One of the approaches to meet Canada’s Greenhouse gas (GHG) reduction goal is to set emission mitigation plans in mega cities. The Greater Toronto Area (GTA), is the largest mega city in Canada with a population of over 6 million.  To set a mitigation plan, it’s important to first quantify the GHG emissions  in the city by preparing an emission inventory. The next step is to monitor GHG levels in the atmosphere on a regular basis to verify the current emission inventory estimates and observe any trends that might follow specific mitigation policies.  In this study, we first developed a bottom-up methane (CH4) emission inventory with a high spatial resolution for the GTA that complements the existing CO2 emission inventory. By monitoring GHG mixing ratios in the GTA, using stationary and mobile in-situ measurements as well as total column measurements using Fourier transform spectrometers and satellite measurements we try to improve our estimates in the existing bottom-up inventories.