Recently, highly sensitive near-IR laser absorption spectrometers have been employed to measure ammonia (NH3) emissions. These instruments allow in-situ measurements of highly time-resolved NH3 emission levels in automobile exhaust. However, the effect of the automobile exhaust CO2 in NH3 measurements has not been studied in detail. Because the CO2 concentration in automobile exhaust is 2 to 3 orders of magnitude higher than the NH3 concentration, there is a possibility that spectral overlap by CO2 lines and/or the spectral broadening of NH3 by CO2 could affect the measured NH3 levels.This study had two major objectives. First, the effect of CO2 on the measured NH3 concentration was assessed using our developed near-IR laser absorption spectrometer. The second objective was to provide on-board NH3 measurements in the hybrid gasoline automobile exhaust using the developed spectrometer.As a result, the CO2 in automobile exhaust was found to affect the measured NH3 concentration. The NH3 concentrations in automobile exhaust differed by few percent at 0%-20% CO2 concentrations. The NH3 concentration values in automobile exhaust were corrected by using these data. On-board measurements of NH3 concentration in the hybrid gasoline automobile exhaust system showed that the NH3 emission prevailed under the conditions in which the automobile was running rich or during overly aggressive driving (e.g. on highway).