Coronavirus shutdowns clear the air, temporarily

As the novel coronavirus has spread around the globe and people have been advised—or ordered—to stay home, the dramatic curtailment of human activities has created an unintended experiment for studying air quality. “We have seen a decrease in air pollution on short time frames,” says atmospheric scientist Helen Worden of the National Center for Atmospheric Research in Boulder, Colorado. Air quality is a complex mix of human-caused and natural emissions and weather, she says. The pandemic-driven atmospheric experiment is allowing scientists to explore how well observations agree with predictions. “It could be a learning opportunity.”

Atmospheric concentrations of particulate matter, aerosols, and many molecular species that contribute to global warming and poor air quality have plunged nearly everywhere; the biggest changes often correlate with places that enforce the strictest lockdowns or are the most polluted. Researchers have focused particularly on nitrogen dioxide, because the molecule’s lifetime of a day or less is ideal for obtaining a local, nearly real-time measure of human activity. The molecule “is a good proxy for pollutant emissions more generally,” says Jenny Stavrakou, an atmospheric researcher at the Royal Belgian Institute for Space Aeronomy. Power generation, other industrial activities, and transportation account in roughly equal measure for NO₂ emissions on a global scale, she says, although in cities “traffic is by far the dominant source.”

Beginning in late January, marked decreases in NO₂ levels were recorded over regions in China, and then, as the virus spread, the levels sank in other parts of the world. The reductions reached 50% in Milan and in parts of India; 30% in areas around Paris, Barcelona, and Madrid; and 20% for the regions around Brussels, Frankfurt, and Hamburg. In the third week of April, the NO₂ level in New York City was 28% lower than a year earlier; other US cities had smaller reductions.

Meanwhile, with restrictions relaxing in China, by mid-April NO₂ levels were already rebounding. A similar thing happened after China halved emissions in preparation for the 2008 Olympic Games in Beijing, and two years later when it cleared the air for Expo 2010 Shanghai; both times, air pollution bounced back rapidly once restrictions on activities were lifted.

Stavrakou uses data from the tropospheric monitoring instrument (TROPOMI) on the European Space Agency’s Sentinel 5 Precursor satellite. A spectrometer sensitive from UV to near-IR wavelengths, TROPOMI makes high-spatial-resolution (3.5 km by 5.5 km) measurements of vertically integrated concentrations of ozone, methane, formaldehyde, NO₂, sulfur dioxide, and other molecules in the atmosphere. The instrument collects measurements at the same points around the globe at the same time every day. (Once commissioned, a spectrometer aboard the recently launched South Korean satellite Cheollian 2B will provide the first-ever monitoring of atmospheric species throughout the day.)

It’s trickier to pinpoint the sources of more persistent molecules, such as carbon monoxide and carbon dioxide. And clouds, winds, and rain affect the measurement and movement of all pollutants. Although qualitative changes are evident from observations, “we need modeling studies to understand how observations are linked to emissions,” Stavrakou says. “We need to gather traffic data, aircraft data, data about industrial activities, and to incorporate the data into our models.”

In one unpublished result since the pandemic began, Worden and colleagues found that, in a corridor between Wuhan and Beijing, peak values of CO were down by 30–45% following Chinese New Year this year compared with the same period in 2019. “We have to do inverse modeling to figure out how much was due to meteorology and how much to human activity,” says Worden, the US principal investigator for MOPITT, a Canadian instrument on NASA’s Terra satellite that monitors CO in the troposphere.

Worden notes that last year’s fires in Brazil and Australia primed researchers in her field to work under time pressure. “We hadn’t previously had as much motivation to calculate emissions so quickly until those events,” she says. “But with lots of emergencies, we’ve learned how to ramp up quickly.”

If the models reproduce the observations, researchers will gain confidence about other molecular species that are not directly observed by satellites, says Stavrakou. For example, they hope the models will shed light on why levels of ozone and particulate matter increased over China even when other pollutants were down. Making observations during the lockdowns is valuable for understanding air pollution and taking steps to reduce it, she says. In the longer term, she adds, such observations and modeling will also provide atmospheric insights that could be relevant to mitigating global warming.

Denise Mauzerall is an atmospheric chemist at the Woodrow Wilson School of Public and International Affairs at Princeton University. “I’m sure we can learn things from the pandemic lockdowns,” she says. “What are the most important sources of the pollution? How does the better air benefit health?”

But Mauzerall says her main concern is “a rebound effect” once the lockdowns end. Will we focus on short-term economic goals, she asks, or “can this crisis initiate a way forward that is greener and better for air quality, climate, and the environment?”