Vehicles, not farms, are likely source of smog-causing ammonia

March 30, 2017

It has long been thought that the high levels of ammonia in the atmosphere, which often causes a haze of brown, noxious smog in cities, is the result of agriculture, especially fertilization.

But new research suggests that this ammonia-suffused smog is more likely the result of automobile exhaust.

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

Zondlo and his team of researchers came to this conclusion after using laser sensors to measure ammonia levels in six cities — Philadelphia, Denver and Houston in the United States, and Beijing, Shijiazhuang and Baoding in China. The researchers found that vehicle emissions of ammonia occurred alongside the emission of nitrogen oxides, which, when combined, form ammonium nitrate—which the causes the brown haze.  

Daven Henze, an associate professor at University of Colorado and researcher with NASA’s Health and Air Quality Applied Sciences Team, said the findings were an important step to better understanding air pollution over cities. 

“It’s really useful to have his [Zondlo’s] work making headway on clarifying the magnitude of the source,” Henze said.

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