SALT LAKE CITY—Winters can be toxic here. For days or even weeks, a thick haze settles over this city of skiers and hikers, as polluted air becomes trapped in a basin ringed by mountains. It can be hard to see the next car on the road. Hospital visits for pneumonia and asthma spike, schools suspend outdoor recess, and even healthy residents complain of scratchy throats and coughing fits.
Meteorologists say the phenomenon, known as an inversion, is easy to explain: A high-pressure system traps cold air in the basin, placing a lid over the pollution. But the smog's specific ingredients, and how they interact in the atmosphere, have been something of a mystery. And there is growing pressure to solve it: The U.S. Environmental Protection Agency (EPA) has judged the city to be in "serious" violation of clean air standards for part of each year, obligating state officials to come up with a plan for reducing the threat—something they've so far been unable to do.
Last year, in an effort to help develop that plan, researchers from six universities and several state and federal agencies launched an unprecedented effort to better understand the precise chemical makeup and sources of the pollution. During two inversions that lasted a total of 17 days, they gathered data from aircraft, balloons, and ground stations.
The broad strokes of what they found came as little surprise. The haze was mostly composed of tiny particles, less than 2.5 microns in diameter (PM2.5), which can lodge in the lungs and contribute to premature death. Some of the particles were dust, smoke, or soot, but about three-quarters were made up of ammonium nitrate. It forms when nitrogen oxides produced by vehicles, furnaces, and industrial equipment combine with ammonia, which typically wafts from farms that use ammonia-based liquid fertilizers or produce heaps of animal manure.
The researchers were startled, however, by the levels of pure ammonia they measured, given that Utah's farms are mostly idle in winter. "We don't typically think of the winter months as being big months for ammonia," says chemist Jennifer Murphy of the University of Toronto in Canada, who participated in the study. Researchers and regulators are now trying to nail down exactly why those levels were so high, and whether cutting those emissions could help clear the air over what some residents have come to call "Smog Lake City."
Despite its abundance, the role of the colorless, sharp-smelling, and eye-watering gas in deadly air pollution is poorly understood. In part, that's because it is notoriously difficult to track. Ammonia molecules are "sticky" and eagerly combine with other compounds, making it difficult for monitoring instruments to capture them. And the gas can have a very short life span—sometimes just a few days. "Ammonia is awful," says environmental engineer Mark Zondlo of Princeton University. "It's truly one of the worst gases to measure in the atmosphere."
Around the world, new ground-, air-, and space-based sensors are helping bring the sources, movements, and fate of ammonia into clearer focus. The improved monitoring comes as some nations, including the United Kingdom, are moving to slash ammonia emissions. But others, including the United States, have not made limiting ammonia a priority, in part because of uncertainty surrounding sources, as well as concerns that costly controls might do little to improve air quality. Instead, regulators have often opted to target other key smog ingredients, including oxides of nitrogen and sulfur created by combustion.
But the focus on ammonia is likely to intensify. Global emissions of the gas have doubled over the past 70 years and are forecast to continue to rise, in large part because of growing demand for chemical fertilizers. That has put pressure on researchers and regulators to better understand the implications for air quality.
This summer, atmospheric scientist Jeff Collett of Colorado State University in Fort Collins stood in a clearing in Rocky Mountain National Park surrounded by instruments that highlighted just how challenging it is to track ammonia. Other air pollutants, such as ozone and carbon monoxide, are generally monitored by networks of automated instruments that collect and relay data in real time. But to track ammonia, Collett's team must make an hourlong trip from campus to the field several times a week to manually collect samples from their instruments.
One is a simple bucket that collects rainwater, which researchers analyze to see how much ammonia has become trapped in water vapor. Another relies on a sponge coated with an acid to absorb the gas. (Ammonia, a base, eagerly reacts with acids.) There is also an acid-coated glass spiral, which strips the sticky ammonia molecules out of air samples before separating out other components of particulate matter.
It's a finicky process, but the samples are vital to Collett's effort to document how ammonia drifts from farms about 80 kilometers away in Greeley, Colorado, into the park, where the nutrient can damage sensitive ecosystems, and into Denver, where it contributes to smog. The work, underway since 2011, has helped sharpen the picture of the region's ammonia sources and movements. For example, when Colorado farm groups argued that golf courses were playing an outsize role in ammonia emissions because of their liberal fertilizer use, Collett stuck a monitor near a local golf course and showed that wasn't correct; farms were the bigger source. The monitoring has also enabled the state to establish a system that warns farmers when weather conditions are predicted to push ammonia toward Denver, encouraging them to voluntarily limit fertilizer applications and cover manure piles.
Elsewhere, other monitoring efforts—including a 66-site national network run by EPA that reports readings every 2 weeks—have painted a bigger, continent-wide picture, including how ammonia emissions can vary by weather and season. Advances in mobile monitoring have made it possible to more quickly collect measurements like Collett's. And since 2008, NASA satellites have provided a global look at ammonia's signature in the atmosphere. Such tools are helping scientists assemble a more complete picture of ammonia sources, including wildfires, which are estimated to produce 10% of global ammonia emissions by releasing the compound from plants.
"A decade ago, we had maybe a dozen long-term measurements around the country, and only one or two aircraft measurements ever," says atmospheric chemist Daven Henze of the University of Colorado in Boulder. "Now, we're able to regularly get information about timing, magnitude, variability, and sources."
