Municipal wastewater treatment plants (WWTPs) generate and emit greenhouse gases (GHG) via a number of activities and operations such as pumping, aeration, mixing, and sludge dewatering and digestion. As part of the collective efforts to achieve a 2050 "net-zero" emission goal, public utilities have started including quantitative sustainability indices as important factors to be evaluated when major capital improvements and O&M modifications are being planned for their water infrastructures. GHG emissions is one important sustainability index. At a typical WWTP, GHG emissions are fugitive, heterogenous, and time-varying, which make it extremely difficult to measure. Utilizing the widely adopted "emission factors", such as the IPCC database, for estimating the GHG emissions is rather empirical or sometimes even speculative. A robust field measurement approach is needed that is capable of capturing the dynamic, fugitive emissions at WWTPs. In this project, we have developed a hybrid method integrating the long range open-path Optical Remote Sensing (ORS) technology and inverse Dispersion Modeling (iDM) to continuously quantify GHG emissions from the wastewater treatment facilities on a real-time basis. The ORS instrument used in the field campaign consisted of an open-path tunable diode laser spectroscopy (OP-TDL) for CH4, a CO2 sensor, a portable meteorological station and 3-D sonic anemometer to record temperature, air pressure, relative humidity, and wind vector, and an open-path Fourier transform infrared (OP-FTIR) for N2O. Both the OP-TDL and OP-FTIR scanned along multipaths to capture the "big picture" of GHG plumes for dispersion model input for emission estimation. Backward Lagrange Stochastic model (bLS) was used to estimate the methane emission rate in real time. The system was first applied at Bonnybrook WWTP in Canada. This presentation will describe the newly developed monitoring methodology and present the results of continuous GHG measurement from the field campaigns conducted during different seasons. The site-specific emission factors (EFs) developed based on the measurements have been compared with the literature and the IPCC values, which will also be presented to show the inaccuracy of using empirical, non-site-specific emission factors.