Characteristics Of PM_2.5 And Chemical Compositions In Chongqing,China

Air pollution attracts the most attention in China and even in the world.PM_2.5 has become one of the most important atmospheric environmental problems in many areas.Chengyu urban agglomeration was made a compound pollution area,and was embedded in the key areas of joint prevention and control of air pollution."Measures for the implementation of atmospheric pollution prevention action plan by Chongqing government"required that the PM_2.5 mass concentration in urban districts decreased above 16%by the end of 2017 than 2013.Although it was found that the PM_2.5concentration has been decreased from 2013 to 2015,the PM_2.5 concentration still exceed the standard seriously,with a value of 57?g·m^-3 in 2015,which was close to the national average.In order to study the pollution characteristics of PM_2.5 and chemical components in Chongqing,PM_2.5 samples were collected in urban,suburban and background areas during October 2015 to August 2016.Among the sampling sites,Shapingba and Yubei represented the urban sites,Jinyun Mountain and Simian Mountain surrounded as a suburban site and a background site,respectively.And then the spatial-temporal variability of PM_2.5 mass concentration,water soluble ions?WSIs?and carbon components were analyzed.Besides,the backward trajectory analysis was used for the influence of air mass transmission on sampling sites during period of typical pollution.The main conclusions are as follows:?1?The seasonal and spatial distributions of PM_2.5 concentration were distinctive.The urban sites,i.e.,Shapingba and Yubei suffered from the most severe PM_2.5 pollution(54.9?g·m^-3 and 58.8?g·m^-3),followed by the suburban site,i.e.,Jinyun Mountain(33.9?g·m^-3)and the background site,i.e.,Simian Mountain(21.9?g·m^-3).Temporally,the PM_2.5 concentrations were characterized with higher in autumn and winter,and the lower in spring and summer.?2?The analysis of reconstructed chemical mass showed that OM and SO₄^2-were the highest contents of PM_2.5 at all the sampling sites.NO₃^-reached the highest value in winter among the four seasons and the content was similar to SO₄^2-.Spatially,the carbon fractions at the urban sites were higher than that at the suburban and background sites.OM contributed 34.4%and 30.2%at Shapingba and Yubei,respectively,and 24.5%and26.6%at the suburban and background sites.EC accounted for about 7.6%and 7.2%at Shapingba and Yubei,respectively,while only 3.0%at the suburban and background sites.?3?The spatial-temporal variability of WSIs were similar to that of PM_2.5.The total water soluble ions accounted for about 37.2%⁴6.1%of PM_2.5 for the full year.Spatially,WSIs accounted for 46.1%of PM_2.5 at Jinyun Mountain,the highest among the four sites.SO₄^2-,NO₃^-and NH₄⁺are dominant ions,accounting for about 87.1%⁹3.1%of total WSIs.And these three ions mainly existed in the forms of?NH₄?₂SO₄ and NH₄NO₃ at the urban sites,while in the forms of?NH₄?₂SO₄,NH₄HSO₄ and NH₄NO₃ at the suburban and background sites.?4?The distributions of OC concentrations were presented as follows:Shapingba(13.3?g·m^-3)>Yubei(10.9?g·m^-3)>Jinyun Mountain(5.56?g·m^-3)>Simian Mountain(3.69?g·m^-3),and the EC distributions showed Shapingba(4.61?g·m^-3)>Yubei(4.20?g·m^-3)>Jinyun Mountain(1.08?g·m^-3)>Simian Mountain(0.64?g·m^-3).The highest and the lowest concentrations of OC and EC occurred in winter and summer,respectively.Both the increase of coal consumption and cold start of motor vehicle in winter could lead to increase of the OC and EC emissions.In addition,the stable atmospheric boundary layer and the high frequency of temperature inversion were not conducive to the spread of pollutants,which would increase the concentration of pollutants.The content of SOC is of great significance to carbonaceous aerosol pollution.The minimum OC/EC ratio method was used to evaluate SOC concentrations in this study.The highest SOC concentration occurred in summer at Simian Mountain site,while the peak occurred in winter at the other sites.?5?A continuous pollution process occurred in Chongqing from December 28,2015to January 6,2016.The results by the backward trajectory showed that the pollution was mainly caused by the cumulative effect from short distance transport,while clean air from the West alleviated the pollution of atmospheric fine particles.