Both nitrous oxide (N2O) and nitric oxide (NO) (as a N2O precursor during denitrification) have negative effects on the environment. N2O is also a potent greenhouse gas. N2O and NO emissions in full-scale wastewater treatment plants (WWTPs) are mostly from mainstream activated sludge processes, but there are fewer emission studies from biofilm systems. In this study, online monitoring of NO and N2O emissions from a full-scale biological aerated filter (BAF) for secondary nitrification was performed for the first time over 12 months. Impacts of influent quality (influent NH4+-N, NO3--N, NO2--N) and environmental factors (pH, dissolved oxygen (DO), and water temperature) on N2O and NO emissions were also evaluated. The results show that N2O and NO emissions from the BAF occurred even though DO was high at 6.94-8.86 mg/L; they greatly fluctuated with the season that much greater in spring than other seasons (summer, autumn and winter). N2O and NO emission factors were 0.017-1.261% and 0.00017-0.0041% of influent total nitrogen load, respectively, with an average ammonia removal rate of 60%. The N2O emission factor for the secondary nitrification BAF was comparable to that of mainstream activated sludge systems in WWTPs, and much lower than that of biofilm reactors in lab-scale. Nevertheless, the low influent ammonia loading of 0.004-0.05 kg NH4+-N/m(3) d contributed to the reduced N2O emissions for the secondary nitrification BAF, compared with that from mainstream wastewater treatment processes. Nitrite seems to be the most important cause of NO and N2O production, and nitrifler denitrification pathway was postulated to be the main contributor to N2O and NO production in the studied BAF. (C) 2016 Elsevier B.V. All rights reserved.