New estimates from NASA reveal ozone air pollution in May and June of 2020 dropped by 2 percent, largely due to emission reductions in Asia and the Americas.
That might not sound like much, but experts say it's a global decrease that would otherwise take at least 15 years to achieve, even under the most aggressive emission reduction schemes proposed by the Intergovernmental Panel on Climate Change.
"I was really surprised at how large the impact on global ozone was," says Jessica Neu, who researches the chemical composition of the atmosphere at NASA's Jet Propulsion Laboratory.
"We expected more of a local response at the surface."
Ozone in the atmosphere isn't necessarily a bad thing. Higher up, these molecules shield our planet from the full power of the Sun. But lower down, they can irritate the lungs and increase the risk of people dying from cardiovascular or respiratory disease.
Ozone isn't a pollutant we humans directly emit into the atmosphere; it's formed when sunlight interacts with nitrogen oxides (NOx), which are released into the air from automobiles, factories, power plants, and refineries.
Even though we know this, the relationship between nitrogen oxides and ozone in the lower atmosphere is a tricky one to predict.
Reactions are subject to the whims of weather and the presence of other pollutants and chemicals. In some scenarios, a drop in nitrogen oxides can actually increase ozone pollution.
For instance, when China reduced its emissions of fine particulate matter a few years ago, the changes caused an unexpected increase in ozone air pollution.
Given this uncertainty, researchers saw last year's lockdowns as a "scenario-of-opportunity" to figure out what would happen to the atmosphere if there was a rapid and large reduction in human activity and our pollutants. We can then use this knowledge to create more effective environmental policies.
Feeding multiple satellites' data from 2020 into four models of atmospheric reactions, the team found NOx emissions ebbed and flowed with the world's lockdowns. In April and May, for instance, global emissions dropped by at least 15 percent.
Those nations that had the strictest lockdowns ultimately showed the greatest emission reductions in nitrogen oxides.
In China, for instance, lockdown orders at the beginning of the year produced a 50 percent drop in these particular pollutants.
When quarantine measures were later enacted in the United States, Europe, the Middle East, and West Asia, NOx emissions fell between 18 and 25 percent in April and May.
The resulting impact on the atmosphere was surprisingly global and rapid. After lockdowns, models and satellite data reveal a rapid drop in ozone production, which quickly spread around the world, cleaning the air of ozone pollution for as high as 10 kilometers (6 miles).
Here, in the troposphere, ozone can not only decrease air quality, it can also trap heat and contribute to global warming. As a result, the authors think last year's COVID-19 lockdowns had benefits for both air quality and climate change.
"I was very happy that our analysis system was able to capture the detailed changes in emissions across the world," says atmospheric scientist Kazuyuki Miyazaki from NASA's Jet Propulsion Laboratory.
"The challenging and unprecedented nature of this work is a testament to improvements in satellite monitoring in service of societal needs."
In the time of a climate crisis, and when air pollution is worse than ever before, the sooner we can figure out how emissions impact the atmosphere, the better.
According to the World Health Organization, air pollution kills roughly 7 million people each year. It's been described as a silent pandemic, even more dangerous than all the violence in the world and most disease.
In March of last year, estimates in China found just two months of pollution reduction probably saved the lives of 4,000 children under the age of 5 and 73,000 adults.
At the time, that number was more than the global death toll from COVID-19.
Last year's lockdowns have shown us how quickly humans can make a positive impact on the atmosphere and human health, but unless we sustain these practices, the benefits are likely to be short-lived.
As the world opens back up again, global ozone, like our other emissions, will surely rise again.
The study was published in Science Advances.