Atmospheric CO2 mixing ratios and carbon and oxygen isotope composition were measured at 18 m above the ground in Salt Lake City, Utah, United States, for a one-year period. Mixing ratios were highest in the wintertime with maximum values approaching 600 μmol.mol−1 during atmospheric inversions. Nighttime carbon and oxygen isotope ratios of source CO2 showed a seasonal pattern with isotopically depleted values in the wintertime and isotopically enriched values in the spring and summer. The effects of gasoline combustion, natural gas combustion, and biogenic respiration of plants and soils on CO2 mixing ratio were quantified with a mass balance calculation using dual carbon and oxygen isotopic tracers. The calculations showed large contributions of natural gas combustion in the winter and significant nighttime biogenic respiration in the spring and late summer/early fall. The isotope-tracer technique used shows promise for quantifying the impacts of urban processes on the isotopic composition of the atmosphere and partitioning urban CO2 sources into their component parts.