Pollution Characteristics And Source Analysis Of PM_2.5 In Chengguan And Xigu Districts Of Lanzhou City

Objective: To study the pollution characteristics,the chemical compositions,the concentration varying patterns and spatial distribution of PM_2.5 in Chengguan and Xigu Districts（hereinafter referred to as two districts）of Lanzhou City.To preliminarily analyze the major sources of PM_2.5 pollution.Methods: Collecting PM_2.5 concentrations and routine meteorological data of Lanzhou city in the year 2015.PM_2.5 samples were gathered and determined from Chengguan and Xigu Districts;The water-soluble ions in PM_2.5 samples were determined by ion chromatography;The heavy metal elements in PM_2.5 samples were determined by inductively coupled plasma mass spectrometer;The polycyclic aromatic hydrocarbons in PM_2.5 were detected by high performance liquid chromatography.The data collected were analyzed by nonparametric test and correlation analysis.The sources of PM_2.5 were analyzed by principal component analysis-linear regression model（PCA-MLR）.Results:1.The overall variation characteristics of PM_2.5 in Lanzhou,2015The annual average PM_2.5 concentration was 51.71 μg/m3,which was 0.48 times over than the secondary standard limited value of our country.It’s range of variation was between 19¹47 μg/m3.One seventh of 365 days in 2015 failed to meet the standards of PM_2.5 daily mean value.The average PM_2.5 concentration of Lanzhou varied in different areas and different time,that was Xigu District > Chengguan District（P<0.05）,and heating period > normal period（P<0.01）,and Winter > Spring > Autumn > Summer（P<0.01）.2.The chemical compositions and characteristics of PM_2.5 in two districts（1）The pollution characteristics of PM_2.5 in two districts.During the sampling time,PM_2.5 concentrations of two districts changed deeply.PM_2.5 concentration in Chengguan District was higher than Xigu District during Summer and Non-heating period（P< 0.01）.In the same district,PM_2.5 concentration varied in different periods,usually Winter > Spring > Autumn/Summer（P<0.01）,and heating period > normal period（P<0.01）.（2）The chemical compositions and the overall variation characteristics of PM_2.5 in two districts.The average concentrations of different chemical constituents in PM_2.5 showed the same order,water soluble ions > heavy metals > polycyclic aromatic hydrocarbons（PAHs）.The total mass concentrations of PAHs in PM_2.5 had an obvious spatial distribution,Chengguan District > Xigu District（P<0.05）.While the total mass concentrations of the other two kinds of chemical constituents in PM_2.5 had no difference of spatial distribution（P>0.05）.（3）The pollution characteristics of water soluble ions in PM_2.5.The average concentrations of 4 kinds of water soluble ions in PM_2.5 from high to low were: SO2-4>NO-3>Cl->NH+4.In the same area,there was a significantly positive correlation among 4 kinds of water-soluble ions（P<0.01）,and the pollution of water soluble ions were most serious in winter（P<0.05）and heating period（P<0.01）.（4）The pollution characteristics of heavy metal elements in PM_2.5.The contents of 7 heavy metal elements in PM_2.5 were Al > Pb > Mn > As > Sb/Cd > Tl.During sampling period,As was a standard exceeded pollutant whose average mass concentrations were 0.89 and 0.88 times higher than national limits in Chengguan District and Xigu District respectively.The mass concentrations of Cd,Pb and Mn were higher in Chengguan District than Xigu District（P<0.05）.In the same area,the mass concentrations of 7 PM_2.5 heavy metal elements had seasonal differences（P<0.05）,and each kind of metal elements had a different seasonal changing trend.（5）The pollution characteristics of PAHs in PM_2.5.The key components of PAHs were Bb F,Flua,Ant,Pyr,Nap,Chr,DBah A and Ba A.During sampling period,Ba P exceeded bid badly.The average mass concentrations of Ba P were 1.21 and 0.85 times higher than national limits in Chengguan District and Xigu District respectively.The mass concentrations of Nap and Icd P were higher in Chengguan District than Xigu District（P<0.05）.In the same area,the mass concentrations of some PAHs had seasonal differences（P<0.05）,and each kind of PAHs had a different seasonal changing trend.3.The preliminary analysis of PM_2.5 pollution sourceThe primary pollution source in Chengguan District was a mixed pollution of coal and traffic,followed by soil dust and biomass burning source;The pollution sources of Xigu District were relatively complex,including the mixed pollution of coal combustion,traffic emission,and soil dust pollution,while the industrial pollution,as well as a secondary pollution sources also played a big part.Conclusion:（1）The pollution of PM_2.5 as well as its chemical compositions were still grim in Lanzhou,and they varied in different time and space.The government should strengthen the PM_2.5 monitoring and preventing of Xigu District during the heating period.Meanwhile,the government should pay special attention to control As pollution and Ba P pollution,to reduce health risks.（2）The primary pollution source in Chengguan District was a mixed pollution of coal combustion and traffic emission,while the pollution sources of Xigu District was relatively complex.So the relevant government departments should further strengthen the monitoring of air pollution in Lanzhou city,attach great importance to the governance of issues like motor vehicle emissions and coal combustion,etc.And some emission control and limitation measures should be introduced.At the same time,in view of the complex PM_2.5 pollution sources in Xigu District,more measures should be designed to meet the needs,and to ensure that the air quality could change better,and make Lanzhou city soon become a green city.