Quantifying urban nitrogen emission and deposition through optical sensing techniques
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 (PM2.5), 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 (NOx) emissions and the lack of NH3 emission control, new vehicles manufactured now emit more NH3 molecules than NOx 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/NOx regimes at the global scale enabled by satellite observations will also be discussed.
1Harvard-Smithsonian Center for Astrophysics, firstname.lastname@example.org