The carbon isotope composition of atmospheric CO2 in Paris

One characteristic of air pollution in the urban environment is high CO2 concentrations resulting from human activities. Determining the relative contributions of the different CO2 sources can be addressed simply and elegantly by combining isotope and concentration measurements. Using this approach on atmospheric CO2 samples collected in Paris, its suburbs and the open country provides fairly accurate conclusions. Our results show that air pollution within the first few metres above ground results basically from binary mixtures among which road traffic is the main contributor and, in particular, vehicles using unleaded gasoline (∼90% of the total). Heating sources, which account for 50% of the CO2 input below the atmospheric inversion level, and vehicles using diesel contribute very little. Human respiration has a recognisable signature at street level under certain circumstances. The combined isotope and concentration analysis provides a sensitive tracer of local variations, even detecting the occasional prevalence of human respiration and the onset of actions in which natural gas is burnt. It also detects surprising inlets of ‘clean air’ (CO2-wise) in the very centre of the city.