Abstract: During the rainy season (April-September) of 2012, the atmospheric particulate matter with a diameter less than 2.5 mm (PM_2.5) were sampled from the forest parks in the urban area, suburban area, and rural area of Guangzhou City.  The  mass concentration of PM_2.5 and its watersoluble ions (SO₄^2-, NO₃^-, NO₂^-, Cl^-, F^-, Na⁺, NH₄⁺, Ca²⁺, K⁺, and Mg²⁺) contents were also measured. In the forest parks in the urban area, suburban area, and rural area, the diurnal variation of PM_2.5 mass concentration was 21.8-161.7, 19.4-156.3, and 17.2-66.5 μg·m^-3, with an arithmetic average being 55.9, 49.8, and 44.4 μg·m^-3, respectively. SO₄^2-, Na⁺, and NH₄⁺ were the main components of watersoluble ions in the PM_2.5, and the SO₄^2- had the highest content and decreased gradually from urban to rural forest parks. The contribution of the SO₂ and NO_x in the PM_2.5 from coal combustion to the forest parks was larger than that from vehicle exhaust, but presented a decreasing trend from urban to rural forest parks, indicating that vehicle exhaust had a greater contribution to the atmospheric SO₂ and NO_x in the urban forest park. In the sampling period, the contribution of sea salt to the water soluble fractions (especially K⁺) of the PM_2.5 was greater for the suburban forest park than for the other two parks. The equivalent concentration of the NH₄⁺ in the PM_2.5 was far less than those of the SO₄^2- and NO₃^-, with a neutralization ratio being much lower than 1.0, which suggested that the PM_2.5 had a higher acidity. The PM_2.5  acidity had an increasing trend from rural to urban forest parks.

Key words: PM_2.5, water-soluble ion, particulate acidity, forest park, Guangzhou.