COVID-19: Reduction in Tropospheric NOx and Ozone Corresponding to Worldwide Lockdowns

When the world went into lockdown to slow the spread of COVID-19, air pollution emissions started to rapidly decrease leaving a global atmospheric fingerprint detected by a team of scientists at NASA’s Jet Propulsion Laboratory using satellite measurements. These traces provided an unexpected window into what low-emissions world could look like, thus providing a means for identifying effective environmental policies.

While many countries in the last few decades have implemented environmental policies to reduce human health risk from air pollution by controlling emissions, the impacts of those policies have not always been clear. The global lockdowns in response to COVID-19 represent a well-observed “scenario-of-opportunity” that allows us to assess how atmospheric emission and composition responds to reduced human activity.

COVID-19 lockdowns effectively showed how reducing NOx emissions affects the global atmosphere. Its identifying signature shows up as in the atmosphere’s altered ability to produce harmful ozone pollution and ozone’s reduced influence on Earth’s heat balance that affects climate. These effects are not uniform across the world and depend on the location and season of the emission reductions.

The results of this research indicate that in order to design effective environmental policies which benefit both air quality and climate, decision-makers need to carefully consider the complex relationships between emissions and atmospheric composition.

At the country scale, the evolution of NOx emission reductions over time is strongly correlated with the COVID-19 Government Response Stringency Index, an indicator of the severity of government lockdown measures to slow transmission of COVID-19.

Chinese NOx emissions rapidly declined from late January through late February, corresponding to China’s first lockdown. This was followed by a rapid recovery to their normal levels for March and April. In May, the emissions again started to decrease corresponding to a second lockdown in some parts of the country.

In Italy, the early implementation of lockdown led to large emission reductions before other European countries, from late February to early May.

Most of the states in the United States announced emergency stay-at-home orders in late March. The estimated emissions show declines beginning in late February and early March, with maximum reductions of about 25% in April and May, followed by a moderate recovery in June.

The team found that, globally, emissions of NOx decreased 15%.