COVID-19 lockdowns had strange effects on air pollution across the globe

Earlier this year, governments around the world imposed travel and business restrictions to slow the spread of COVID-19. Like many people, the world’s atmospheric chemists were anxiously staying at home, trying to keep their households running while working remotely. But amid the upheaval, many saw an opportunity.

The first signs came from China. When the country imposed a strict lockdown in late January, most people could not leave their homes at all for about 3 weeks. And direct emissions of air pollution sharply fell at a rate and scale never observed before. “We knew changes in emissions would be big and worth studying,” says Joost de Gouw, a chemist at the Cooperative Institute for Research in Environmental Sciences. “The pandemic is a tragedy, of course—but the science we can do is important.”

Those researchers who could do so put spectrometers on their roofs or checked on sensor networks to make sure they were in good condition to capture what unfolded in the skies as human economic activity—particularly car traffic—rapidly dropped. “It’s a weird opportunity,” says Nga Lee “Sally” Ng, who studies organic aerosols at the Georgia Institute of Technology. “Everything is so awful, but then you get to do this experiment you have been dreaming of doing.”

Running atmospheric chemistry experiments to answer fundamental questions is usually very difficult. How would the air quality change if passenger traffic were halved? Ozone reacts through several tangled paths; what mix of conditions and human emissions determines the outcome? To get at these questions, chemists must use complex reaction chambers or computer models because human behavior usually changes very slowly, and so do our pollution patterns.

In that respect, as in so many, 2020 has been an unprecedented year. “We have a chance to really test our ideas without waiting 5 or 6 years for concentrations to decrease,” says Ronald Cohen, an atmospheric chemist at the University of California, Berkeley. Some of those tests met expectations—and some have taken researchers by surprise. Scientists including Cohen hope their findings will not only deepen our understanding of the mechanisms that rule atmospheric chemistry but also provide guidance for policy makers who want to improve air quality and address climate change.

Tumbling traffic emissions

When the pandemic hit, city streets were eerily quiet. Passenger traffic plummeted, and traffic-related emissions—particularly carbon dioxide and nitrogen dioxide—dropped accordingly. Gas-powered vehicles directly emit CO₂ and NO₂ as combustion by-products. CO₂ is a greenhouse gas that plays a major role in global warming. NO₂ plays a key role in atmospheric reactions that produce ozone and fine particulates, both of which harm health.

Chemists around the world are collaborating to analyze how CO₂ emissions changed earlier this year. In New Zealand, where stringent movement restrictions bolstered the country’s effective containment of COVID-19, traffic volume fell by about 80% during the height of the lockdown, says Jocelyn Turnbull, a radiocarbon scientist at geological research company GNS Science. Her team is still working to analyze data from roadside CO₂ sensors in Wellington.

Turnbull and her colleagues are also evaluating CO₂ levels nationally through radiocarbon analysis of grass from 120 citizen scientists’ lawns around the country. As plants grow, they sequester CO₂ from the air. CO₂ produced by burning fossil fuels has a distinct isotopic signature, and plants record that signature as they take up the gas during photosynthesis.

Preliminary results from both sets of data suggest that fossil fuel CO₂ emissions in New Zealand seem to have fallen at the same rate as traffic, about 80%, Turnbull says.

Data from the Berkeley Environmental Air-quality and CO₂ Network (BEACON) show a similar trend in the San Francisco Bay Area. Cohen, who leads the project, says the emission differences were especially stark during weekday commute times, when roads typically have bumper-to-bumper traffic. Things changed dramatically in mid-March when Bay Area health officials ordered people to shelter in place.

During the first 6 weeks that the orders were in place, traffic was down by about 45%. BEACON data show that total CO₂ emissions fell by about a quarter during that period compared with the previous 6 weeks. Cohen says these results are currently under review at a journal.

The emission changes were short lived, but they paint a picture. “This is a pretty good model for what the world would be like if half of us were driving electric cars,” Cohen says. He says it’s one thing for him and other scientists to demonstrate this change computationally. Having real-world data to confirm their predictions will, he expects, be persuasive to policy makers.

Turnbull agrees. The scale of the drop in CO₂ emissions is very small relative to the amount of CO₂ in the atmosphere, she says. But these data bolster scientists’ arguments that they can monitor changes in CO₂ emissions—a necessary component in determining whether regulations are having their intended effects. Many countries, states, and cities have committed to lowering their emissions, whether through the Paris Agreement or independently, and they will need data.

The other traffic-driven pollutant that scientists saw drop dramatically is NO₂.

