What was done
Recognizing the potential of large metropolitan areas to act as major sources of anthropogenic carbon dioxide emissions, the authors devised a measurement program to detect and quantify the spatial variability of CO2 levels in the near-surface air over the Phoenix metropolitan area.  To accomplish their objective, air temperatures, relative humidities, and atmospheric CO2 concentrations were measured at a height of 2 m at approximate 1.6-km (1.0-mile) intervals prior to sunrise and in the middle of the afternoon across four transect routes through the Phoenix metro area during a five-day period in January of 1998.

What was learned
The data revealed the presence of what the authors called an "urban CO2 dome," where concentrations in the city center reached as high as 555 ppm.  Concentrations diminished from the city center toward the outlying rural areas, where they were reported to be around 370 ppm.  Pre-dawn CO2 values inside the dome were found to be considerably higher than mid-afternoon values, and temperature and relative humidity were found to exhibit little influence on the magnitude or location of the CO2 dome.

What it means
This paper identifies a largely unexplored area of research - urban carbon dioxide concentrations - and will undoubtedly lead to a number of future studies that further quantify the urban CO2 dome's existence as well as its biological impacts.  Among several potential effects of the urban CO2 dome, the authors note that "high CO2 concentrations within the dome may help to ameliorate the deleterious effects of urban air pollution on vegetation growing within the city" by reducing the leaf stomatal openings by which pollutants gain entrance to plants.