Although　anthropogenic　emissions　decreased,　polluted　days　still　occurred　in　the　Beijing-Tianjin-Hebei　(BTH)　region　during　the　initial　outbreak　of　the　coronavirus　disease　(COVID-19).　Analysis　of　the　characteristics　and　source　distribution　of　large-scale　air　pollution　episodes　during　the　COVID-19　outbreak　(from　23　January　to　April　8,　2020)　in　the　BTH　region　is　helpful　for　exploring　the　efficacy　of　control　measures　and　policy　making.　The　results　indicated　that　the　BTH　region　suffered　two　large-scale　air　pollution　episodes　(23-28　January　and　8-13　February),　which　were　characterized　by　elevated　PM2.5,　SO2,　NO2,　and　CO　concentrations,　while　the　O-3　concentration　decreased　by　1.5%-33.9%　(except　in　Shijiazhuang,　where　it　increased　by　16.6%　during　the　second　episode).　These　large-scale　air　pollution　episodes　were　dominated　by　unfavorable　meteorological　conditions　comprising　a　low　wind　speed　and　increased　relative　humidity.　The　transport　pathways　and　source　distribution　were　explored　using　the　Hybrid　Single　Particle　Lagrangian　Integrated　Trajectory　(HYSPLIT),　potential　source　contribution　function　(PSCF),　and　concentration　weighted　trajectory　(CWT)　models.　The　air　pollution　in　the　BTH　region　was　mainly　affected　by　local　emission　sources　during　the　first　episode,　which　contributed　51.6%-60.6%　of　the　total　trajectories　in　the　BTH　region　with　a　PM2.5　concentration　ranging　from　146.2　mg/m3　to　196.7　mg/m(3).　The　short-distance　air　masses　from　the　southern　and　southwestern　areas　of　the　BTH　region　were　the　main　transport　pathways　of　airflow　arriving　in　the　BTH　region　during　the　second　episode.　These　contributed　51.9%-57.9%　of　the　total　trajectories　and　originated　in　Hebei,　Henan,　central　Shanxi,　and　Shaanxi　provinces,　which　were　the　areas　contributing　the　most　to　the　PM2.5　level　and　exhibited　the　highest　PSCF　and　CWT　values.　Therefore,　on　the　basis　of　local　emission　reduction,　enhancing　regional　environmental　cooperation　and　implementing　a　united　prevention　and　control　of　air　pollution　are　effective　mitigation　measures　for　the　BTH　region.　(c)　2020　Elsevier　Ltd.　All　rights　reserved.