[Trend of atmosphreic PM_(2. 5) and their components in two districts of Lanzhou City from 2015 to 2019].

Objective: To analyse the variation of the fine particulate matter(PM_(2. 5)) concentration and its chemical composition in two districts of Lanzhou City from 2015 to 2019.

Methods: From January 2015 to December 2019, two sampling points, namely residential district(Chengguan District) and industrial district(Xigu District) were selected in Lanzhou City, to collect PM_(2. 5) samples, and the mass concentrations, metals and metalloid elements, water-soluble ions and polycyclic aromatic hydrocarbons(PAHs) contents were determined.

Results: A total of 838 samples were collected in Chengguan District and Xigu District in Lanzhou City during 2015-2019, the average annual mass concentration and the daily average concentration of the PM_(2. 5) were decreasing year by year(P<0. 05). The higher metal content in PM_(2. 5) were Al, Pb, Mn and As during 2015-2019. Sb, Al, As, Cd, Cr, Pb, Mn and Tl element content were higher in 2015 and 2016 than in other years(P<0. 05), the average annual concentration of As were 7. 92 and 8. 63 ng/m~3 respectively, which exceeded the reference concentration limit of the ambient air quality standard. The average annual mass concentration of Cr from 2015 to 2019 was much higher than the annual average reference concentration limit of the ambient air quality standard. From 2015 to 2019, except for 2018, the highest average concentration of PAHs in atmospheric PM_(2. 5) in the two districts of Lanzhou City was Ant, B(b)F and I(cd)P mass concentration were highest in 2015, N(a)P, FI, Ace, Phe, Ant, Flu, Pyr, B(k)F and B(a)P mass concentration were highest in 2017, Acy, Chr and B(ghi)P mass concentration were highest in 2018(P<0. 05). From 2017 to 2018, the average annual content of B(a)P in the two districts of Lanzhou City were 1. 16 and 1. 11 ng/m~3, respectively, which exceeded the annual average concentration limit of the ambient air quality standard. The concentration of Cl~- in PM_(2. 5) in the two districts of Lanzhou City was decreasing year by year, the annual average concentrations of SO~2-_4, NO~-_3 and NH~+_4 from 2017 to 2019 showed a downward trend year by year. SO~2-_4, Cl~- of both districts were the highest in 2015, and NO~-_3, NH~+_4 were the highest in 2017(P<0. 05).

Conclusion: The air quality of Lanzhou City has been improved obviously, but the pollution problem is still serious.