Size-resolved aerosol water-soluble ions at a regional background station of Beijing,Tianjin,and Hebei,North China

The characteristics of water-soluble ions in size-resolved particulate matter were investigated usingion chromatography at Shangdianzi,a regional background station of Beijing,Tianjin,and Hebei.Seasonal total concentrations of ions(Na+,Mg2+,K+,Ca2+,NH4+,Cl-,SO42- and NO3-) were75.5±52.9 μg/m3 in spring,26.5±12.3μg/m3 in summer,22.7±20.4μg/m3 in autumn,and31.1±23.9 μg/m3 in winter,respectively.The secondary ions(NO3-,SO42- and NH4+),mainly associated with fine particles,accounted for 84.2%in spring,82.1%in summer,81.5%in autumn and 76.3%in winter of all ions.Strong correlations were found between NH4+ and SO42-（r=0.95,p<0.01） as well as NH4+ and NO3-（r=0.90,p<0.01） in fine particles;while in coarse particles,correlations between Mg2+ and NO3-（r=0.80,p<0.01）,and Ca2+ and NO3-（r=0.85,p<0.01） were found.The concentrations of Na+,K+,Mg2+,Ca2+,NH4+,Cl-,NO3-,and SO42- were 2.02,0.81,0.36,1.65,9.58,4.01,18.9,and 18.4 μg/m3 in particulate matter from southeast-derived air masses,which were typically 1.58-3.37 times higher than in northwest trajectories.Thus,concentrations of water-soluble ions at this background station were heavily influenced by regional transport of serious pollution derived from biomass burning,coal combustion,industrial and vehicle exhaust emissions from Beijing,Tianjin,and Hebei.