Chemical Composition of Water-soluble Ions and Carbonate Estimation in Spring Aerosol at a Semi-arid Site of Tongyu, China

Fine aerosol samples (PM2.5) were collected daily at Tongyu, a semi-arid site in northeast China from April to June 2006, when Asian dust events occurred frequently. The concentrations of nine water-soluble ions were determined in the filter samples. The results showed that the mean mass level of PM2.5 at Tongyu station was 260.9 mu g/m(3), and the total mass concentration of the nine kinds of water-soluble ions occupied similar to 17% of the PM2.5 mass. Nine dust storm (DS) events were observed during the sampling period with mean concentrations of 528.0 mu g/m(3) for PM2.5 and 39.5 mu g/m(3) for total ions; during non-dust storm (NDS) periods, these values were 111.7 and 19.6 mu g/m(3), respectively. The mass concentrations of most of the water-soluble ions in the DS samples were much higher than those in the NDS samples. The ratios of ions to Fe were stable in DS samples, while they were more scattered in NDS samples, suggesting that the anthropogenic influence was significant in NDS samples collected at Tongyu. Moreover, the observed stable values of the K+/Fe, Ca2+/Fe, Na+/Fe, NO3-/Fe, and SO42-/Fe ratios during the DS events should be associated with the ionic composition characteristics of soil dust in northeastern regions. Ion balance calculations for the NDS samples suggest that those aerosol particles were alkaline while DS samples were strongly alkaline. A deficit of measured anions during DS implies the presence of carbonate, which can be used to evaluate the carbonate concentration. A good relationship between the calculated and detected carbonate concentration was observed, which demonstrates that the ion balance supplies another method for estimating the carbonate content during DS.