Although agriculture is the dominant NH₃ source at continental to global scales, in urban areas a significant NH₃ source is gasoline vehicles equipped with three-way catalysts (TWC). The presence of NH₃ in vehicle exhaust greatly enhances the formation and growth of secondary inorganic aerosols. With the growing efficiency of TWC to reduce NO_x emissions and the recent introduction of selective catalytic reduction (SCR) system in diesel vehicles, NH₃ is now the dominant reactive nitrogen species emitted by vehicles produced in the recent decade.

Except for the Euro VI standard on heavy duty diesel vehicles, there are no vehicle emission standards to regulate NH₃ worldwide. Reductions in fleet NH₃ emissions are slow or insignificant in US cities in recent years due to modest reduction in NH₃ emissions from new vehicles and increasing emissions from older vehicles (though with their TWCs still active). In contrast, the emissions of SO₂ and NO_x have been regulated effectively in many countries and are projected to decrease even further in the upcoming decades.

While high concentrations of NH₃ measured in cities have been attributed to vehicle emissions, current inventories rely on laboratory studies and tunnel/roadside measurements to estimate vehicle emission factors (NH₃ emitted per unit mass of fuel). Vehicle NH₃ emissions depend on road grade, driving mode, and vehicle age. Therefore, the representativeness of laboratory tests or stationary measurements at single locations for an entire metropolitan area is not well-characterized.

—Sun et al.

The researchers outfitted vehicles with sensors to detect ammonia levels and focused on six cities: Philadelphia, Denver and Houston in the United States, and Beijing, Shijiazhuang and Baoding in China. By measuring ammonia levels during various times of the day at different points of entry into the cities, the team was able to paint a picture of a “breathing” city, where levels of pollutants rise and fall, depending on traffic and conditions.

The research was aided by the use of open-path quantum cascade laser ammonia sensors developed by the group of Mark Zondlo, an associate professor of civil and environmental engineering at Princeton University, within Princeton’s Center for Mid-Infrared Technologies for Health and the Environment (MIRTHE). The laser-based sensors were smaller, easier to work with and more accurate than previously used sensors, Zondlo said. They also allowed for more efficient mobile testing.

Previously, vehicles had to be specially outfitted for data gathering. Holes were often drilled into the bodies of the vehicles to attach sensors. Banks of equipment were needed in the vehicle as well. Because the new sensors are relatively small, they can be mounted onto a luggage rack on top of the vehicle and then plugged into a laptop. This is the basis for their Princeton Atmospheric Chemistry Experiment mobile laboratory, a conventional SUV equipped with chemical and meteorological sensors.

You can really see the plumes coming out from the vehicles in real time. So basically you can tell if the car in front of you was in a bad operating condition. You can really see the plumes coming out from it.

—Da Pan, co-author

Among the conclusions from the study:

• Vehicle NH₃:CO₂ emission ratios in the US 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 US 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 US vehicles (0.26 ± 0.07 Tg/yr) are more than twice those of the National Emission Inventory (0.12 Tg/yr), while Chinese NH₃ vehicle emissions (0.09 ± 0.02 Tg/yr) are similar to a bottom-up inventory.

• Vehicle NH₃ emissions are greater than agricultural emissions in counties containing near half of the US population and require reconsideration in urban air quality models due to their colocation with other aerosol precursors and the uncertainties regarding NH₃ losses from upwind agricultural sources. Ammonia emissions in developing cities are especially important because of their high emission ratios and rapid motorizations.

Pan said the research opens up two avenues for further study: How the proximity of ammonia and nitrogen and sulfur compounds in emissions affects fine particulate matter production, and how the data eventually could influence vehicle emissions regulations. Zondlo is also investigating how satellite observations of ammonia can help understand these emissions as part of the NASA Health and Air Quality Applied Sciences Team.

Besides Zondlo and Pan, authors of the paper included Denise Mauzerall, a professor of civil and environmental engineering and public and international affairs at the Woodrow Wilson School at Princeton; Kang Sun, Lei Tao, David Miller and Levi Golston of Princeton; Robert Griffon, H.W. Wallace and Hu Jun Leong of Rice University; M. Melissa Yang of NASA Langley Research Center; Yan Zhang of Nanjing P&Y Environmental Technology Co.; and Tong Zhu of Peking University.

Support for the project was provided in part by Council for International Teaching and Research at Princeton University with funds from the Fung Global Forum, National Geographic Air and Water Conservation Fund, the National Science Foundation, NASA and the Houston Endowment.

Resources

• Kang Sun, Lei Tao, David J. Miller, Da Pan, Levi M. Golston, Mark A. Zondlo, Robert J. Griffin, H. W. Wallace, Yu Jun Leong, M. Melissa Yang, Yan Zhang, Denise L. Mauzerall, and Tong Zhu (2017) “Vehicle Emissions as an Important Urban Ammonia Source in the United States and China” Environmental Science & Technology 51 (4), 2472-2481 doi: 10.1021/acs.est.6b02805