What was done
The authors made spatially and temporally intensive measurements of atmospheric CO2 concentration across an urban-to-rural CO2 gradient comprised of Salt Lake City's downtown business district, a residential neighborhood, and a non-urbanized rural location within the Salt Lake Valley (Utah, USA) from 2004 to 2006, as well as measurements of CO2 and water isotopic composition at the same locations over a one-year period.

What was learned
The highest CO2 concentrations, occasionally exceeding 500 ppm, were recorded during wintertime inversions, with the downtown site showing the highest CO2 concentrations at night, but exhibiting similar values to the neighborhood site in the daytime, while the rural site, according to the four researchers, "showed consistently lower and relatively constant values." This pattern was also observed in the summer, although they found that "absolute CO2 concentrations were lower at all three sites, particularly during afternoon, well-mixed atmospheric conditions." In addition, they report that the "inverse analysis of CO2 sources and the O isotope composition of ecosystem respiration showed large contributions (>50%) of natural gas combustion to atmospheric CO2 in the wintertime, particularly at the city center, and large contributions (>60%) of biogenic respiration to atmospheric CO2 during the growing season, particularly at the rural site."

What it means
Pataki et al. say their results "illustrate that spatial and temporal patterns of urban CO2 concentrations and isotopic composition can be used to infer patterns of energy use by urban residents as well as plant and soil processes in urban areas," concluding that "these measurements have great potential for inferring patterns of human activities and biological respiration in complex, human-dominated ecosystems."