A　coupled　ARPS-CMAQ　modeling　system　was　applied　to　investigate　the　atmospheric　assimilative　capacity　(AAC)　of　PM10　in　the　Beijing　metropolitan　region　of　China.　The　AAC　was　defined　as　the　maximum　allowable　pollutant　emission　that　can　be　discharged　to　the　atmosphere　without　violating　the　desired　air　quality　objective　in　the　planning　region.　The　coupled　modeling　system　was　firstly　evaluated　through　comparing　the　simulation　results　of　PM10　concentration　with　the　corresponding　observations　during　four　representative　months　in　2002,　which　showed　an　acceptable　modeling　performance.　By　using　a　trial-and-error　approach,　the　validated　modeling　system　was　run　through　gradually　reducing　the　emission　strength　within　the　modeling　domain　from　a　GIS-based　pollutant　emission　database　until　a　desired　air　quality　objective　was　achieved.　The　air　quality　objective　was　characterized　by　the　air　quality　guideline-satisfaction　ratio　(AQGSR)　which　was　defined　as　the　ratio　of　days　when　the　daily　PM10　concentrations　are　below　the　air　quality　standard　during　a　certain　time　period.　The　PM10　emission　within　the　modeling　domain　after　such　trial-and-error　reduction　under　the　condition　of　satisfying　a　certain　AQGSR　was　considered　its　corresponding　assimilative　capacity.　Accordingly,　the　atmospheric　assimilative　capacities　of　PM10　in　the　study　region　were　identified　corresponding　to　the　AQGSR　of　60,　65,　70,　75　and　80%,　respectively.　The　relation　between　such　capacity　and　guideline-satisfaction　ratio　was　then　established　through　regression　analysis.　The　results　could　provide　sound　basis　for　decision　makers　in　terms　of　effective　air　quality　management　by　understanding　the　allowable　pollutant　discharge　under　different　desired　air　quality　objectives.