Estimates of fossil fuel emissions on local to regional spatial scales and hourly to weekly temporal scales are increasingly useful in studies of the carbon cycle and mass and energy flow in cities. We used a tunable diode laser absorption spectrometer (TDL) to measure CO2 mixing ratios and carbon isotope composition of CO2 in order to estimate the contribution of gasoline versus natural gas combustion to atmospheric CO2 in Salt Lake City. The results showed a pronounced diurnal pattern: the proportional contribution of natural gas combustion varied from 30--40% of total anthropogenic CO2 during evening rush hour to 60--70% at pre-dawn. In addition, over a warming period of several days, the proportional contribution of natural gas combustion decreased with air temperature, likely related to decreased residential heating. These results show for the first time that atmospheric measurements may be used to infer patterns of energy and fuel usage on hourly to daily time scales.