| Summary: | PM2.5 and PM10 samples were collected simultaneously in each season in Beijing, Tianjin and Shijiazhuang to identify the characteristics of water-soluble ion compositions in the North China Plain. The water-soluble ions displayed significant seasonal variation. The dominant ions were NO3(-), SO4(2-), NH4(+) and Cl(-), accounting for more than 90% and 86% to the mass of total water-soluble ions in PM2.5 and PM10, respectively. The anion/cation ratio indicated that the ion acidity of each city varied both between sites and seasonally. Over 50% of the ion species were enriched in small particles ≤1 µm in diameter. The [NO3(-)]/[SO4(2-)] ratio indicated that vehicles accounted for the majority of the particulate pollution in Beijing. Shijiazhuang, a city highly reliant on coal combustion, had a higher SO4(2-) concentration.
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