COVID-19 Pandemic and Air Pollution Emissions: What Do We Know?

Benjamin Brown-Steiner, Archana Dayalu, Matthew Alvarado, Thomas Nehrkorn, Jennifer Hegarty, Karen Cady-Pereira

Soon after the novel coronavirus was identified in December 2019, nations around the globe imposed heavy travel restrictions and a variety of social-distancing guidelines. These responses severely impacted daily human activities, particularly through shifts in nonessential business operations (including work-from-home orders, restaurant closures or transitions to takeout only) and movement of schools to online learning. These travel restrictions and declining economic activities also led to a dramatic and clear decline in anthropogenic emissions of greenhouse gases and other air pollutants.

China was one of the first nations to impose economic and travel restrictions, and thus was one of the first regions in which emissions reductions were noted. Satellite observations indicated a sharp decline in carbon monoxide (CO) and nitrogen oxide (NO_x) concentrations over key economic regions in China.^[1] In the ensuing months, many other regions imposed similar restrictions, resulting in a worldwide decline in road and airline traffic. Many news media sources began reporting on clean air and improved visibility not seen in decades, and initial estimates of decreases in air quality-related mortality and morbidity were subsequently released.^[2] Suddenly, we found ourselves conducting a global-scale emissions reduction experiment. This article summarizes what we know about these emissions reductions, including their atmospheric and societal impacts, and explores some possible future scenarios.

Economic activity and change in air quality

All economic sectors are likely impacted by the coronavirus response as national economies face a 5% to 10% reduction in economic activity,^[3] but those most directly impacted are the transportation, industrial, and energy production sectors. The transportation sector has seen the clearest impact so far, as global air travel is down roughly 10% to 15% as of mid-March^[4] and some airlines are reducing summertime international flight volume by up to 60%.^[5] Major urban areas are seeing up to 35% declines in congestion and traffic levels.^[6] Many nonessential industries and commercial entities are decreasing or significantly scaling back their levels of production, which is translating to decreased energy use and resultant emissions of both greenhouse gases and chemical species directly relevant to air quality. Some of this decline is likely offset by increased energy use in the residential sector, although there is a high level of regional-scale heterogeneity and thus the net change in energy use and associated emissions is unclear.^[7]

Travel restrictions, social-distancing measures, and declining economic activities have led to a dramatic and clear decline in anthropogenic emissions of greenhouse gases and other air pollutants.

There are two categories of air pollutant emissions that we are interested in: greenhouse gas emissions and emissions relevant to air quality. Greenhouse gas emissions include emissions of long-lived species, such as carbon dioxide (CO₂) and chlorofluorocarbons (CFCs), as well as emissions of shorter-lived species, such as methane (CH₄) and nitrous oxide (N₂O). The chemical species related to air quality include those that impact surface ozone (O₃) levels, such as emissions of NO_x, CO, and others, as well as those that impact surface aerosol levels, such as sulfur dioxide (SO₂) and primary aerosol emissions.

From the perspective of greenhouse gas emissions, a decline in economic activity and transportation is clearly reducing emissions of CO₂,^[8] with some regions such as China seeing declines up to 25%,^[9],[10] although, due to the long-lifetime and timescale of global mixing, there is as of yet no clear change in atmospheric CO₂ observations.^[11] Ralph Keeling, professor at the Scripps Institute of Oceanography, notes that “emissions would need to drop by about 10 percent for at least six months in order to be detected” by the Mauna Loa observation station in Hawaii.^[12] Emissions of CH₄, N₂O, and CFCs are likely also perturbed, but whether this perturbation is negative or positive is currently unknown.^[13]