Few efforts to inventory ammonia, however, have been as thorough as the one undertaken in the Salt Lake City region in the winter of 2017. The two inversion events documented by the ground and air campaign each lasted more than a week, and the researchers were able to gather observations in each of the area's three major valleys: Salt Lake, Cache, and Utah.
Existing tallies of Utah's ammonia sources suggested ammonia levels would be similar in each of the three valleys. In fact, the researchers found that levels varied by geography—and that the readings were higher than they expected.
Now, Murphy and allied researchers are working to understand that variation and figure out where the ammonia is coming from. The team is using a network of ground monitors, combined with aerial measurements, to map ammonia concentrations within the city. They are examining wind patterns to see how ammonia might drift in from nearby agricultural areas. And they are looking for sources that may have been overlooked.
Cars in urban areas, for example, may be contributing more ammonia than previously understood. In one recent study, Zondlo deployed mobile instruments that use lasers to measure ammonia plumes released by vehicles in cities in the United States and China. He found that vehicles—which produce ammonia as a byproduct of their emissions-cleaning catalytic converters—were emitting about twice as much ammonia as assumed. "In the grand scheme of things, vehicles were a fairly small source," he notes. Still, the emissions could play an important role in particulate pollution in cities, he says, because the ammonia is being produced in close proximity to other combustion compounds that fuel the creation of PM2.5.
In Utah, state regulators hope a better understanding of Salt Lake City's ammonia sources will help them build improved computer simulations of air pollution events, which can be key to identifying solutions. For example, if it turns out that ammonia is drifting into the city from farms in neighboring valleys, the state could try to curb those sources—perhaps by asking farmers to limit fertilizer use—when the weather is ripe for inversions. But that strategy might not make sense if urban ammonia sources like cars turn out to be playing a bigger role in driving the chemistry that produces smog. "With so many factors, we need to understand the full picture," Murphy says.
Regulators also want to be sure that potentially costly controls on farms or other ammonia sources will produce a benefit, which means cracking the chemical makeup of the smog. In the United States, for example, existing air pollution regulations have sharply reduced atmospheric concentrations of nitrogen oxides, meaning fewer molecules of those compounds are available to combine with ammonia and form particulates. So, reducing ammonia emissions might not make much of a difference in areas where other smog ingredients are already in short supply. In other areas, however, choking off ammonia plumes could be key to reducing particulates. "We're still not in a place," says Murphy, "where we can even say that difficult measures are going to [have] an impact."
The situation is very different in Europe, where environmental regulators have long put a spotlight on ammonia, in part because of concerns about its impact on ecosystems. (Ammonia can leach into streams and rivers, for instance, where it can be toxic to aquatic organisms.) The Economic Commission for Europe, a United Nations offshoot, set ammonia limits in 2012, and European countries have used a variety of strategies to reduce overall agriculture emissions by 24% since 1990. Germany, for example, placed per-hectare limits on the use of certain kinds of fertilizers, and the Netherlands created financial incentives for more efficient fertilizer use.
Earlier this year, the United Kingdom unveiled a sweeping air quality plan that includes plans to cut the nation's ammonia emissions from agriculture by 16% by 2030. The move came in the wake of a U.K. Environment Agency finding that ammonia was the nation's only major air pollutant to increase since 2013, and that emissions from farms would continue to rise without "urgent action." That trend threatened the government's bid to halve, by 2025, the number of people breathing air with PM2.5 levels deemed unsafe by the World Health Organization (WHO). (The WHO particulate standard is 10 micrograms [µg] of PM2.5 per cubic meter of air, averaged over a year; the U.S. annual standard is 12 µg/m3.)
To achieve the ammonia cut, the government plans to require farmers to limit fertilizer applications and cover manure piles, and it will impose stricter controls on dairy operations. The agriculture industry, which was consulted on the plan, has been largely receptive. Farmers have already voluntarily taken similar steps, industry officials noted, and they welcomed government plans to help fund the deployment of ammonia-control technologies.
Other nations with hefty ammonia emissions aren't yet ready to follow the United Kingdom's lead. China, which is known to be a global ammonia emissions hot spot but doesn't have a reliable inventory of sources, does not regulate the compound. Neither does the United States, although EPA does consider ammonia to be a precursor to PM2.5.
One big issue facing U.S. regulators is a lack of comprehensive data on ammonia sources. "It's hard to regulate something if you're not measuring it," Collett says. U.S. farm groups have, to date, rebuffed efforts to require farmers to report ammonia emissions, arguing the effort would be needlessly burdensome. In 2013, EPA did launch a 2-year ammonia monitoring study, in concert with the pork, dairy, and poultry industries, involving 24 sites in nine states. But the project was halted after agency science advisers criticized the quality of data that were being collected.
If EPA did pursue ammonia regulations, politics would likely pose a stumbling block. Farm groups have argued that, because the gas has many sources and can drift long distances, any controls would have to be carefully designed; a fix would not be as straightforward as, for instance, installing a chemical scrubber on a power plant. They also note farmers have already taken voluntary steps to limit emissions, such as reducing the amount of ammonia precursors used in animal feed and changing manure management practices.
Still, U.S. regulators could face pressure to act if studies from Salt Lake City and elsewhere provide evidence that ammonia has become an important driver of particulate pollution. And at least one scientist believes answers could come sooner than later—in "years, not decades," predicts Henze, who sits on the EPA advisory panel considering the issue. "EPA has not been willing to push the ball forward because of the uncertainty" surrounding ammonia, he says. "Now we're pushing past the uncertainty."