Even in urbanized regions, carbon dioxide (CO2) emissions are derived from a variety of biogenic and anthropogenic sources and are influenced by atmospheric transport across borders. As policies are introduced to reduce the emissions of CO2, there is a need for independent verification of emissions reporting. In this work, we use carbon isotope (12CO2 and 13CO2) simulations in combination with atmospheric measurements to distinguish between CO2 sources in the Greater Toronto Area (GTA). This is being done by developing an urban δ13C framework based on CO2 emissions data and forward modelling. We developed the UofT/ECCC inventory, a CO2 inventory for southern Ontario at a very fine spatial and temporal resolution (0.02ox0.02o and hourly, respectively). The inventory is run with the GEM-MACH chemistry transport model and results are used in our framework in combination with region-specific δ13C signatures of the dominant CO2 sources; the product is compared against highly accurate 13CO2 and 12CO2 ambient data made at sites across southern Ontario. The strength of this framework is its potential to estimate contributions of both locally-produced and regionally-transported CO­2. Locally, anthropogenic CO­2 in urban areas is often derived from natural gas combustion (for heating) and gasoline/diesel combustion (for transportation); the isotopic signatures of these processes were measured to be significantly different (approximately d13CVPDB = -44 ‰ and -28 ‰ respectively) in the GTA and can be used to infer their relative contributions. Utilizing our δ13C framework and differences in sectoral isotopic signatures, we quantify the relative contribution of CO2 sources on the overall measured concentration and assess the ability of this framework as a tool for tracing the evolution of sector-specific emissions.