Stringent　mitigation　measures　have　reduced　wintertime　fine　particulate　matter　(PM2.5)　concentrations　by　42.2%　from　2013　to　2018　in　the　Beijing-Tianjin-Hebei　(BTH)　region,　but　severe　PM　pollution　still　frequently　engulfs　the　region.　The　observed　nitrate　aerosols　have　not　exhibited　a　significant　decreasing　trend　and　constituted　a　major　fraction　(about　20%)　of　the　total　PM2.5,　although　the　surface-measured　NO2　concentration　has　decreased　by　over　20%.　The　contributions　of　nitrogen　oxides　(NOX)　emissions　mitigation　to　the　nitrate　and　PM2.5　concentrations　and　how　to　alleviate　nitrate　aerosols　efficiently　under　the　current　situation　still　remains　elusive.　The　WRF-Chem　model　simulations　of　a　persistent　and　heavy　PM　pollution　episode　in　January　2019　in　the　BTH　reveal　that　NOX　emissions　mitigation　does　not　help　lower　wintertime　nitrate　and　PM2.5　concentrations　under　current　conditions　in　the　BTH.　A　50%　reduction　in　NOX　emissions　only　decreases　nitrate　mass　by　10.3%　but　increases　PM2.5　concentrations　by　3.2%,　because　the　substantial　O3　increase　induced　by　NOX　mitigation　offsets　the　HNO3　loss　and　enhances　sulfate　and　secondary　organic　aerosols　formation.　Our　results　are　further　consolidated　by　the　occurrence　of　severe　PM　pollution　in　the　BTH　during　the　COVID-19　outbreak,　with　a　significant　reduction　in　NO2　concentration.　Mitigation　of　NH3　emissions　constitutes　the　priority　measure　to　effectively　lower　the　nitrate　and　PM2.5　concentrations　in　the　BTH　under　current　conditions,　with　35.5%　and　12.7%　decrease,　respectively,　when　NH3　emissions　are　reduced　by　50%.　©　2021　Elsevier　Ltd