Agricultural activity and rising temperatures fueled a rise in atmospheric ammonia in the U.S., a pollution trend driven by the Midwest, South and West regions, according to a new analysis of satellite data.

The scientists behind the comprehensive study tracking these pollution levels between 2002 and 2016 said their work provides an "essential scientific basis" for U.S. policymakers to plan how to reduce agricultural emissions of ammonia, which can create aerosols that worsen air quality.

According to the study, published May 25 in Science of the Total Environment, atmospheric levels of ammonia in the U.S. grew by about 15% over the 14-year period, from 1.85 to 2.13 parts per billion, with some regions experiencing even faster growth.

Agricultural activity is a major source of air pollution, and the poor air quality it creates kills nearly 18,000 people in the U.S. each year. One of its significant pollutants is ammonia, a nitrogen-based compound that is released into the environment from the waste of livestock and from both natural and synthetic fertilizer.

Ammonia reacts with acidic gases such as sulfuric and nitric acid to create fine particulate matter, a dangerous air pollutant that can contribute to smog. It can also seep into soil and water and reduce the biodiversity of ecosystems. Some of these effects are caused by nitrogen oxides, which are usually produced during the combustion of fossil fuels.

"It's a health problem, for the ecosystem and also people," said Rongting Xu, the lead author of the new study and a postdoctoral researcher in Oregon State University's Department of Forest Ecosystems and Society.

Ammonia levels in the air are frequently investigated by researchers, according to Xu, but she and her co-authors wrote that previous studies had generally used limited data on fertilizer use and livestock manure and made assumptions about their relationship to ammonia in the atmosphere.

At the center of the recent study is a somewhat new statistical method called geographical and temporal weighted regression, which can track the potentially changing relationship between multiple variables through both space and time. Before this paper, this approach had not been applied to ammonia emissions and potential contributors.

Satellite data dating between 2002 and 2016 of atmospheric ammonia, climate, fertilizer use and livestock manure were all plugged into this and other statistical tools to determine how and where ammonia levels changed and what caused these changes.

According to the researchers' statistical analysis, levels of atmospheric ammonia grew annually by about 0.03 parts per billion over the study period. This is an even faster increase than what is implied by the directly measured difference of 0.28 parts per billion between 2002 and 2016 levels, given the sizable variability in ammonia emissions between years.

Although the U.S. ammonia concentration of 2.16 parts per billion is still less than in other countries such as China and India, Xu said, it can still cause environmental problems.

"The government is trying to reduce this kind of air pollution," she said, "but … this ammonia concentration increased a lot in the past 20 or 30 years, because of agriculture."

The emission trends differed widely based on geographical regions within the U.S. While changes on the East Coast were negligible, according to the paper, ammonia levels rose by a "striking" 0.074 parts per billion per year in the Midwest, the country's largest fertilizer consumer and manure producer and a known ammonia hotspot. This statistically calculated rate is similar to the Midwest's measured difference of ammonia, which ballooned from 1.53 to 2.59 parts per billion between 2002 and 2016.

The stretch of Southern states between Texas and Tennessee, as well as the Western states California, Oregon, Washington and Montana, also saw increases in ammonia concentration at a rate similar to the national average, a previously undetected trend. 

The researchers were also able to tie these increases to agricultural practices and the changing climate. Both nitrogen-based fertilizers and livestock manure contributed to the large increases in the Midwest and South, while only the effects of manure were strong in the West.

They noted that high temperatures could trigger ammonia emissions from fertilizer and manure, which was borne out in their study: Hot summers in the Midwest and South also accounted for ammonia increases, while warmer winters did the same in the West.

Although the study is comprehensive in its national scope and exploration of possible contributors, there were some limitations to the data it used. For a large portion of the West around the Rocky Mountains, data was not available because of the high elevation or cloudy days when it was collected, possibly missing ammonia hotspots in Colorado and Utah that were identified in previous regional research.

The map resolution of the study was relatively low — each pixel has a side length of one degree latitude and one degree longitude, or between roughly 45 and 70 miles (72 to 113 kilometers) — which does not easily allow for identifying ammonia hotspots such as individual farms, Xu said.

The postdoc also expressed interest in tracking the concentrations of acidic gases, which could indicate where ammonia may be "missing" due to reacting with the gases to form particulate matter.

As for reducing ammonia emissions, Xu made a few suggestions, such as farmers using less natural fertilizer and the U.S. Department of Agriculture implementing strategies for more efficient uses of ammonia and other forms of nitrogen on farms to reduce nitrogen runoff. She also said livestock can be fed differently so they create less ammonia, which occurs in part when the animals eat too much protein.

Consumers eating less meat or going vegetarian may also help.

"Meat consumption can really affect this ammonia emission," Xu said. "If everybody can reduce meat consumption, they can really help the environment, the air quality."

The study ,"Satellite-detected ammonia changes in the United States: natural or anthropogenic impacts," published May 25 in Science of the Total Environment, was authored by Yaqian He, University of Central Arkansas; Rongting Xu, Oregon State University; Stephen A. Prior, National Soil Dynamics Laboratory (USDA); Di Yang, University of Wyoming; Anni Yang, Colorado State University and National Wildlife Research Center; and Jian Chen, Auburn University.