Regional　transport　plays　an　important　role　in　the　serious　PM2.5　pollutions　in　the　Beijing-Tianjin-Hebei　(BTH)　region,　China.　Practical　regional　joint　emission　control　strategies　require　quantitative　assessments　of　the　transport　contribution　among　cities　and　regions.　The　Community　Multiscale　Air　Quality　model　equipped　with　the　Integrated　Source　Apportionment　Model　is　used　to　simulate　the　contributions　from　5　major　emission　sectors　in　13　cities　of　the　BTH　region,　and　4　surrounding　provinces　outside　BTH　for　the　year　2014.　Annual　averaged　local　contribution　ranges　from　32%　to　63%　for　the　13　cities　in　the　BTH　region,　where　secondary　components　contributing　more　than　primary　components.　Regional　contribution　ratio　becomes　larger　and　the　transport　distance　longer　in　July　and　October　than　in　January　and　March.　For　Beijing,　local　contributions　are　62%　and　69%　in　January　and　March　respectively,　and　the　regional　transports　are　mainly　from　nearby　cities　such　as　Zhangjiakou,　Baoding　and　Langfang.　In　July　and　October,　local　contributions　in　Beijing　are　only　33%　and　38%　respectively,　and　a　large　range　of　regions　in　the　south　have　substantial　contributions,　where　Shandong　Province　and　Henan　Province　contribute　3.6-5.3　mu　g/m(3).　Analysis　of　daily　contributions　suggests　that　regional　transport　is　stronger　under　higher　PM2.5　concentrations.　During　heavy　pollution,　local　emissions　in　Beijing　contribute　61%,　49%,　23%　and　25%　in　January,　march,　July　and　October　respectively,　while　during　the　clean　days,　the　ratios　are　88%,　88%,　76%　and　57%　respectively.　Southerly　regional　transport　during　the　rising　phase　of　＂saw　tooth＂　pattern　might　be　enhanced　by　weak　cold　high　pressure　and　its　easterly,　northerly　moving　path.　Among　the　major　emission　sectors,　in　winter,　local　domestic　combustion　is　the　most　important　source　for　Beijing,　Tianjin　and　Shijiazhuang.　In　summer,　transportation　and　domestic　combustion　are　two　important　local　sources　for　Beijing,　while　joint　control　in　other　cities　should　focus　on　industry.　(C)　2019　Elsevier　B.V.　All　rights　reserved.