What was done
Between 15 Dec 2004 and 20 Jan 2005 at a height of 18 meters above the ground on the campus of the University of Utah in Salt Lake City, the authors measured atmospheric CO2 concentration and its stable carbon isotope composition (δ¹³C) by means of tunable diode laser absorption spectrometry, conventional isotope ratio mass spectrometry, and infrared gas analysis.

What was learned
In terms of maximum CO2 concentrations observed, toward the end of the measurement period values as high as 600 ppm were recorded, coinciding with a major inversion event. On a diurnal basis, there was a pattern of "relatively larger contributions of natural gas combustion in early morning, pre-dawn hours representing about 60-70% of total fossil fuel-derived CO2, and smaller contributions of about 30-40% during late afternoon and evening rush hour," which findings, according to Pataki et al., are "consistent with greater natural gas use during cold nighttime hours and increased gasoline combustion during evening rush hour." In addition, they report there was a pattern of "decreasing relative contributions of natural gas combustion over [a] week-long measurement period that corresponded to increasing ambient air temperature," which the researchers say is likely due to "reduced natural gas usage for residential heating during a warming period."

What it means
The study demonstrated "for the first time," in the words of the scientists who conducted the work, that "atmospheric measurements may be used to infer patterns of energy and fuel usage on hourly to daily time scales," and that they can provide "insight into urban energy use patterns and drivers." In addition, they shed more light on the origin of the urban CO2 dome, highlighting the major roles played by the heating of homes and other buildings by the burning of natural gas and the powering of cars and other vehicles by the burning of gasoline.