What was done
The authors measured atmospheric CO2 concentrations continuously, and carbon and oxygen isotopic composition weekly, 18 meters above the ground in Salt Lake City, Utah, USA, for a period of one year, after which the seasonal cycles of ð¹³C and ð¹⁸O were used to assess the proportional contributions of natural gas combustion, gasoline combustion, and biogenic respiration to the urban CO2 concentration in excess of the rural background concentration at different times of the year.

What was learned
Urban CO2 concentrations were highest in the winter, with maximum nighttime values approaching 600 ppm.  Nighttime average values, however, were considerably lower, ranging from approximately 390-480 ppm in the winter and from 375-400 ppm in the spring and summer.  Afternoon values, on the other hand, were typically within 5 ppm of the rural background value of 372 ppm.

The isotopic measurements revealed that approximately 60% of the winter urban-rural CO2 differential came from natural gas combustion, while 40% was derived from the burning of gasoline.  This latter component remained a large portion of the urban CO2 dome in summer, while the natural gas component vanished and biogenic plant and soil respiration had their largest effect in the spring and late summer.

What it means
As has been demonstrated in most other urban CO2 dome studies [see Urban CO2 Dome in our Subject Index], the vast majority of the urban CO2 concentration in excess of the rural background concentration owes its existence to the burning of fossil fuels.