Direct remote sensing of vehicle exhaust emissions under real-world driving conditions is desirable for a number of reasons, including: identifying high emitters, investigating the chemical composition of the exhaust, and probing fast reactions in the plume. A remote sensor, incorporating IR and UV spectrometers, was developed. The IR spectrometer consists of a grating system mounted on a synchronous motor, optically interfaced to a room temperature PbSe detector. UV-vis measurements are made with a CCD array spectrometer. Eight optical passes through the exhaust plume allow rapid and sensitive monitoring of the exhaust stream emitted by moving vehicles on a car-by-car basis. The combination of these two techniques resulted in unprecedented, direct measurement capability of over 25 pollutants in the exhaust plume. Emissions from a fleet of vehicles powered by a range of fuels (gasoline, diesel, natural gas, and methanol) were tested. The exhaust from hot gasoline- and methanol-powered cars contained high levels of NH3, up to 1500 ppm. These emissions were up to 14 times higher than the corresponding NOx emissions. Unlike most previous work, NOx was measured as the sum of NO and NO2; N2O was also measured. Field testing at a southern California freeway on-ramp was conducted over a one week period, totaling >4,500 measurements. It was found that 66.4% of the emitted NH3 was produced by 10% of the fleet, following the (gamma) - distribution that has been reported for criteria pollutants. Mean NH3 emission rates were calculated at 138 mg km-1, nearly twice as high was previous estimates.