PROVIDENCE — National regulations are needed to limit harmful ammonia emissions from agricultural sources and prevent knock-on soil and water degradation in sensitive estuary ecosystems, such as Narragansett Bay, according to a study recently published in Atmospheric Environment.

The study, completed by a team at Brown University’s Institute at Brown for Environment and Society, analyzed wet-deposited ammonium, or ammonium incorporated into rainfall, in Providence in 2018 to find that agricultural rather than city-based sources are most likely to blame for recent increases in urban ammonium levels.

“The ammonium we measured in precipitation has a significant non-local contribution, which does seem to be transported from what we believe to be agricultural regions,” said lead author Emmie Le Roy, who conducted research as a Brown University undergraduate and will begin a Ph.D. at the Massachusetts Institute of Technology this fall.

The levels of nitrate deposition, which come from nitrogen oxide emissions, in the Narragansett Bay area have decreased substantially in the past decades because of the federal Clean Air Act and other regulatory changes, according to co-author Emily Joyce, who published another paper on the topic last year.

Improvements in wastewater treatment systems, too, have reduced nitrogen input to Narragansett Bay by nearly 60 percent, Joyce said. “That was definitely what they needed to focus on” to get a handle on the problem then, she said, but there’s been “nothing done to atmospheric deposition.”

Joyce’s atmospheric measurements, the first done in 30 years, showed ammonium deposition had risen to six times the amount in 1990.

“If there’s more ammonium, then there’s going to be more algae blooms and fish kills,” Joyce said. “And so from a water quality standpoint … you really want to figure out where that’s coming from and try to mitigate that.”

But to mitigate ammonium, regulators need to know where to focus their efforts — and this is what Le Roy’s research sought to address.

Her team studied the stable isotopes, or chemical signatures, of ammonium in more than 200 precipitation samples collected at Brown University from January to November 2018 to determine where the emissions came from.

“You can think of them as being sort of like a fingerprint that is distinct for different source types,” Le Roy said of the isotopic signatures.

Ammonium from close-range, urban sources, namely vehicle emissions and fossil-fuel combustion, would typically have a higher isotopic composition, according to Wendell Walters, Brown University associate professor and a co-author of the study. While ammonium from agricultural sources, including animal waste and fertilizer, would have a lower isotopic composition.

By scouring weather station databases, the team also found storm systems that developed over land carried significantly more ammonium than those originating over marine or coastal areas. This data supported long-range agricultural sources as the origin of ammonium deposits.

During the past seven decades global emissions of ammonia have more than doubled from 23 to 60 teragrams annually — one teragram is a billion kilograms or 2.2 billion pounds. Researchers say the increase is tied to rising ammonia emissions from industrial agriculture. The ability to grow crops depends on nitrogen, a critical plant nutrient. However, an overabundance of nitrogen, in animal waste and in excess fertilizer, can turn into gaseous ammonia.

When ammonia enters the atmosphere, it combines with pollutants — mainly nitrogen and sulfuric oxide compounds produced by the burning of fossil fuels — to form fine-particle air pollution that can travel long distances.

Though the exact location is hard to pinpoint, Le Roy said wind patterns indicate that ammonium emissions are showering down onto Providence from states as far away as California, or even across the Pacific Ocean.

The wet deposition of ammonium is a global-scale problem, Walters said, and one that needs regulatory attention to prevent further acidification and eutrophication of sensitive ecosystems.

“It’s not something that the city of Providence could tackle on their own since there’s this large intrastate transport phenomenon occurring with the deposition,” Walters said. “But we may need to address the importance of incorporating ammonia regulations in the future — and that would have to be more at the national scale.”