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 novel remote sensor, incorporating IR and UV dispersive spectrometers, was developed. 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. Subsequent on-road remote sensing measurements on over 4,500 vehicles showed that NH3 emissions follow a μ-distribution, with 10 % of the fleet being responsible for 66 % of the total emissions. Mean NH3 emissions for the measured fleet were 78.6 ppm, significantly higher than previously estimated.