Pollution Characteristics And Source Analysis Of PM_2.5 At Background Site In Central China

PM_2.5samples were collected in Dongting Lake from September 2017 to August 2018,and the pollution characteristics of 11 water-soluble inorganic ions were measured.Based on correlation analysis,positive matrix factorization analysis and backward trajectory clustering,the sources of water-soluble inorganic ions in PM_2.5in Dongting Lake site were analyzed.The results are as followed:（1）During the sampling period,the average daily concentration of PM_2.5was 45.73μg/m³,and the seasonal change order was:winter>autumn>spring>summer.The PM_2.5in autumn,winter and spring exceeded the national standard.The daily average mass concentration of PM₁₀was 69.05μg/m³,which was higher than the national standard value of PM₁₀,and its seasonal variation characteristics were the same as PM_2.5.The ratio of PM_2.5/PM₁₀was higher than 50%during the sampling period,and was high in autumn and winter but low in spring and summer.Therefore,the air pollution in this site was mainly affected by fine particles,with serious PM_2.5pollution events mainly occurring in late autumn,winter,and early spring.（2）The average mass concentration of water-soluble ions was 30.83±22.45μg/m³,with the seasonal variation as winter>autumn>spring>summer.The order of the average mass concentration of each ion was NO₃^->SO₄^2->NH₄⁺>Ca²⁺>Cl^->K⁺>Na⁺>Mg²⁺>F^->NO₂^->Br^-,and each ion showed significant seasonal characteristics.Among them,SO₄^2-,NO₃^-and NH₄⁺（simplified as SNA）were the three richest ions in PM_2.5in Dongting Lake area,with total average concentration of 27.97μg/m³,accounting for90.51%of total water-soluble inorganic ions.Compared with SO₄^2-,NO₃^-showed a significant difference between the cold and warm seasons.The PM_2.5was mainly contributed by NO₃^-in the cold season,but by SO₄^2-in the warm season,in that SO₄^2-is not volatile but NO₃^-is unstable and volatile which leads to a decrease in its proportion in spring and summer.During the whole sampling period,the PM_2.5pollution was mainly contributed by NO₃^-.（3）Temperature and relative humidity were important in the process of SO₂conversion.The formation mechanism of SO₄^2-was very complicated.Both NO₃^-and NOR decreased with increasing temperature,suggesting that the formation of NO₃^-depended on temperature.NH₄⁺was almost completely neutralized by SO₄^2-and NO₃^-.（NH₄）₂SO₄was the main form of existence of NH₄⁺and SO₄^2-,with a small part existing as NH₄HSO₄and（NH₄）₂SO₄.NO₃^-is mainly formed by the reaction of excess NH₃and HNO₃.（4）The PMF model analysis identified six factors,including four primary factors,named industry,biomass burning,vehicle,and dust;and two secondary factors,which were secondary sulfate and nitrate,with industry and the two secondary factors being the major factors.（5）Backward trajectory clustering results showed that,in the warm months,the air masses mainly came from the south and flows through Changsha;while in the cold months,the air masses mainly came from the central and north China plains and passed through Wuhan.Therefore,serious PM_2.5pollution in the study site was caused by the main regional transmission emission source.