Scientists look to answer questions about urban ammonia emissions

In the early days of the COVID-19 pandemic, something previously unimaginable happened: traffic on Los Angeles’s streets thinned. As people stayed home to curb the spread of the virus SARS-CoV-2, the number of cars on the road dropped significantly.

The disappearance of LA’s traffic came with an unintended side effect. The city’s omnipresent smog began to wane.

For scientists, the situation provided a rare, fleeting glimpse into an atmosphere that, for the first time in modern history, wasn’t being filled with car exhaust. Researchers took advantage of this unusual situation and investigated how different human activities, especially driving, affected air quality.

While most atmospheric scientists focused on how the lack of traffic was affecting common urban pollutants like nitrogen oxides and particulate matter, Daven Henze of the University of Colorado Boulder focused on a compound that’s frequently ignored in cities: ammonia. Ammonia can affect urban air quality by reacting with other urban pollutants to form particulate matter 2.5 μm in diameter or smaller (PM_2.5). That’s “the size range that is really hazardous to human health,” Henze says. Long-term exposure to PM_2.5 can lead to asthma, heart attacks, and even premature death.

During the pandemic, Henze and his team used satellites to measure ammonia levels. They noticed that by the end of March 2020, the levels were 35% lower than ammonia concentrations measured before the stay-at-home orders went into place (Environ. Sci. Technol. 2022, DOI: 10.1021/acs.estlett.1c00730). But the reason for the drop wasn’t what most scientists expected.

Although many experts believed that ammonia primarily wafted into cities from nearby agricultural sources, Henze’s study suggests that cars were the more likely culprit. He calculated that these mobile sources would have had to be responsible for between 60 and 90% of the compound’s emissions to explain its decline during the pandemic. In contrast, the US Environmental Protection Agency attributes around 20% of LA’s ammonia to cars.

Understanding urban ammonia sources has been hard to do because ammonia is a finicky molecule to study. That finickiness, along with limited funding, have prevented researchers from deploying robust, long-term ammonia-monitoring networks on par with ones that are used to study other hazardous atmospheric pollutants. More monitoring data could help atmospheric scientists better pinpoint where ammonia is coming from in cities and how each source contributes to ammonia’s impact on human health. A better understanding of urban ammonia could also help regulators elucidate ways to lower ammonia emissions.

What limited data scientists have on ammonia suggest that controlling its emissions may greatly benefit air quality. Henze and his team calculate that, in the US, vehicular emissions of ammonia could be responsible for 10,000 premature deaths per year, equal to the toll associated with ozone.

Yet, while the US has thousands of monitors dedicated to tracking ozone levels, only a handful focus on ammonia.

A sticky compound

Ammonia is a frustrating species to measure. Although researchers can easily detect its presence in the atmosphere using spectroscopy and chemiluminescence methods, getting an accurate reading of its atmospheric concentration is difficult, says James Schwab, an atmospheric scientist at the University at Albany.

Sampling ammonia is a problem because of its high polarity and propensity for forming hydrogen bonds. As a result, “ammonia is very sticky,” Schwab says. This stickiness causes some of the compound inevitably sticks to the walls of analytical equipment before it can ever reach the detector, which affects the instrument’s measurements. The machine measures a smaller amount of ammonia than what is actually in the air.

Over the past decade, however, scientists like Mark Zondlo, an environmental engineer at Princeton University, have developed instruments that can overcome this sticky predicament. For example, Zondlo uses a method that forgoes the need to draw ammonia into an enclosed system where the compound can stick to surfaces and instead relies on a laser-based sensor that samples the air as it blows by (Atmos. Meas. Tech. 2014, DOI: 10.5194/amt-7-81-2014). Satellite-based instruments, like the ones Henze used to study LA’s air, work in a similar manner. A satellite can measure how much sunlight different pollutants absorb as the light bounces from Earth’s surface.

But both of these measurement techniques have downsides. The equipment is expensive and researchers usually need specialized training to analyze the results. These limitations make it hard to set up long-term ammonia-monitoring networks using these techniques. To establish such a network for a pollutant, agencies like the US EPA require low-cost solutions so that they can afford to install hundreds of monitors across the country.

One of the few networks for ammonia is the Ammonia Monitoring Network (AMoN). It relies on a low-cost, low-tech method for measuring the pollutant that uses ammonia’s stickiness as an advantage. Established by the EPA and the National Atmospheric Deposition Program (NADP) in 2007, AMoN has deployed 106 monitors, each of which contains a sampling device that lets ammonia slowly diffuse onto its surface for 2 weeks. At the end of that period, NADP scientists collect each sampler and analyze them in a laboratory using a simple, well-established technique that converts ammonia into a colored compound. The intensity of the color can then be correlated with the amount of ammonia collected by the samplers.

