What was done
In May of 1996 the authors collected samples of the aerial parts of several species of grass growing in Paris, France, within 2 to 40 meters of a major high-traffic highway that was traveled by about 8,000,000 vehicles per day, as well as samples of grasses that grew in remote rural areas of the country.  ð¹³C values of the samples were then obtained, after which the relative contributions of background atmospheric carbon and fossil-fuel-derived carbon were calculated.

What was learned
Fossil-fuel-derived carbon in the urban grasses was found to comprise 23.3 ± 0.8% of the total carbon they contained, which suggests that during the daylight hours of this time of year (May), near-surface atmospheric CO2 concentrations at the studied site were likely elevated by about the same percentage above background rural values.  By comparison, over the complete daylight period of the same time of year in a suburban residential area of Phoenix, Arizona, USA, atmospheric CO2 concentrations averaged 10% more than the surrounding rural values (Idso et al., 2002).  The higher values implied for Paris are likely due to sampling close to a major high-traffic highway, as well as the fact that the urban CO2 dome of Paris may be even stronger than that of Phoenix, based upon measured maximum central-city CO2 concentrations on the order of 620 ppm in Phoenix (Idso et al., 2002) and 950 ppm in Paris (Widory and Javoy, 2003).

What it means
The authors say their findings suggest that the assessment of the ¹³C isotopic composition of grasses "represents a promising new tool for the study of the impact of fossil fuel CO2 in major cities."  We also note that their results demonstrate the importance of fossil-fuel-derived carbon for the growth and development of urban vegetation, and that the elevated CO2 concentrations of major cities may help plants to better cope with the deleterious effects of air pollution [see Ozone (Effects on Plants) and Air Pollution (Non-Ozone) - Effects on Plants in our Subject Index] as well as the additional summer heat stress produced by the urban heat island phenomenon [see Growth Response to CO2 With Other Variables (Temperature) in our Subject Index].

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.

Widory, D. and Javoy, M.  2003.  The carbon isotope composition of atmospheric CO2 in Paris.  Earth and Planetary Science Letters 215: 289-298.