Characterisation and Source Identification of Inorganic Water-Soluble Ions in PM_2.5 in a Small-Size Steel City of China

Ambient PM2.5 samples were collected in Laiwu, a small industrial city in China known for steel production, to measure and evaluate the mass concentrations of PM2.5 and the inorganic water-soluble ions (IWS ions). The highest PM2.5 value recorded was 251.28 mu g/m(3), which occurred during winter and was lower than other major Chinese cities. The spatial distribution showed that the center of the city, which is connected to the downtown area, was heavily polluted (170 mu g/m(3) higher) compared to the suburbs. With the exception of winter, the weight of PM2.5 was dominated by IWS ions, with three secondary ions (NO3-, SO42-, and NH4+) accounting for more than 70% of the total ions in mass. This percentage is higher than that of big cities. An increase in sulfur oxidation ratios (SOR), nitrogen oxidation rate (NOR), and anion equivalent/cation equivalent (AE/CE) indicated that the atmosphere in Laiwu was more oxidized than other big cities. The ion analysis revealed that PM2.5 was highly correlated with NO3-, SO42-, and NH4+. The principal component analysis indicated that district heating during winter and industrial output and secondary transformation during summer were the major sources of the water-soluble ions. The backward trajectory study identified the north of Shandong Province as the likely source-area that affected air quality in Laiwu.