Satellites designed to support the monitoring of fossil fuel CO2 emissions aim to measure systematically atmospheric plumes of column-integrated CO2 concentration generated by the intense emissions from large cities, power plants and industrial sites. Atmospheric transport models are used to assimilate these data by inverting the emissions that best match the atmospheric observation. We show that the plumes emitted by cities and powerplants contain not only fossil fuel CO2 but also significant amounts of CO2 released by human respiration and by the burning of biofuels, which will leave some ambiguity in the retrieval of fossil fuel CO2 emissions from satellite concentration measurements.