Vehicles Responsible For Much Higher Proportion Of Ammonia Air Pollution Than Previously Thought

While agriculture has generally been assumed to be the primary source of ammonia air pollution in cities, it seems that quite a lot of it is actually originating with vehicles in the cities themselves, according to new research.

To state that again: more of the ammonia air pollution present in big cities is the result of vehicle activity in the cities themselves, rather than being from nearby agricultural operations.

Photo of smog in Beijing by Kang Sun, Princeton University

“Ammonia doesn’t have to come all the way from the Midwest to Philadelphia or New York,” stated Mark Zondlo, an associate professor of civil and environmental engineering at Princeton University. “Much of it is being generated here. … It’s actually coming from the vehicles.”

Since the ammonia emissions are being emitted by vehicles, that means that they are being co-emitted with nitrogen oxides — which easily transforms into ammonium nitrate, a primary component of the brown urban smog/haze that many cities are home to.

The press release provides more:

To conduct his research, Zondlo and his team outfitted vehicles with sophisticated 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 Zondlo’s group 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.

The use of the new sensors led immediately to some notable observations.

“You can really see the plumes coming out from the vehicles in real time,” commented Da Pan, a team member and fourth-year doctoral student at Princeton. “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.”

Commenting on earlier relatively successful efforts to reduce vehicle nitrogen and sulfur emissions, and what this new work should lead to, Zondlo stated: “We’ve pulled these levers as much as we can. But there’s one big lever that we haven’t touched at all and that’s been ammonia. And if we’re really serious about attacking fine particulate matter and improving air quality, we need to start understanding and eventually limiting these ammonia emissions.”

The new findings are detailed in a paper published in the journal Environmental Science and Technology.