Why pollution is plummeting in some cities — but not others

A night view of an empty road in Wuhan

An empty road in Wuhan, China, which went into lockdown in January.Credit: AFP/Getty

The lockdowns implemented worldwide to stem the spread of the new coronavirus have caused an economic downturn, but also seem to have an upside — cleaner air over urban regions normally affected by heavy pollution. Scientists are now rushing to analyse why the effect is more pronounced in some places than in others. But they caution that the drop might not last long if the global economy ramps back up after the crisis.

Still, shutting off a large portion of the economy serves as a natural control experiment by creating a world that has very few emissions, says Dan Westervelt, a climate and air-pollution researcher at Columbia University’s Lamont–Doherty Earth Observatory in Palisades, New York. “What’s happening right now could give us a sense of what might be feasible under extreme clean-air-policy scenarios in the future,” he says.

Satellite observations have shown a marked drop in nitrogen dioxide (NO2) concentrations over China and northern Italy since the outbreak of the coronavirus crisis in January (see ‘Pollution drop-off’). The two countries were the first to introduce broad lockdowns as their coronavirus infections soared. NO2, an airborne pollutant created mainly by vehicles and power plants that burn fossil fuels, can cause respiratory diseases in humans. There are signs that atmospheric concentrations of other harmful pollutants, including particulate matter, toxic sulphur dioxide and carcinogenic formaldehyde emitted by industrial sources, have dropped too. But data are still preliminary, and some of the improvement could be down to natural weather variations.

Pollution drop-off: Maps of France and Italy showing nitrogen dioxide concentration in troposphere in March 2019 and March 2020.

Source: European Space Agency TROPOMI instrument

Waning pollution

Atmospheric NO2 concentrations have also dropped over metropolitan regions in the United Kingdom, Germany and the Netherlands — all of which are under partial or total lockdown, says Henk Eskes, an atmospheric scientist at the Royal Netherlands Meteorological Institute in De Bilt, who analyses observations by a trophospheric monitoring instrument called TROPOMI on the European Space Agency’s Sentinel-5 Precursor satellite.

But thorough analysis of space observations of air pollution is still lacking, and scientists must investigate how lower atmospheric concentrations of various pollutants correspond to decreasing traffic and industrial emissions.

In the northern hemisphere, atmospheric NO2 typically decreases by up to 50% between January and May because of the Sun’s angle, so researchers are looking for reductions in excess of the natural decrease. The concentration of NO2 and other airborne pollutants, including near-surface ozone, which chemically interacts with nitrates, is subject to large day-to-day variability. Favourable weather in early spring might have contributed to the observed drop in local air pollution in recent weeks, Eskes says.

But compared with previous years, the decreases in NO2 concentrations in China and Italy appear to be unprecedented, says Eskes. “The changes observed are so large that we feel confident they cannot be explained by weather-induced variability alone,” he says. “But it will need a lot more research, including surface measurements and atmospheric transport modelling, to establish the causes and quantify the scale of any changes more accurately.”

Unclear causes

The situation is fuzzier in the United States. Reports that air pollution has dropped in New York and other large cities because of COVID-19 precautions imposed in March are premature, says Dan Goldberg, an atmosphere researcher at Argonne National Laboratory in Lemont, Illinois.

Weather Factor: Shows nitrogen dioxide levels in 7 US cities from January to March 2020.

Source: European Space Agency TROPOMI instrument (data); Dan Goldberg, Argonne National Laboratory (analysis)

“I haven’t seen any statistically significant changes in air pollution in most US cities, which is contrary to the claims in some media articles,” he says. According to the TROPOMI data, the only US city that is seeing a statistically significant improvement in air quality is Los Angeles, California (see ‘Weather factor’). The caveat, he says, is that it has been unusually rainy there in the last few weeks. So it is still unclear what fraction of the observed improvement in pollution is due to favourable weather and what fraction is due to COVID-19 precautions. “Probably both are simultaneously helping,” he says.

Observations of pollutants at surface level don’t seem to be showing a huge change in the United States either. Preliminary data show a slight decrease in particulate-matter pollution in New York City, says Westervelt.

In other places, fine-particle pollution has in fact increased. Long-range transport of particles from agricultural burning in Mexico, for example, has yielded worse-than-average air quality in much of the central United States in March, says Goldberg.

The immediate benefits of reduced emissions for air quality and human health might become clear only as shutdowns continue. But progress could be countered by China and the United States suspending the enforcement of environmental rules — such as those requiring energy-utility companies to monitor pollution and repair leaky equipment — to help factories to make up for lost production. So the long-term effects could be negligible.

“The current crisis is a unique learning experiment,” says Westervelt. “But stuff like that makes me worry that pollution will just ramp up when the worst is over.”

Nature 580, 313 (2020)

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