In terms of direct impacts on air quality, NO_x concentrations in major industrial and urban areas are seeing a significant decline of up to 70%.^[14] These are seen most clearly in China and Italy, as those countries were the first to face significant coronavirus infections and thus, the first to impose heavy economic restrictions.^10,[15],[16]Other urban regions such as Washington, D.C. and New York City are seeing similar trends^[17], although this may be due to meteorological factors and not emissions related factors.^[18] CO levels are also observed to have decreased over regions of heavy industrial and economic activity.¹⁴ Aerosol concentrations are also declining in many urban areas and industrial regions, and major cities are noting dramatic improvements in visibility metrics.² A decrease in aerosol emissions is expected to have a regional warming effect, although it’s too early to tell if seen in observations.^[19] There is also some level of “fake news” related to the emissions of certain species. For instance, a reported increase in SO₂ emissions due to increased cremation of coronavirus victims is not supported by evidence.^[20]

Future scenarios impacting emissions depend primarily on two factors: (1) the timing and magnitude of peak coronavirus infections both within individual nations and globally; and (2) individual national responses to the coronavirus and, in particular, changes in economic activity resulting from these responses. Estimates as to peak coronavirus infections vary widely, from the spring of 2020 all the way out until 2021.³ The resulting economic activity scenarios are dependent largely on the behavior of political and economic systems, and thus outside of the scope of this article, but we present four scenarios most relevant to the emissions of greenhouse gases and air quality-relevant species.

Scenario 1: Return to Business as Usual

The first scenario is a “Return to Business as Usual” scenario in which the decline in emissions of NO_x and aerosol-relevant species due to land-based travel restrictions and stay-at-home orders will likely reverse as soon as these restrictions and orders are lifted, while airplane travel may rebound more slowly. Soon after these restrictions are lifted, urban and industrial-scale air quality will return to normal or near-normal levels, and the dramatic improvements in urban-area visibility will disappear. If airline travel restrictions are similarly lifted, airplane emissions will rise rapidly, and the changes to clouds and visibility will revert to pre-coronavirus levels, although research examining these impacts are ongoing.^[21]

Scenario 2: Gradual Recovery

The second scenario is a “Gradual Recovery” scenario in which many local, regional, and national-scale economies will ramp up to pre-coronavirus levels as quickly as they can. The time scale of this return to normal is unknown and highly heterogeneous, but overall emissions of CO₂ and other greenhouse gases are likely to return to normal over the following months and years. In the long-term, we’re likely to see a “blip” in CO₂ levels similar in scale to the 2008 Economic Recession, although the full magnitude of this blip depends on the overall level of economic disruption that results from the coronavirus response at the global scale. Additionally, there may be a short-term “overshoot,” as various economies attempt to over-correct and overproduce so as to not result in longer-term disruptions to their overall economic activity.

Scenario 3: Partial Recovery

The third scenario is a “Partial Recovery” scenario. If there’s a large-scale economic recession or depression that results from the coronavirus pandemic, individual economies may not be able to return to pre-coronavirus levels. Alternatively, long-lasting modifications to economic structures may fundamentally shift individual economies and their emissions. The ultimate impact of this scenario depends largely on the ways in which economies recover, or try to recover, and whether the coupling of emissions to economic activity is altered.

Scenario 4: Seasonal Disruption as a New Norm

The fourth and final scenario is a “Seasonal Disruption as a New Norm” scenario. If the coronavirus becomes a disruptive seasonally or annually recurring pandemic, and if treatments or vaccinations are not developed or capable of lessening the overall impact, the societal and economic disruptions we’re currently experiencing may become a seasonal occurrence. There are current concerns that the coronavirus will return with greater overall infection rates later in 2020^[22], as was seen with the 1918 Spanish Flu pandemic.^[23] The disruption to individual economies, and their resultant emissions, might then become a seasonal cycle. Currently, the likelihood of this scenario is unknown.

It’s too early to tell if there will be significant long-term air quality or climate impacts from the 2019-2020 coronavirus pandemic. What is certain is that the present economic slowdown has produced a concurrent slowdown in the emission of greenhouse gases and other pollutants. Once travel restrictions and stay-at-home orders are lifted, emission rates will likely return to a pre-pandemic level, although this recovery may be slow. For instance, some industries most severely affected by the pandemic are not planning on returning to pre-pandemic levels in the short-term.^[24] The overall impact of the coronavirus pandemic on human behavior, society, and regional and global economies is likely to be long-lasting, and precisely what a post-coronavirus “new normal” will look like is unknown.