To study the characteristics and sources of the fine particulate matter (PM_2.5) in the middle and lower levels of pollution in the atmosphere, an online monitoring system for atmospheric gas composition and aerosol were used to track ρ(PM_2.5)and the concentration of the watersoluble ions in Pudong New District, from January 1,2018, to December 31, 2020. The results showed thatρ(PM_2.5) was the highest in winter, followed by in spring and in autumn, and it was the lowest in summer. The order of watersoluble ion components in PM_2.5 by mass concentration was as follow:ρ(NO^-₃) >ρ(SO₄^2-) >ρ(NH⁺₄) >ρ(Cl^-)>ρ(K⁺)>ρ(Na⁺), while the sum concentration of SO₄^2-,NO^-₃, and NH⁺₄(SNA) accounted for over 94% of the total ion components and over 52% of PM_2.5,which were the key factors in PM_2.5. The percentage of Cl^-was about 3%. The percentages of Na⁺ and K⁺ were both less than 1%. The concentration of NO^-₃ changed the most with the season. In spring, the particulate matter was weakly acidic,while in winter, it developed to weakly alkaline, and that of the whole year was neutral. Based on ion equilibrium calculations, SNA tends to exist in the atmosphere primarily as (NH₄)₂SO₄ and NH₄NO₃. SO₄^2-,NO^-₃ and NH⁺₊had a high correlation, while Cl^- and K⁺ had a high correlation with homology. The sulfur oxidation rate (SOR) and nitrogen oxidation rate (NOR) were both higher than 0.1, and the secondary transformation degree of SO₂and NO₂was higher. Vehicles exhaust and the following secondary particles transmitting and regional transmitting had become the main source of fine particles in the observation period.