Increasing　ammonia　emissions　could　exacerbate　air　pollution　caused　by　fine　particulate　matter　(PM2.5).　Therefore,　it　is　of　great　importance　to　investigate　ammonia　oxidation　in　PM2.5.　This　study　investigated　the　diversity,　abundance　and　activity　of　ammonia　oxidizing　archaea　(AOA),　ammonia　oxidizing　bacteria　(AOB)　and　complete　ammonia　oxidizers　(Comammox)　in　PM2.5　collected　in　Beijing-Tianjin-Hebei　megalopolis,　China.　Nitrosopumilus　subcluster　5.2　was　the　most　dominant　AOA.　Nitrosospira　multiformis　and　Nitrosomonas　aestuarii　were　the　most　dominant　AOB.　Comammox　were　present　in　the　atmosphere,　as　revealed　by　the　occurrence　of　Candidatus　Nitrospira　inopinata　in　PM2.5.　The　average　cell　numbers　of　AOA,　AOB　and　Ca.　N.　inopinata　were　2.82　x　10(4),　4.65　x　10(3)　and　1.15　x　10(3)　cell　m(-3)　air,　respectively.　The　average　maximum　nitrification　rate　of　PM2.5　was　0.14　mu　g　(NH4+-N)　[　m(3)　air.h](-1).　AOA　might　account　for　most　of　the　ammonia　oxidation,　followed　by　Comammox,　while　AOB　were　responsible　for　a　small　part　of　ammonia　oxidation.　Statistical　analyses　showed　that　Nitrososphaera　subcluster　4.1　was　positively　correlated　with　organic　carbon　concentration,　and　Nitrosomonas　eutropha　showed　positive　correlation　with　ammonia　concentration.　Overall,　this　study　expanded　our　knowledge　concerning　AOA,　AOB　and　Comammox　in　PM2.5　and　pointed　towards　an　important　role　of　AOA　and　Comammox　in　ammonia　oxidation　in　PM2.5.