Vehicle Emissions as an Important Urban Ammonia Source in the United States and China

Ammoniated aerosols are important for urban air quality, but emissions of the key precursor NH 3 are not well quantified. Mobile laboratory observations are used to characterize fleet-integrated NH 3 emissions in six cities in the U.S. and China. Vehicle NH 3:CO 2 emission ratios in the U.S. are 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 CO 2 inventories are accurate, NH 3 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), while Chinese NH 3 vehicle emissions (0.09 ± 0.02 Tg/yr) are similar to a bottom-up inventory. Vehicle NH 3 emissions are greater than agricultural emissions in counties containing near half of the U.S. population and require reconsideration in urban air quality models due to their colocation with other aerosol precursors and the uncertainties regarding NH 3 losses from upwind agricultural sources. Ammonia emissions in developing cities are especially important because of their high emission ratios and rapid motorizations.