The mitigation of nitrous oxide (N 2 O) is of primary significance to offset carbon footprints in aerobic granular sludge (AGS) systems. However, a significant knowledge gap still exists regarding the N 2 O production mechanism and its pathway contribution. To address this issue, the impact of varying granule sizes, dissolved oxygen (DO), and nitrite (NO 2 - ) levels on N 2 O production by ammonia-oxidizing bacteria (AOB) during nitrification in AGS systems was comprehensively investigated. Biochemical and isotopic experiments revealed that increasing DO or decreasing NO 2 - levels reduced N 2 O emission factors (by 13.8 or 19.5%) and production rates (by 0.08 or 0.35 mg/g VSS/h) via weakening the role of the AOB denitrification pathway since increasing DO competed for more electrons required for AOB denitrification. Smaller granules (0.5 mm) preferred to diminish N 2 O production via enhancing the role of NH 2 OH pathway (i.e., 59.4-100% in the absence of NO 2 - ), while larger granules (2.0 mm) induced conspicuously higher N 2 O production via the AOB denitrification pathway (approximately 100% at higher NO 2 - levels). Nitrifying AGS systems with a unified size of 0.5 mm achieved 42% N 2 O footprint reduction compared with the system with mixed sizes (0.5-2.0 mm) under optimal conditions (DO = 3.0 mg-O 2 /L and NO 2 - = 0 mg-N/L).