[Characteristics of rain season atmospheric PM2.5 concentration and its water-soluble ions contents in forest parks along an urban-rural gradient in Guangzhou City of South China].

During the rainy season (April-September) of 2012, the atmospheric particulate matter with a diameter less than 2.5 mm (PM2.5) were sampled from the forest parks in the urban area, suburban area, and rural area of Guangzhou City. The mass concentration of PM2.5 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 PM2.5 mass concentration was 21.8-161.7, 19.4-156.3, and 17.2-66.5 microg x m(-3), with an arithmetic average being 55.9, 49.8, and 44.4 microg x m(-3), respectively. SO4(2-), Na+, and NH4+ were the main components of water-soluble ions in the PM2.5, and the SO4(2-) had the highest content and decreased gradually from urban to rural forest parks. The contribution of the SO2 and NOx in the PM2.5 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 NOx in the urban forest park. In the sampling period, the contribution of sea salt to the water soluble fractions (especially K+) of the PM2.5 was greater for the suburban forest park than for the other two parks. The equivalent concentration of the NH4+ in the PM2.5 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 PM2.5 had a higher acidity. The PM2.5 acidity had an increasing trend from rural to urban forest parks.