As the coronavirus pandemic slowed global trade in early 2020, emissions of nitrogen oxides – which create ozone, a danger to human health and the climate – fell by 15% globally , with local reductions of up to 50%, according to a study released Wednesday, led by scientists at NASA’s Jet Propulsion Laboratory.
Due to declining NOx emissions, by June 2020, global ozone levels had fallen to a level that policymakers believed it would take at least 15 years to reach through conventional means, such as regulations.
The study shows that innovative technologies and other solutions to reduce NOx locally have the potential to rapidly improve air quality and climate globally. It was published Wednesday in Science Advances.
Ozone protects us from destructive solar radiation when it is well above the Earth in the stratosphere. Closer to the ground, however, it has other lasting impacts. It is estimated that surface ozone caused 365,000 deaths worldwide in 2019 by damaging the lungs of vulnerable people, such as young children and asthmatics. Likewise, it damages the respiratory systems of plants – their ability to photosynthesize – reducing plant growth and crop yields. And at the top of the troposphere, it’s a powerful greenhouse gas, raising global temperatures.
When the world entered containment, scientists had an unprecedented opportunity to study how human activity interacts with natural processes in the Earth system on a regional and global scale.
A team of international researchers led by JPL scientist Kazuyuki Miyazaki took the opportunity to research the two main nitrogen oxides: nitrogen oxide and nitrogen dioxide, collectively referred to as NOx. They traced the chain of events from reducing fossil fuel consumption during shutdowns to reducing local NOx emissions and finally reducing global ground-level ozone pollution.
The stricter the lockdown imposed by a country, the greater the reduction in emissions. For example, stay-at-home orders from China in early February 2020 resulted in a 50% drop in NOx emissions in some cities in a matter of weeks; most US states saw a 25% drop later in the spring.
The total result of the reduction in NOx emissions was a 2% drop in global ozone – half the amount that the most aggressive NOx emission controls considered by the Intergovernmental Panel on the Climate Change, the authoritative body of international climate experts, were expected to produce more than a 30-year period.
Ozone reductions from reduced NOx emissions quickly spread both around the globe and from the surface upward for more than six miles – 10 kilometers.
“I was really surprised at the magnitude of the impact on global ozone,” said Jessica Neu, JPL scientist, co-author of the new study. “We expected more of a local response on the surface.”
The reactions that turn NOx into ozone require sunlight and depend on many additional factors, such as weather conditions and other chemicals in the air. These factors interact in so many ways that under certain circumstances reducing NOx emissions actually increases ozone. Researchers therefore cannot fully understand or accurately predict ozone concentrations from NOx emission data alone. It requires further analysis, like this study, Neu said.