Atmospheric CO2 inversion studies infer surface sources and sinks from observations and models. These studies usually require determination of the fossil fuel component of the observation, which can be estimated using anthropogenic tracers such as CO. The objective of this study is to demonstrate a new CO tracer method that accounts for overlapping forest fire and photochemical CO influences, and to quantify several aspects of the uncertainty in the CO tracer technique. Photochemistry model results and observations from the International Consortium for Atmospheric Research on Transport and Transformation experiment are used to quantify changes in the fossil fuel CO2 prediction from the CO tracer method with and without the inclusion of CO from biomass burning and photochemistry. Although the chemical sources and sinks tend to offset each other, there are regions where the chemical reactions change fossil fuel CO2 predictions by up to ±4 ppm. Including biomass burning lowers fossil fuel CO2 by an average of 12 ppm in plumes heavily influenced by long-range transport of forest fire CO. An alternate fossil fuel CO2 calculation is done in a power plant plume using SO2 as a tracer, giving a change in 20 ppm from the CO method, indicative of uncertainty in the assumed CO:CO2 ratio.