C4 plant isotopic composition (δ13C) evidence for urban CO2 pollution in the city of Cotonou, Benin (West Africa)

The carbon isotopic composition (δ13C) of plants can reveal the isotopic carbon content of the atmosphere in which they develop. The δ13C values of air and plants depend on the amount of atmospheric fossil fuel CO2, which is chiefly emitted in urban areas. A new indicator of CO2 pollution is tested using the δ13C variation in a C4 grass: Eleusine indica. A range of about 4‰delta units was observed at different sites in Cotonou, the largest city in the Republic of Benin. The highest δ13C values, from −12‰ to −14‰, were found in low traffic zones; low δ13C values, from −14‰ to −16‰, were found in high traffic zones. The amount of fossil fuel carbon assimilated by plants represented about 20% of the total plant carbon content. An overall decrease in plant δ13C values was observed over a four-year monitoring period. This decrease was correlated with increasing vehicle traffic. The δ13C dataset and the corresponding geographical database were used to map and define zones of high and low 13C-depleted CO2 emissions in urban and sub-urban areas. The spatial distribution follows dominant wind directions, with the lowest emission zones found in the southwest of Cotonou. High CO2 emissions occurred in the north, the east and the center, providing evidence of intense anthropogenic activity related to industry and transportation.