In　this　study,　large　eddy　simulation　(LES)　is　adopted　to　evaluate　the　aerodynamic　and　deposition　effects　of　street　trees　on　fine　particulate　matter　(PM2.5)　concentration　within　street　canyons.　The　Extended　Nonperiodic　Domain　LES　for　Scalar　Transport　(ENDLESS)　is　used　to　allow　exploration　of　vegetation　effects　at　a　neighborhood　scale　(up　to　100　canyons)　while　maintaining　a　reasonable　resolution　required　to　resolve　flow　patterns　within　each　canyon.　We　investigate　three　emission　scenarios:　(i)　only　local　traffic　emissions　within　the　canyon　(the　first　canyon　in　the　urban　environment);　(ii)　only　background　pollution　originating　from　the　upwind　canyons;　and　(iii)　a　combination　of　scenarios　(i)　and　(ii).　Numerical　results　show　that　the　presence　of　trees　has　different　effects　on　the　PM2.5　level　within　canyons　in　different　emission　scenarios　and　at　different　spatial　scales.　At　the　street　scale　with　only　local　traffic　emissions,　aerodynamic　effect　of　trees　results　in　an　increase　in　the　concentration　near　leeward　walls　and　a　decrease　in　the　concentration　near　windward　walls,　which　overwhelms　the　deposition　effect.　On　the　other　hand,　trees　have　a　negligible　impact　on　the　transport　of　background　pollution　into　the　canyon　or　its　distribution　within　the　canyons.　The　deposition　has　beneficial　effects　that　only　manifest　in　a　considerable　way　at　the　neighborhood　scale.　Finally,　the　effect　of　trees　on　a　simple　operational　urban　pollution　model　(OUPM)　is　investigated.　After　modifications　for　aerodynamic　and　deposition　effect　of　trees,　the　predictions　of　the　OUPM　show　good　agreement　with　LES　results.