As the third most abundant nitrogen species in the atmosphere, ammonia (NH₃) is a key component of the global nitrogen cycle. Since the industrial revolution, humans have more than doubled the emissions of NH₃ to the atmosphere by industrial nitrogen fixation, revolutionizing agricultural practices, and burning fossil fuels. Ammonia is a major precursor to fine particulate matter (PM_2.5), which has adverse impacts on ecosystem and human health. On-road vehicles are key sources of NH₃ emissions and depositions in urban areas. Due to the successful control of oxidized nitrogen (NO_x) emissions and the lack of NH₃ emission control, new vehicles manufactured now emit more NH₃ molecules than NO_x molecules. To quantify NH₃ emissions at an urban scale, a quantum cascade laser-based NH₃ sensor was integrated into a mobile laboratory, which participated in extensive field campaigns, including the NASA DISCOVER-AQ campaigns and the CAREBeijing-NCP campaign in China (400 h on-road time, 18,000 km total sampling distance). Vehicle NH₃:CO₂ emission ratios in the U.S. are found to be similar between cities (0.33–0.40 ppbv/ppmv, 15% uncertainty) despite differences in fleet composition, climate, and fuel composition. While Beijing, China has a comparable emission ratio (0.36 ppbv/ppmv) to the U.S. cities, less developed Chinese cities show higher emission ratios (0.44 and 0.55 ppbv/ppmv). If the vehicle CO₂ inventories are accurate, NH₃ emissions from U.S. vehicles (0.26 ± 0.07 Tg/yr) are more than twice those of the National Emission Inventory (0.12 Tg/yr). Vehicle NH₃ emissions are greater than agricultural emissions (which is highly uncertain as well) in counties containing near half of the U.S. population and require reconsideration in urban air quality models. Future opportunities of characterizing NH₃ emissions and investigating the transition of NH₃/NO_x regimes at the global scale enabled by satellite observations will also be discussed.