Progressively narrow the gap of PM2.5 pollution characteristics at urban and suburban sites in a megacity of Sichuan Basin, China

•The dominant pollutant changed from SO42− to NO3− in most seasons of Chengdu.•SO42− was likely generated more easily in urban areas due to Mn-catalytic heterogeneous reactions.•OC concentrations were higher in urban areas than in suburban areas, other than EC.•The contribution of combustion origins and secondary aerosols increased from CP to PP.