The goal of the project is develop an independent method of evaluating source specific CO2ff emissions at the urban scale, with quantifiable uncertainties. The project will result in identification of trace gases that are related to specific CO2ff source sectors and quantify CO2ff from those source sectors. A new methodology for forward modeling and inversion of atmospheric trace gas ratios will be developed. The proposed study will build a framework for estimating fossil fuel CO2 (CO2ff) by sector for urban regions. This study will leverage the existing Indianapolis Flux Project (INFLUX) sampling infrastructure, observational dataset, bottom-up data products and modeling tools. The analysis framework will include careful assessment of multi-species trace gas measurements to identify emissions ratios specific to seven urban emissions sectors. This study will also combine high-resolution transport models and bottom up emissions models with the multi- species trace gas measurements in an inverse analysis, which will provide emissions estimates for each sector and the associated uncertainties.