Beijing　has　achieved　significant　air　quality　improvement　over　the　past　decade.　In　this　study,　we　applied　a　simulation　technique　on　pollutant　monitoring　data　to　investigate　the　contribution　of　emission　reduction　(ERE)　and　meteorological　effects　(ME)　to　the　improvement　of　air　quality.　We　measured　the　daily　PM2.5　concentrations　during　four　months　(representing　the　four　seasons)　in　2013　and　2018.　We　found　that　the　concentrations　of　NH4+,　NO3,　and　SO42-　decreased　from　2013　to　2018,　but　the　proportion　of　NO3　in　PM2.5　showed　an　apparent　increase.　Multiple　scenarios　were　simulated　using　the　WRF-CAMx　model　to　determine　the　impact　of　ERE　and　ME　on　air　pollution.　The　results　showed　that　both　variables　positively　contributed　to　the　annual　air　quality　improvement,　with　ERE　contributing　more　than　70%　to　the　total　PM2.5　reduction.　However,　their　seasonal　contributions　showed　notable　differences.　In　winter,　ME　contributed　more　than　40　mu　g/m(3)　to　the　PM2.5　decrease,　which　was　much　higher　than　that　in　other　seasons.　This　suggests　that　the　positive　effect　of　pollutant　emission　reduction　on　air　quality　may　have　been　weakened　under　extremely　adverse　meteorological　conditions.　Hence,　the　monitoring　and　modeling　results　were　coupled　to　evaluate　the　secondary　conversion　degree.　We　found　that　the　emissions　of　precursors,　including　SO2　and　NOx,　notably　decreased,　but　both　the　local　and　regional　conversion　degrees　increased　from　2013　to　2018.　Therefore,　for　effective　air　pollution　reduction,　we　suggest　that　future　air　pollution　mitigation　measures　should　focus　on　reducing　secondary　pollution　in　Beijing.