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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-) were 75.5 ± 52.9 μg/m3 in spring,26.5 ± 12.3 μg/m3 in summer,22.7 ± 20.4 μg/m3 in autumn,and 31.1 ± 23.9 μg/m3 in winter,respectively.The secondary ions (NO3-,SC42-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 SC42-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 bing,coal combustion,industrial and vehicle exhaust emissions from Beijing,Tianjin,and Hebei.