Background
It is generally assumed that humans are responsible for nearly all of the increase in the air's CO2 content that has occurred since the inception of the Industrial Revolution, and that this increase resulted primarily from the burning of fossil fuels that accompanied the rise of mechanized industry and the concomitant explosive growth of the planet's human population.  This being the case, one might expect large population centers to exhibit elevated near-surface atmospheric CO2 concentrations; and a number of studies confirm this expectation, as may be verified by visiting the Urban CO2 Dome section of our website.

What was done
To determine the relative contributions of different CO2 sources to the urban CO2 dome of Paris, the authors measured CO2 concentrations and carbon isotope abundances in air samples collected from various locations in the city (streets, gardens, etc.), its suburbs and the surrounding open countryside, as well as from vehicles, heating sources, power stations, etc., and from laboratories and classrooms where the human component of the CO2 dome would be expected to be dominant.

What was learned
The near-surface atmospheric CO2 concentration throughout the countryside surrounding Paris averaged 415 ± 9 ppm, while values in the city sometimes reached as high as 950 ppm.  These higher values were driven primarily by vehicular exhaust, as the authors report that "road traffic is the main contributor and, in particular, vehicles using unleaded gasoline (~90% of the total)."  In enclosed spaces, however, the CO2 derived from human respiration dominated.  In a 150-m³ classroom containing 20 students for a period of four hours, for example, the CO2 concentration rose as high as 4630 ppm, which "corresponds well," in the words of the authors, "to an average individual respiration flux of 5 l/min containing ~3.7% CO2."

What it means
The results of this interesting study confirm the central conclusion of studies of the urban CO2 dome of Phoenix, Arizona, i.e., that it owes its existence primarily to the CO2 that comes from vehicular exhaust (Idso et al., 2002; Koerner and Klopatek, 2002).

References
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.

Koerner, B. and Klopatek, J.  2002.  Anthropogenic and natural CO2 emission sources in an arid urban environment.  Environmental Pollution 116, Supplement 1: S45-S51.