui,northern Zhejiang,northwestern Jiangxi,northeastern Guizhou,western Guangdong and eastern Hubei.