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As coronavirus cases popped up across California in March 2020, the previously impossible happened in Los Angeles County: The region’s normally bumper-to-bumper traffic slowed by roughly 24%. Lucky drivers were now, suddenly, able to make it from Burbank to Santa Monica at rush hour on the 101 and 405 in less than 50 minutes. 

A team of scientists led by �鶹��Ѱ�����Boulder are using the once-in-a-lifetime event to answer an unusual question: How much do vehicles in a city like Los Angeles add to the ammonia emissions that can hang in the air and sicken residents?

The group’s findings, published Nov. 23 in the journal , may spell bad news for a region that loves its cars. Ammonia is a common pollutant that can react to form small particles in the air that are a major cause of respiratory and cardiovascular disease, especially in densely populated areas. The researchers show that city vehicles may spew a lot more of these molecules than state and federal agencies have believed.

The study is the first to explore how vehicles churn out ammonia across an entire urban center using satellites in space.

“The tricky question has always been: How do we separate out ammonia concentrations owing to traffic from the ammonia emitted from sources like agriculture?” said Daven Henze, a co-author of the new study and professor in the Paul M. Rady Department of Mechanical Engineering at �鶹��Ѱ�����Boulder. “Then the COVID lockdown suddenly provided us with a natural experiment.”

In other words, the pandemic gave the researchers an accidental before and after picture—with a smoggy, car-filled Los Angeles on one side and a clearer, relatively empty urban area on the other.

Henze and his colleagues took advantage of that situation by drawing on satellite images to track the ammonia concentrations in the air above Los Angeles before and during the lockdown of March 2020. The team discovered that cars may churn out as much as 95% of this harmful pollutant throughout the city at any one time.

“Our estimates for vehicle ammonia emissions are higher than federal and state inventories by a factor of two to five,” said Hansen Cao, lead author of the study and a postdoctoral researcher at �鶹��Ѱ�����Boulder.

Hidden chemical danger

The research sheds light on what may be an underappreciated pollutant: Molecules of ammonia, the odor you detect when you pass a smelly farm.

Henze explained that scientists have long known that agricultural operations, from corn fields to chicken farms, churn out huge amounts of this chemical. Once in the air, ammonia can mix with nitrogen oxides to form what researchers call “fine particulate matter.”

“Those emissions come from, to put it politely, the downstream processes,” Henze said. “The feedlots, poultry and swine manure—they all give off a lot of ammonia.”

There’s another source of that pollution, too: Your car’s tailpipe. Estimates suggest that ammonia emissions from vehicles can lead to roughly 15,000 premature deaths across the United States every year. Recent research has also hinted that those numbers may miss the real toll of urban pollution. 

Ammonia hotspots

Henze and his colleagues tapped data from two satellites, the United States’ Suomi National Polar-orbiting Partnership and Europe’s Sentinel-5 Precursor to further explore the ammonia question.

In early March 2020, the team spotted two clear hotspots for ammonia in the Los Angeles region—one above downtown L.A. and one just north of Riverside, California, a hub for livestock and agriculture. By the end of the month, the hotspot over downtown had all but disappeared as traffic petered out.

“I think we were almost surprised that we could see the downtown hotspot and the impact of the pandemic,” Henze said. “We weren’t just taking measurements at one road. We were looking at the entire urban area from space.”

That ultimate bird’s eye view paid off. The team calculated that vehicles produce at least 60% of the ammonia emissions in urban Los Angeles. Estimates from state and national regulators, in contrast, had pegged those numbers at less than 25%. Next, Cao said she and her colleagues want to apply the same techniques to explore the effects of the pandemic on the air above other cities.

The results could underscore the importance of regulating cars and their engines so that they churn out less of this dangerous pollutant, she said. 

“Vehicles can be the dominant sources of ammonia emissions over urban areas,” she said. “If we’re underestimating those emissions, then previous estimates of premature deaths owing to ammonia emissions might also be underestimated.”

Colin Harkins and Muhammad O. Nawaz, both graduate students at �鶹��Ѱ�����Boulder, were co-authors of the new study. Other co-authors included researchers at Atmospheric and Environmental Research, Inc.; the U.S. National Oceanic and Atmospheric Association; University at Buffalo; NASA’s Jet Propulsion Laboratory; and the Southern University of Science and Technology in China.