Analysis Of Urban Air Pollution Status And PM_2.5 Sources In Hanzhong City

Based on the"One city,One Policy"project in Hanzhong City and the actual situation of energy structure,industrial distribution and residents’lifestyle in the study area,manual sampling of environmental receptor samples and collection of source samples were carried out in winter 2021 and summer 2022.At the same time,the air pollutant monitoring data and meteorological data of Hanzhong City from 2017 to 2022were collected to analyze the pollution status and influencing factors.Through chemical component analysis of the samples,pollution source spectrum and receptor component spectrum were constructed.CMB model and PMF model were used to explore the contribution rate and sharing rate of local sources.Traj Stat software was used to analyze the transport path and potential source area of particulate matter,and the main conclusions were drawn as follows:（1）From 2017 to 2022,the number of good days in Hanzhong increased year by year.In the past six years,SO₂,NO₂ and CO were all lower than the secondary concentration limit,while PM₁₀,PM_2.5 and O₃＿8h exceeded the limit,but all showed a trend of decreasing year by year.The concentrations of PM_2.5,PM₁₀,SO₂,NO₂ and CO were higher in spring and winter than in summer and autumn.O₃ shows the opposite trend.There is a significant correlation between the concentration of pollutants and meteorological factors,indicating that the concentration of pollutants is greatly affected by meteorological factors.（2）The average PM_2.5 concentration during the winter sampling period was 104.99μg/m³,and the average PM_2.5 concentration during the summer sampling period was61.18μg/m³.The average PM_2.5 concentration of the four sampling points in winter was higher than the secondary daily average concentration limit stipulated in the Ambient Air Quality Standards,and the average PM_2.5 concentration of the four sampling points in summer was lower than the secondary daily average concentration limit stipulated in the Ambient Air Quality Standards（75μg/m³）.（3）The composition analysis of PM_2.5 particles shows that the concentration of water-soluble ions in PM2.5 is 38.68μg/m³ during winter sampling and 9.4μg/m³during summer sampling.The concentration of secondary ions(NO₃^-,NH₄⁺,SO₄^2-)is significantly higher than that of other ions.Especially in winter,the proportion of the total ion concentration is as high as 77.23%;In winter and summer,the concentration of Al,Ca,Fe and other inorganic elements is higher,and they belong to crustal elements,indicating that the dust source contributes more to PM_2.5 during the sampling period.The average mass concentration of OC and EC is 11.48μg/m³ and 2.63μg/m³,and the ratio of OC and EC at each sampling point is between 2.5 and 10.5,which indicates that the source of pollutants at each sampling point during the sampling period is the possibility of vehicle exhaust dust and coal sources.The Pearson correlation coefficient between OC and EC is 0.916,indicating that OC and EC have a good correlation and the possibility of the same source is high.（4）The CMB model and PMF model were used to analyze the source of PM_2.5.In the CMB analysis results,the total contribution of dust sources（including urban secondary dust）to PM_2.5 concentration in Han Zhong City reached about 40%（the average contribution of all stations was 43.24%）.The main contribution source during winter sampling was road dust,while the largest contribution source during summer sampling was vehicle exhaust dust,and the analytical results of all stations were relatively consistent.The PMF simulation results showed that the contribution of vehicle exhaust dust to PM_2.5 during the sampling period was the highest,reaching 44.7%,followed by the contribution of dust reaching 17.2%,and the contribution of secondary particulate matter（secondary sulfate+secondary nitrate）to PM_2.5 was also higher,reaching16.4%.Through comprehensive analysis of CMB and PMF simulation results,it can be seen that dust source and vehicle exhaust dust are the main pollution sources in Hanzhong City,and the contribution of secondary sources is also large.（5）The backward trajectory results show that the air flow from the northwest direction in Hanzhong City is mostly long and moving fast,while the air flow from the south and other directions is mostly short and moving slowly.The pollution trajectory from the south is relatively large and carries a high concentration of PM_2.5.According to the clustering analysis results,it can be concluded that the southward air flow trajectory is short and the air mass moves slowly and steadily.The diffusion condition is poor,and the southward air flow is generally warm and wet,and the influence of static and stable weather is more likely to cause the accumulation of PM_2.5.This is consistent with the fact that the high value areas of PM_2.5 WPSCF and WCWT are mainly concentrated in Hanzhong and the northern part of Sichuan adjacent to Hanzhong,and the external potential sources are mainly from Guangyuan,Bazhong and Dazhou of Sichuan.The low-value areas of WPSCF and WCWT are mainly distributed in Gansu,Ningxia and Guanzhong and Northern Shaanxi in the north of Hanzhong,which may be caused by the obstruction of Qinling Mountains,which makes it difficult for pollutants to be transmitted.This indicates that local pollution and short distance transmission of pollutants have a greater impact on PM_2.5 pollution in Hanzhong.