Each AMoN monitor costs around $2,000 per year to operate and maintain. However, the network has a few downsides. By collecting the ammonia over multiple days, the technique doesn’t allow researchers to determine where the ammonia is coming from. “Over the course of those 2 weeks, you’re probably getting air blowing from a lot of different directions,” essentially blurring together signals from different sources, Henze says. Also, AMoN’s monitor placement is heavily weighted toward rural areas, a consequence of researchers choosing to add the ammonia samplers to the NADP’s preexisting monitoring sites, which were focused on studying nonurban levels of acid rain. Scientists would like to establish AMoN in urban areas, but a lack of funding prevents it, says David Gay, the program coordinator for the NADP.

An ironic source

Although there are no AMoN monitors in cities, scientists have known since the late 1990s that ammonia can be produced by cars. Ironically, vehicles release the compound as the by-product of technologies designed to stop emissions of other vehicular pollutants, like nitric oxides (NO_x), explains Chelsea Preble, a research engineer at the University of California, Berkeley.

In gas-powered passenger cars, the three-way catalytic converter reduces NO_x to produce nitrogen gas and water. But the catalytic converter can occasionally transform NO_x into ammonia. This reaction happens more often during driving conditions common in cities, like stop-and-go driving in traffic, says Naomi Farren, an air pollution researcher at the University of York. It can also happen more often as cars begin to age (Atmos. Environ. X 2021, DOI: 10.1016/j.aeaoa.2021.100117).

Diesel trucks can also emit ammonia. Due to stringent NO_x regulations, the US requires diesel engine manufacturers to equip trucks with selective catalytic reduction (SCR) systems. This equipment uses a urea-based diesel exhaust fluid that is converted to ammonia to reduce NO_x to nitrogen and water. Sometimes, more ammonia is added to the engine than needed, and the unreacted portion can be released from the tailpipe in a phenomenon known as ammonia slip.

As of 2021, just over half of trucks in the US were equipped with SCR systems. That number is expected to grow. For example, by the end of 2022, California will require every truck in the state to be equipped with this engine technology. “So we’re going to have a fleet now that is fully equipped, theoretically, with these SCR systems,” Preble says. Some researchers worry this will increase ammonia emissions.

Most researchers think these exhaust treatment systems can be improved to reduce ammonia emissions. So far, most of the work has focused on the problem in diesel trucks. To reduce ammonia emissions from diesel engines, manufacturers can add ammonia slip catalysts downstream of the SCR system to remove any ammonia coming through.

This technology was likely developed in response to recent European Union regulations that limit ammonia emissions from diesel trucks. But there is no requirement to regulate ammonia in the US, so manufacturers have no incentive to include these catalysts in their SCR systems.

In general, there are no incentives to find or implement technology to address ammonia emissions from vehicles in the US. “It’s not something that is mandated or required,” says Arvind Thiruvengadam, a mechanical engineer at West Virginia University. “As a result, there’s not much of a push” to do something about it, he says.

Regulation catch-22

According to experts, regulating ammonia emissions in the US isn’t currently a priority. Ammonia “hasn’t garnered the interest that regulated [pollutants] have,” the University at Albany’s Schwab says. One reason that ammonia has been ignored is that until recently, there were more problematic pollutants to focus on, like NO_x, ozone, and better-quantified sources of particulate matter. “Not unwisely, policy tends to go for the lowest-hanging fruit,” CU Boulder’s Henze says.

However, as emissions of those other pollutants drop because of implemented air quality controls, “it’s now increasingly important to also target the other important precursors to atmospheric air pollution that is harmful to people’s health,” UC Berkeley’s Preble says. “That includes pollutants like ammonia.” But even if regulators wanted to turn to ammonia, there’s currently too much uncertainty surrounding its sources and chemistry, especially in cities. “Anyone could say, ‘Why are we regulating ammonia if we don’t know its emissions very well?’ ” Princeton’s Zondlo says.

Unfortunately, this argument can often become a catch-22. Regulators need better measurements from scientists before they can consider acting, but because ammonia is a low regulatory priority, there isn’t adequate funding to allow researchers to collect more data.

“It’s a funny world we live in,” Schwab says. “If it’s not a regulated quantity, you’re scrambling here, there, and everywhere to get the support you need to measure.”

Not understanding where ammonia comes from in cities also limits regulators’ ability to know which levers need to be pulled to reduce emissions. For example, improving catalytic converters in cars could help, but some studies have suggested that cars aren’t to blame for urban ammonia emissions and have instead pointed to alternative sources like urban green space (Environ. Sci. Technol. 2017, DOI: 10.1021/acs.est.7b00328).

The lack of data on ammonia emissions and chemistry can also affect existing air quality policies. For example, PM_2.5 regulations that focus on presursors like NO_x or sulfur dioxide, will be less effective than if regulators properly factored in ammonia emissions. The former is “only looking at two-thirds of the problem,” says Zondlo.

Still, Zondlo says it’s inevitable that ammonia will be regulated in the future, especially as countries work to control PM_2.5 levels. Last year, the World Health Organization lowered its recommended level for PM_2.5 because of the health effects associated with it. The EPA is also reexamining its PM_2.5 standards and is expected to announce whether it will further tighten them by 2023.

West Virginia University’s Thiruvengadam agrees with Zondlo. If air quality standards for PM_2.5 keep getting stricter, “then they will probably start looking into whether ammonia needs to be regulated.”