Diurnal variations of water-soluble ions in PM2.5 in Shanghai

Thirty-six daily time interval PM205 samples were collected in different seasonal dates in urban Shanghai, and the concentrations of four anions (Cl-, NO3-, SO42-, C2O42-) and five cations (NH4+, Na+, K+, Ca2+, Mg2+) were analyzed with ion chromatography. Sulfate, nitrate and ammonium were found to be the dominant species, accounting for about 80% of the total ions. The daily nitrate to sulfate mass ratio ranged from 0.31 to 0.82, indicating that coal combustion was still the main pollution source in Shanghai. The equivalent ratio of ammonium to sum of nitrate and sulfate showed fixed diurnal variation pattern in all the sampling days with higher values in the nighttime, suggesting that fine particles in the night were more neutralized. The oxalate to sulfate ratio was lower in the winter sampling days than that in hotter summer and autumn sampling days. Oxalate was significantly correlated with sulfate in winter sampling days, but not in the summer and autumn, suggesting that the formation mechanism of oxalate and sulfate was similar in winter, however different in hot days.