In most places, traffic is the main source of NO₂, accounting for 45–50% of emissions of the gas, says Vincent-Henri Peuch, head of the European Commission’s Copernicus Atmosphere Monitoring Service (CAMS).

The NO₂ effect was particularly stark in China in late January and early February. CAMS satellite data show that in eastern China, for example, NO₂ dropped by about 65% compared with the same period in 2019. This drop was in large part due to a decrease in traffic. “For 2 to 3 weeks, almost nobody could go outside in China,” says Yuan Wang, who studies aerosols and haze formation at the NASA Jet Propulsion Laboratory (JPL) in California.

Similar trends were seen around the world, in Delhi, Los Angeles, northern Italy, and elsewhere, although percentages varied. India went on lockdown on March 24, and by early April, people living in the northern part of the country were excitedly tweeting photos of the Himalayas—a view some said they hadn’t seen so clearly in decades. The drop in NO₂ levels likely led to a significant decrease in haze in the area.

Air pollution anomalies

When viewed broadly and globally, air pollution dropped during the lockdowns. A study based on data from satellites and more than 10,000 ground-based monitoring stations around the world found that average global air quality during lockdowns improved relative to the same periods in 2019 (Proc. Natl. Acad. Sci. U.S.A. 2020, DOI: 10.1073/pnas.2006853117).

But global averages can tell us only so much; what matters is what people in specific locations are actually breathing. And even when primary emissions look good, secondary reactions between those molecules and others in the atmosphere can complicate the air quality picture. The same study found that even as NO₂ emissions plummeted 60% and fine particulate matter dropped 31%, global average ozone went up slightly around the world. Another study, not yet peer reviewed, found that particulate matter didn’t drop consistently across the US (ChemRxiv 2020, DOI: 10.26434/chemrxiv.12275603.v7).

“It’s not so linear,” says Rima Habre, who studies the connection between health and air pollution at the University of Southern California. When human emissions go down, she says, “pollutants like ozone that form in the air can actually go up in nonstraightforward ways.”

In fact, dramatic cuts in primary emissions seem to have triggered severe air pollution events. In northern China and in the Los Angeles area, for example, the strictest lockdown periods saw unusually intense ozone spikes.

The air pollution events in China were “a surprise for everybody,” says Aijun Ding, an atmospheric chemist at Nanjing University. After all, he says, “we almost entirely shut down the traffic and the factories.” The explanation is a nonlinear connection between nitrogen oxides (NO₂ and NO) and secondary pollutants such as ozone.

“The exact response of ozone to lockdowns depends on what regime we were in to begin with,” explains Jesse Kroll, an aerosol chemist at the Massachusetts Institute of Technology. All things being equal, if levels of nitrogen oxides are already relatively low, lowering them more causes a decrease in ozone.

If levels of nitrogen oxides are generally high—as they are in large cities like Los Angeles and Beijing—all bets are off. Abundant nitrogen oxides may sop up hydroxyl radicals, preventing them from reacting with volatile organic compounds in the air to form ozone. And when nitrogen oxides are abundant enough, they actually start reacting with ozone itself, removing that pollutant from the atmosphere (Nat. Chem. 2020, DOI: 10.1038/s41557-020-0535-z). Consequently, in urban areas where this chemistry is dominant, lowering NO₂ levels actually causes ozone to rebound.

The special relationship between NO₂ and ozone has been known since at least the late 1980s. One way scientists have been able to study it is by looking at the differences in air quality throughout the week: nitrogen oxide emissions fluctuate with traffic patterns, going up on weekdays and down on the weekends. But the emission differences during the pandemic were much, much greater—and, it seems, the resulting anomalies were weirder.

“The chemistry is nonlinear—we know that,” says JPL’s Wang. “We just didn’t expect such nonlinearity.”

Both NO₂ and ozone, in turn, have their own nonlinear relationships with particulate matter. NO₂ can react with several different gases in the atmosphere to yield these particles, including pathways involving ammonia from agricultural emissions and interactions with volatile organic compounds. Wang’s analysis of CAMS data extends the weirdness to airborne particulates. He found that even as NO₂ emissions dropped dramatically in China—by as much as 93% in Wuhan—data from the country’s air quality monitors showed particulate matter hot spots during the lockdown period, particularly in the northern part of the country, where Beijing is located (Science 2020, DOI: 10.1126/science.abb7431).

Ding’s group is using air quality data and atmospheric modeling to untangle the air pollution chain reaction that caused periods of high particulate matter. So far, he says, the models suggest that enhanced ozone led to the production of abundant NO₃ radicals, which in turn formed particulates made of nitrates (Natl. Sci. Rev. 2020, DOI: 10.1093/nsr/nwaa137).

