Daily　PM(2.5)　and　water-soluble　inorganic　ions　(Na(+),　NH(4)(+),　K(+),　Mg(2+),　Ca(2+),　Cl(-),　NO(3)(-)　and　SO(4)(2-))　were　collected　in　Xi＇an　(34.23　degrees　N,　108.88　degrees　E),　China　from　March　2006　to　March　2007.　PM(2.5)　was　collected　using　battery-powered　mini-volume　samplers.　And　the　ions　were　determined　by　ion　chromatography　from　the　measured　aerosol　mass.　The　annual　average　mass　concentration　of　PM(2.5)　was　found　to　be　194.1　+/-　78.6　mu　g　m(-3),　which　exceeded　substantially　the　international　guidelines　for　health　concerns.　The　seasonal　average　mass　concentration　of　PM(2.5)　was　highest　in　winter　(266.8　mu　g　m(-3))　and　lowest　in　summer　(138.6　mu　g　m(-3)).　The　three　highest　abundant　ions　were　SO(4)(2-),　NO(3)(-),　and　NH(4)(-),　with　average　concentrations　of　35.6　+/-　19.5　mu　g　m(-3),　16.4　+/-　10.1　mu　g　m(-3),　and　11.4　6.8　mu　g　m(-3),　which　were　accounted　for　18.7%,　8.0%,　and　5.7%　of　the　PM(2.5)　mass,　respectively.　The　major　ions　were　in　the　species　of　(NH(4))(2)SO(4),　NH(4)HSO(4)　and　NH(4)NO(3),　and　their　concentrations　were　highest　in　winter,　due　to　high　coal　combustion.　The　concentrations　of　Ca(2+)　were　higher　in　spring　than　other　seasons,　due　to　the　higher　mineral　dust　concentrations.　Ca(2+)　was　strongly　correlated　with　CO(3)(2-),　which　was　calculated　as　the　difference　in　the　measured　cations　minus　anions.　Ion　balance　calculations　indicate　that　the　PM(2.5)　was　acidic,　and　this　result　is　consistent　with　the　measurement　of　pH　values.　Sulfur　oxidation　ratio　was　higher　in　summer　and　autumn,　which　implies　that　the　formation　of　secondary　sulfate-rich　particles　is　favored　by　warm　and　relatively　moist　weather.　Nitrogen　oxidation　ratio　was　highest　in　autumn.　(C)　2011　Elsevier　B.V.　All　rights　reserved.