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.
The team focused on dropping levels of nitrogen oxides, or NOx, which are emitted largely from power plants and car exhaust. It also reacts to form surface-level ozone – an invisible contributor to smog and a harmful pollutant to human health.
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%.
Ozone reductions corresponding to emission decreases from lockdowns in each region of the world show distinct patterns, both locally where the lockdowns occurred and remotely due to the global circulation of the atmosphere. The results demonstrate that where and when the lockdowns occurred is very important in determining the impact on atmospheric composition.
To identify regional and seasonal changes in the ozone response, the team conducted model simulations for each region separately. For each model simulation shown here, only the NOx reductions for that region were used, leaving the rest of the globe at the status quo. This allowed the investigators to avoid seeing overlapping effects from more than one region at once.
Each region shows distinct patterns:
Ozone production from NOx is most efficient in the tropics because of warmer and sunnier climate.
The South American emission reductions result in a long tail of decreased ozone along the mid-latitude westerlies in the Southern Hemisphere.
The European and Australian emission influences on ozone are mostly limited to the region poleward of 30°, linked to the movements of large air masses, like cold or warm fronts, that scale thousands of miles.
Overall, the lockdowns in the northern midlatitudes had the largest impact on ozone concentrations because reductions in NOx emissions were largest there.
The team looked at the lockdowns’ impacts on global ozone pollution in the troposphere—the lowest layer of the atmosphere, from the surface to about 40,000 feet (roughly were passenger jets fly). As opposed to the protective ozone layer in the stratosphere (the layer above the troposphere), the troposphere ozone is an important greenhouse gas.
This map, created using measurements of atmospheric gases from NASA and European Space Agency Earth-observing satellites, shows how lower atmospheric ozone reductions occurred worldwide.
NASA satellites showed that the drop in NOx resulted in a 2% global drop of ozone. While this may seem small, this drop in ozone, which occurred over 8 months, would take 15 years to achieve under even aggressive emission control scenarios.
Ozone depends on a number of factors and not just NOx emissions alone, which is why the team pulled together a variety of air quality measurements and combined them with a model to understand what happened.
Asian emissions show a distinct pattern, with large ozone reductions in the upper troposphere around 40,000 feet extending throughout the tropics and to the mid latitudes of both hemispheres.
This pattern reflects how in the tropics warm air rises to higher altitudes and spreads north and southward, and also how it is transported by the Asian monsoon. The results suggest human activity in Asia has a substantial impact on the global environment.
Meanwhile, reduced lockdown emissions in North America reduced ozone up to about 40,000 feet from the ground farther northward, widely over the northern mid and high latitudes into Canada, Europe and the Arctic.
The movement of the signal of reduced ozone levels from Asian and North American emissions also has implications for ozone’s heat-trapping ability as a greenhouse gas. Ozone has the 3rd largest impact on the radiative balance of the atmosphere.
GCMD keywords can be found on the Internet with the following citation:
Olsen, L.M., G. Major, K. Shein, J. Scialdone, S. Ritz, T. Stevens, M. Morahan, A. Aleman, R. Vogel, S. Leicester, H. Weir, M. Meaux, S. Grebas, C.Solomon, M. Holland, T. Northcutt, R. A. Restrepo, R. Bilodeau, 2013. NASA/Global Change Master Directory (GCMD) Earth Science Keywords. Version 126.96.36.199.0