A total of 87 daily PM2.5 samples were collected in the urban area of Suzhou city during 2015, representing spring, summer, autumn, winter, respectively. Mass concentration of PM2.5 was analyzed gravimetrically. Water-soluble inorganic ions, including F-, Cl-, NO3-, SO42-, Na+, NH4+, K+, Mg2+and Ca2+, were determined by ion chromatography. The average mass concentration of PM2.5 was (74.26±38.01) μg·m-3. The seasonal variations of PM2.5 concentrations decreased in the order of winter > spring > autumn > summer. The average total mass concentrations of 9 ions was (43.95±23.60) μg·m-3, and the order of concentration of ions was NO3- > SO42- > NH4+ > Na+ > Cl- > K+ > Ca2+ > F- > Mg2+. Seasonal variation of ion concentrations was significant, with the highest concentration observed in winter and the lowest in summer. The secondary inorganic species, including SO42-, NO3- and NH4+ (SNA) were the major components of the water-soluble ions in PM2.5. SNA's correlations with each other were significant. SO42-, NO3- and NH4+ were probably in the form of NH4NO3 and (NH4)2SO4. The [NO3-]/[SO42-] ratio approaching to 1 implied that mobile sources were as important as stationary sources. Ion balance calculations indicated strong correlations between anion and cation equivalents. The PM2.5 was acidic. Industrial emission, combustion process, secondary formation and fugitive dust were the major sources of the water-soluble ions in PM2.5..
Keywords: PM2.5; Suzhou; seasonal variation; source; water-soluble ions.