An　extremely　severe　and　persistent　haze　event　occurred　over　the　middle　and　eastern　China　in　January　2013,　with　the　record-breaking　high　concentrations　of　fine　particulate　matter　(PM2.5).　In　this　study,　an　online-coupled　meteorology-air　quality　model,　the　Weather　Research　and　Forecasting　Model　with　Chemistry　(WRF/Chem),　is　applied　to　simulate　this　pollution　episode　over　East　Asia　and　northern　China　at　36-　and　12-km　grid　resolutions.　A　number　of　simulations　are　conducted　to　examine　the　sensitivities　of　the　model　predictions　to　various　physical　schemes.　The　results　show　that　all　simulations　give　similar　predictions　for　temperature,　wind　speed,　wind　direction,　and　humidity,　but　large　variations　exist　in　the　prediction　for　precipitation.　The　concentrations　of　PM2.5,　particulate　matter　with　aerodynamic　diameter　of　10　mu　m　or　less　(PM10),sulfur　dioxide　(SO2),　and　nitrogen　dioxide　(NO2)　are　overpredicted　partially　due　to　the　lack　of　wet　scavenging　by　the　chemistry-aerosol　option　with　the　1999　version　of　the　Statewide　Air　Pollution　Research　Center　(SAPRC-99)　mechanism　with　the　Model　for　Simulating　Aerosol　Interactions　and　Chemistry　(MOSAIC)　and　the　Volatility　Basis　Set　(VBS)　for　secondary　organic　aerosol　formation.　The　optimal　set　of　configurations　with　the　best　performance　is　the　simulation　with　the　Gorddard　shortwave　and　RRTM　longwave　radiation　schemes,　the　Purdue　Lin　microphysics　scheme,　the　Kain-Fritsch　cumulus　scheme,　and　a　nudging　coefficient　of　1　x　10(-5)　for　water　vapor　mixing　ratio.　The　emission　sensitivity　simulations　show　that　the　PM2.5　concentrations　are　most　sensitive　to　nitrogen　oxide　(NO)　and　SO2　emissions　in　northern　China,　but　to　NO　and　ammonia　(NH3)　emissions　in　southern　China.　30%　NOx　emission　reductions　may　result　in　an　increase　in　PM2.5　concentrations　in　northern　China　because　of　the　NH3-rich　and　volatile　organic　compound　(VOC)　limited　conditions　over　this　area.　VOC　emission　reductions　will　lead　to　a　decrease　in　PM2.5　concentrations　in　eastern　China.　However,　30%　reductions　in　the　emissions　of　SO2,　NOx,　NH3,　and　VOC,　individually　or　collectively,　are　insufficient　to　effectively　mitigate　the　severe　pollution　over　northern　China.　More　aggressive　emission　controls,　which　needs　to　be　identified　in　further　studies,　are　needed　in　this　area　to　reach　the　objective　of　25%　PM2.5　concentration　reduction　in　2017　proposed　in　the　Action　Plan　for　Air　Pollution　Prevention　and　Control　by　the　State　Council　in　2013.　(C)　2014　Elsevier　Ltd.　All　rights　reserved.