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 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 1,500 ppm. These emissions were up to 14 times higher than the corresponding NOx emissions. Subsequent on-road remote sensing meawsurements on over 4,500 vehicles showed that NH3 emissios follow a gamma-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.