To　investigate　the　chemical　composition,　size　distribution,　and　mixing　state　of　aerosol　particles　on　heavy　pollution　days,　single-particle　aerosol　mass　spectrometry　was　conducted　during　9-26　October　2015　in　Xi＇an,　China.　The　measured　particles　were　classified　into　six　major　categories:　biomass　burning　(BB)　particles,　K-secondary　particles,　elemental　carbon　(EC)-related　particles,　metal-containing　particles,　dust,　and　organic　carbon　(OC)　particles.　BB　and　EC-related　particles　were　the　dominant　types　during　the　study　period　and　mainly　originated　from　biomass　burning,　vehicle　emissions,　and　coal　combustion.　According　to　the　ambient　air　quality　index,　two　typical　episodes　were　defined:　clean　days　(CDs)　and　polluted　days　(PDs).　Accumulation　of　BB　particles　and　EC-related　particles　was　the　main　reason　for　the　pollution　in　Xi＇an.　Most　types　of　particle　size　were　larger　on　PDs　than　CDs.　Each　particle　type　was　mixed　with　secondary　species　to　different　degrees　on　CDs　and　PDs,　indicating　that　atmospheric　aging　occurred.　The　mixing　state　results　demonstrated　that　the　primary　tracers　were　oxidized　or　vanished　and　that　the　amount　of　secondary　species　was　increased　on　PDs.　This　study　provides　valuable　information　and　a　dataset　to　help　control　air　pollution　in　the　urban　areas　of　Xi＇an.