When governments around the world enacted strict lockdown policies to slow the spread of the coronavirus in early 2020, a massive, unplanned experiment began: a test of what happens to air quality when millions of people suddenly stop driving.
With fewer cars, trucks and buses on the roads, air quality visibly improved in cities worldwide. In Los Angeles, the clear air offered a rare glimpse of the San Gabriel Mountains, while the normally thick haze of smog dissipated in New Delhi.
Washington University researchers have found concentrations of nitrogen dioxide, a harmful air pollutant that comes from burning fossil fuels, dipped dramatically during the first six months of 2020. Using satellite data, the team found the pollutant dropped by more than 30% on average worldwide, compared to 2019.
Though scientists worldwide turned their attention to nitrogen dioxide pollution at the beginning of the pandemic, their research mostly focused on monitoring the pollutant in the Earth’s atmosphere, miles above cities and towns.
But ground-level pollution is what’s affecting human health, atmospheric scientist Matthew Cooper said.
“It doesn't really matter what's going on seven kilometers up, because no one's breathing up there,” said Cooper, who led the study as a visiting researcher at Washington University. “What matters is what's happening down at the ground level where everyone's noses are.”
Measuring air pollution at the Earth’s surface can be difficult, partly because many major cities don’t have air quality monitoring equipment.
Of the 215 cities included in the study, nearly one-third — mostly in sub-Saharan Africa and southeast Asia — did not have monitoring stations.
To create a global map of nitrogen pollution during the pandemic, the team relied on satellite data from the European Space Agency’s TROPOspheric Monitoring Instrument.
Known as a spectrometer, the device measures how much light is reflected back toward the satellite at different wavelengths, allowing scientists to calculate the concentration of specific air pollutants in the atmosphere. They then feed the data into a mathematical model that accounts for weather conditions and nitrogen dioxide emissions, producing a detailed map of fluctuating nitrogen dioxide over time.
The results showed a “dramatic short-term change” in nitrogen dioxide concentrations worldwide during the pandemic, said Randall Martin, a Washington University engineering professor.
Certain technological advances, such as better fuel efficiency, have slowly reduced air pollution over time — but pandemic-related changes in transportation had a much larger and more immediate effect.
“The reductions that occurred during these COVID lockdowns are comparable to 15 years of technologically driven reductions globally,” said Martin, one of the study’s co-authors.
The team also found large differences in air pollution within cities.
In Atlanta, for example, the overall concentration of nitrogen dioxide dropped by 28% from April 2019 to 2020 — but the citywide average masked important geographic variability.
Compared to downtown Atlanta, residents living near coal power plants and the city’s airport saw much larger declines in nitrogen dioxide, up to 40%. Nearly 1 million people experienced decreases of only 10% or less.
Capturing these fine-scale differences within cities can help identify neighborhoods with poor air quality, Cooper said.
“Even in wealthier countries, most cities have one or two monitors at most and they really only tell you what's going on at that monitor,” Cooper said. “There can be important environmental justice issues even within cities that have monitoring.”
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