The satellites that have been designed to support the monitoring of fossil fuel CO2 emissions aim to systematically measure atmospheric CO2 plumes generated by intense emissions from large cities, power plants and industrial sites. These data can be assimilated into atmospheric transport models in order to estimate the corresponding emissions. However, 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. We show that these amounts represent a significant proportion of the fossil fuel CO2 emissions, up to 40% for instance in cities of Nordic countries, and will thus leave some ambiguity in the retrieval of fossil fuel CO2 emissions from satellite concentration observations. Auxiliary information such as biofuel use statistics and radiocarbon measurement could help reduce the ambiguity and improve the framework of monitoring fossil fuel CO2 emissions from space.