The　Guanzhong　basin　is　a　part　of　the　three　top　priority　regions　in　China＇s　blue　sky　action　as　of　2019.　Understanding　the　chemical　composition,　sources,　and　atmospheric　process　of　aerosol　in　this　region　is　therefore　imperative　for　improving　air　quality.　In　this　study,　we　present,　for　the　first　time,　the　seasonal　variations　of　organic　aerosol　(OA)　in　Xi＇an,　the　largest　city　in　the　Guanzhong　basin.　Biomass　burning　OA　(BBOA)　and　oxidized　OA　(OOA)　contributed　＞50%　of　OA　in　both　autumn　and　winter.　The　average　concentrations　of　BBOA　in　autumn　(14.8　±　5.1　μg　m−3)　and　winter　(11.6　±　6.8　μg　m−3)　were　similar.　The　fractional　contribution　of　BBOA　to　total　OA,　however,　decreased　from　31.9%　in　autumn　to　15.3%　in　winter,　because　of　enhanced　contributions　from　other　sources　in　winter.　The　OOA　fraction　in　OA　increased　largely　from　20.9%　in　autumn　to　34.9%　in　winter,　likely　due　to　enhanced　emissions　of　precursors　and　stagnant　meteorological　conditions　which　facilitate　the　accumulation　and　secondary　formation.　A　large　increase　in　OOA　concentration　was　observed　during　polluted　days,　by　a　factor　of　~4　in　autumn　and　~6　in　winter　compared　to　clean　days.　In　both　seasons,　OOA　formation　was　most　likely　dominated　by　photochemical　oxidation　when　aerosol　liquid　water　content　was　−3　or　by　aqueous-phase　processes　when　Ox　was　−3),　compared　to　air　masses　from　Northwest　and　West　(10.9–14.5　μg　m−3).　Furthermore,　compared　with　OA　fraction　in　non-refractory　PM1　in　other　regions　of　China,　BBOA　(17–19%)　and　coal　combustion　OA　(10–20%)　were　major　emission　sources　in　the　Guanzhong　Basin　and　the　BTH　region,　respectively,　whereas　OOA　(10–34%)　was　an　important　source　in　all　studied　regions.　©　2020