SO42-,　NO3-　and　NH4+　(SNA)　are　the　major　components　of　particulate　matter　in　Beijing.　In　this　study,　hourly　concentrations　of　SNA　in　PM1　were　measured　through　an　Aerodyne　Aerosol　Chemical　Speciation　Monitor　(ACSM)　at　an　urban　site　of　Beijing,　from　February　3rd　to　20th　of　2017.　The　analysis　of　the　differences　between　two　pollution　cases　during　the　sampling　period　was　further　conducted.　The　results　showed　that　the　concentrations　of　NO3-,　SO42-　and　NH4+　were　38.9　+/-　10.5　mu　g/m(3),　41.9　+/-　22.2　mu　g/m(3)　and　31.3　+/-　10.6　mu　g/m(3)　respectively　in　the　first　case.　A　negative　correlation　was　reflected　between　the　PM2.5　concentrations　and　the　following　two　ratios:　1)　NO3-/SO42-;　2)　[NH4+]　to　([NO3-]+2[SO42-]),　with　a　correlation　coefficient　of　-0.92　and　-0.76　respectively.　In　the　second　case,　the　PM2.5　concentrations　(proportions)　were　much　higher,　with　values　of　49.9　+/-　17.0　mu　g/m(3)　(22.9　+/-　7.7%),　42.3　+/-　15.8　mu　g/m(3)　(20.3　+/-　7.2%)　and　33.1　+/-　10.1　mu　g/m(3)　(15.3　+/-　4.3%).　In　contrast.　The　two　ratios　were　positively　correlated　with　the　PM2.5　concentrations,　along　with　the　correlation　coefficient　of　0.77　and　0.62.　Hence,　it　is　palpable　that　two　pollution　cases　were　sulfate　and　nitrate　dominant　in　respective.　The　emission　characteristics　due　to　the　Chinese　Spring　Festival　before　P1　might　be　a　major　cause.　A　modeling-monitoring　coupled　approach　was　established　for　the　recognition　of　the　secondary　conversion　of　SO42-　and　NO3-.　Results　showed　that　sulfate/nitrate　aged　ratio　(SAR　and　NAR)　and　sulphur　oxidation　ratio　(SOR)　in　the　second　case　were　0.48　+/-　0.23,　0.22　+/-　0.07　and　0.38　+/-　0.23.　They　were　all　higher　than　those　of　the　first　case　whose　values　were　0.32　+/-　0.15,　0.18　+/-　0.05　and　0.22　+/-　0.12,　which　indicated　that　secondary　conversion　of　precursors　was　enhanced　in　the　second　case.　Although　the　OH　concentrations　were　of　no　obvious　difference　in　both　cases,　the　average　concentrations　of　H2O2,　NO2　and　the　relative　humidity　were　much　higher　in　the　second　case,　which　provided　sufficient　conditions　and　time　for　the　formation　of　SO42-　and　NO3-.