What was done
In an attempt to better understand the origin of the urban CO2 dome that resides over Phoenix, Arizona (Idso et al., 1998, 2001, 2002), the authors quantified sources of CO2 emissions across the metropolitan region.

What was learned
In descending order of importance, and including the percentage of the total CO2 emitted to the atmosphere within the confines of the Phoenix metropolitan area, the authors list the following contributors to the urban CO2 dome of Phoenix: vehicles (79.9%), soil respiration (15.8%), power plants (2.2%), human respiration (1.6%), landfills (0.5%), airplanes (< 0.1%).

What it means
In the words of the authors, "anthropogenic sources of CO2 appear to be largely responsible for the CO2 dome present in the Phoenix valley," and of these, they say, "vehicles are by far the largest contributor."  In addition, they note that the major regions of soil CO2 emissions, which are second in importance, "are located in the agricultural regions on the border of Phoenix."  Hence, the character of the central city's urban CO2 dome is almost exclusively a product of vehicular emissions and the region's distinctive meteorology, as noted in the papers of Idso et al. (1998, 2001, 2002).

As an aside, the authors note that urbanization also creates higher temperatures - the urban heat island effect - and that these higher temperatures "may actually cancel out any CO2 fertilization effects on the [urban] ecosystem."  Viewed from a more optimistic perspective, we suggest that the greater atmospheric CO2 concentrations of the urban CO2 dome likely cancel out high-temperature-induced stresses that would otherwise be felt by - and possibly seriously harm - the city's vegetation.

References
Idso, C.D., Idso, S.B. and Balling Jr., R.C.  1998.  The urban CO2 dome of Phoenix, Arizona.  Physical Geography 19: 95-108.

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.