Study On PM_2.5 Components,Secondary Pollution Characteristics And Regional Pollution Transportation In Heating Season In Northern Suburb Of Xi’an

Xi’an has serious air particle pollution and frequent severe and moderate pollution weather,which is one of the key areas of blue sky defense war.The heating season is a key stage of comprehensive prevention and control of air pollution in Xi’an.From November 15,2018 to March 15,2019,a sampling point was set up in the northern suburb of Xi’an to conduct on-line continuous sampling and observation of water-soluble ion components and carbon components in the air PM_2.5of Xi’an City,study the characteristics of PM_2.5components and secondary pollution,and discuss the regional transport path of Xi’an City in combination with backward trajectory and other models,and further analyze the types and sources of pollution,so as to It will provide scientific monitoring data and technical support for air pollution prevention and control and regional joint prevention and control in the northern suburb of Xi’an in the future.The results show that:The mass concentrations of SO₂,CO and O₃in the air pollutants in the northern suburb of Xi’an during the heating season 2018-2019 are lower than the secondary limit value of the ambient air quality standard.According to the intensity of the pollutants exceeding the standard,they are PM_2.5（66.94%）,NOx（54.55%）,PM₁₀（51.24%）,NO₂（33.88%）,PM_2.5is one of the main pollutants in the atmospheric environment during the observation period.The main wind direction is the east wind and the northeast wind with small wind speed Static wind weather is not conducive to the diffusion of pollutants in general;69.23%of PM_2.5is water soluble ion component and carbon component,NO₃^-（27.829±19.94μg/m³）,OC（16.291±9.78μg/m³）,NH₄⁺（14.264±10.62μg/m³）and SO₄^2-（12.765±10.57μg/m³）are the most important components of PM_2.5in the northern suburb of Xi’an during heating season,accounting for 61.01%in total.Sulfates and nitrates in PM_2.5are mainly formed through secondary transformation,and SNA exists in the form of（NH₄）₂SO₄and NH₄NO₃.As a whole,with the increase of relative humidity,sor and nor increase rapidly.The high humidity condition is more conducive to the secondary transformation.When the humidity is greater than 80%,the increase amplitude is significantly reduced.Precipitation plays a major role in the removal of gaseous precursors at this stage;the impact of sulfate is mainly in the daytime,while the impact of nitrate is not only in the daytime,but also in the control of precursors at night System.In PM_2.5,POC accounts for 52.60%of OC,SOC accounts for 47.40%of OC.The changes of POC and EC are obviously related to traffic,and the secondary organic matter is also related to the primary emission delay.The traffic peak has an important impact on carbon components.The air masses in the northern suburb of Xi’an can be roughly divided into six types:the first is from the southwest of Hanzhong,accounting for 25.57%;the second is from the northeast of Weinan,accounting for 21.92%.The two types of air masses are relatively short in distance and slow in transmission,which is more conducive to the secondary generation and aging of particles,with a large impact,accounting for47.49%;the third is from the northwest of Ningxia,accounting for 14.09%;and the fourth is from the northwest of Ningxia The southeast air mass in Luoyang,Henan Province accounts for 10.89%;the northwest air mass from Bayannur,Inner Mongolia accounts for 18.09%;the northwest air mass from Jiuquan,Gansu Province accounts for 9.44%.The two types of air mass are far away and fast in transmission.The areas with the greatest transmission impact on PM_2.5components in the northern suburb of Xi’an are mainly in the south of Yuncheng,Sanmenxia,Luoyang,Weinan,Tongchuan,Xianyang,Shangluo,Ankang,Baoji,Hanzhong,Eastern Guangyuan,Dazhou,northern Bazhong,etc.Secondly,Chongqing urban area and the north of suburban county,Shiyan and the northwest of Nanyang,Qingyang,Yan’an,Yulin,Wuzhong,Erdos and other areas;the most prominent weight contribution areas are Xianyang,Baoji,Hanzhong,Ankang,Eastern Guangyuan,Dazhou,northern Chongqing,followed by Shangluo,Sanmenxia,Weinan,Tongchuan,southern Yuncheng,Western Luoyang,Qingyang,Wuzhong and other areas.During the observation period,the main contribution sources were secondary pollution sources and vehicle emission sources,accounting for 58.72%and 26.54%respectively,followed by coal combustion sources,soil dust sources and biomass combustion sources,accounting for 8.74%,3.86%and 2.14%respectively.