Real-time plant-integrated measurements of CH4 and N2O emissions were conducted at an open-air municipal WWTP using a novel instrument able to measure very small (a few ppb) concentration changes of CH4 and N2O. This high sensitivity enabled downwind plume measurements of both CH4 and N2O, which combined with tracer gas emissions provided real-time plant-integrated emission rates from an open-air WWTP. In the case of N2O, this had not been reported previously. A wide range of emission rates were observed, both between and within sampling campaigns. High emissions were associated with operational problems such as digester foaming and sub-optimal operation of the biological nitrogen removal process, as well as changes in environmental conditions, such as water temperature. For both CH4 and N2O, emission rates increased almost tenfold during these periods. Further research to identify the exact cause of emissions, and to develop mitigation plans, is necessary for minimizing the carbon footprint of WWTPs.