What was done
The authors explored the nature and origin of the urban CO2 dome of Cotonou - the main seaport and largest city (~1.2 million inhabitants) of the West African Republic of Benin, which is located along the coast of the Gulf of Guinea - via direct measurements of screen-level atmospheric CO2 concentration and indirect assessments based on δ¹³C measurements of Eleusine indica, an annual C4 goose-grass that grows randomly throughout the city at roadsides and in lawns and gardens.

What was learned
Noting that direct measurements carried out in Phoenix, Arizona, USA by Idso et al. (2001, 2002) "showed that the peak CO2 concentration in the city center was about 40% greater than that of the surrounding rural area," Kelome et al. report that "the same observation was made in Cotonou, where the mean CO2 concentration can reach 650 ppm compared to the surrounding rural area CO2 concentration of about 380 ppm." They also found that concentrations as high as 900 ppm were periodically obtained in Cotonou's "main trade center, the industrial zone, the harbor area, and the main crossroads in high traffic zones," as well as at a site where "waste collected all over the city is accumulated in marshy zones and burned."

Based on their δ¹³C measurements of Eleusine indica, the five researchers determined that the amount of fossil fuel carbon assimilated by the plants was as great as 20-22% of total plant carbon content in the most polluted areas of the city; and they report that "the proportion of anthropogenic CO2 is positively correlated with the number of polluting vehicles."

What it means
The findings of this study mesh well with those obtained from Phoenix, Arizona, USA, as well as Other Cities around the world, increasing our confidence in the nature and origin of the urban CO2 dome phenomenon. In addition, Kelome et al. suggest that "using Eleusine indica as an indicator provides a useful tool to monitor the sequestration of fossil fuel CO2 in urban vegetation."

References
Idso, C.D., Idso, S.B. and Balling Jr., R.C. 2001. An intensive two-week study of an urban CO2 dome. Atmospheric Environment 35: 995-1000.

Idso, S.B., Idso, C.D. and Balling Jr., R.C. 2002. Seasonal and diurnal variations of near-surface atmospheric CO2 concentrations within a residential sector of the urban CO2 dome of Phoenix, AZ, USA. Atmospheric Environment 36: 1655-1660.