Parsing particulates

Ding is concerned the events of this spring could hamper China’s efforts to improve air quality, because the public may conclude that lowering emissions doesn’t prevent air pollution events. Instead, the spring’s unusual pollution patterns should tell scientists and regulators that they need to account for more factors than primary urban emissions. The COVID-19 lockdown period is “a unique experiment to demonstrate how air pollution mitigation is very complex,” he says.

To get to that complexity, researchers are now doing the hard work of analyzing lockdown-era particulate pollution chemistry. That entails taking a close look at the role of other airborne emissions and chemicals, besides NO₂ and ozone, that react to form particulate matter.

The chemical composition of particulate pollution varies wildly depending on factors such as which primary pollutants interacted to form it and the weather conditions at the time. If scientists can identify the composition, they can then work backward through gnarled reaction pathways and infer the primary source. Luckily, some researchers already had the necessary analytical equipment set up before the pandemic or were able to get it up on their roofs quickly.

One such project has been running in Delhi since 2017. Real-time air quality data are helping provide a baseline and progress report for India’s National Clean Air Program. Launched in 2019, the program aims to decrease particulate matter concentrations in the country’s most polluted cities and regions by as much as 30% by 2024, relative to 2017.

Particulate matter composition in India is unusually complex. Primary particulates come from activities such as wood and coal burning for heating and cooking, construction dust, and waste incineration. Secondary particulates arise from reactions of chemicals emitted by diesel generators, traffic, power plants, and fertilizer application, among many other sources. Agricultural burning is also a major source of air pollution in Delhi.

The team monitoring particulate matter chemical composition in Delhi focuses on particles 2.5 µm and smaller (PM_2.5). Indian Institute of Technology, Kanpur, civil engineer Sachchida Nand Tripathi and his collaborators are still analyzing what happened to the particulates during the country’s lockdowns, which happened in three phases, each 3–4 weeks long. While primary pollution, such as nitrogen oxides and sulfates, dropped significantly during this time, PM_2.5 decreased only moderately. And the third phase saw some spikes in particles that seem to correlate with biomass burning during the wheat harvest in southeast Delhi, he says.

Just as PM_2.5 levels didn’t fall dramatically, their chemical composition didn’t shift radically. “I was really struck by how little the PM_2.5 composition seemed to have changed” in Delhi, says Joshua Apte, an atmospheric chemist at the University of California, Berkeley. “We were looking for a smoking gun” of emission changes, he says, but there wasn’t one.

Apte says the modest changes in air quality in Delhi during lockdowns hold a lesson for regulators: simply reducing urban emissions from traffic is not effective. Half or more of the PM_2.5 in the city of New Delhi comes from elsewhere in the region, he says. To achieve air quality goals, it’s important to focus not just on India’s cities but also on their surrounding communities. For example, rural residents need cleaner heating and cooking systems, and agricultural emissions must be reined in.

Rather than focusing on policy implications, Georgia Tech’s Ng is delving into the basic chemistry at work in particulate matter formation. Her team is particularly interested in the involvement of another important class of emissions: volatile organic compounds. Some of these, like pinene and isoprene, are emitted by vegetation, so they weren’t affected by the lockdown. Others, like benzene, are human emissions. In Ng’s area, Atlanta, the main sources of volatile organics are trees in the region’s forests.

Mimicking the Atlantan air in the lab requires experimental finesse, but Ng’s lab does it, pumping controlled amounts of pinene, NO₂, and other gases into a reaction chamber fitted with 300 ultraviolet lights to mimic the sun. To test what conditions and concentrations nudge air chemistry reactions down one path or another, her team varies one input, such as the humidity or NO₂ level, and watches what happens. What kinds of aerosols form? And how much?

“We’ve been looking at this chemistry for a decade in the lab, and we’ve never been able to do a control experiment outside in the real world,” Ng says. “We know what parameters affect aerosol formation—the question is, Exactly how, from the in-depth mechanistic point of view?”

With a lockdown-induced drop in NO₂ emissions, “now the best experiment is outdoors,” she says. Luckily, she was able to get her field equipment on a roof at Georgia Tech in April, and she says some “amazing” students and postdocs volunteered to come in and babysit it for 5 months straight. After wrapping up the lockdown leg of the project in mid-September, Ng’s team has just begun analyzing the data.

Atmospheric chemists expect to be studying 2020 for years to come—and historic wildfire seasons in the western US and in the Arctic will make the year more noteworthy still.

“The pandemic is just a tragedy, and first and foremost we just hope it goes away,” de Gouw says. “But given that it’s happened, we owe it to ourselves to study this.”