TL;DR—A turbulent year characterized by a global pandemic and natural disasters, 2020 had a silver lining for the air quality management space: the pandemic served as a worldwide laboratory for how policies targeting pollution reduction and behavioral change can impact the air that citizens breathe. A wealth of air quality studies took advantage of the unique cultural and economic conditions of 2020 to develop a better understanding of the behavioral, economic, and policy changes that truly have an impact on air quality. Here we explore some of the air quality trends observed around the world in 2020 and what they mean for air quality management moving forward. 

2020 was a turbulent year for nearly every aspect of life, and air quality was no exception. While last year was challenging in many ways, it also offered opportunities to adjust our behavior as a society and identify where improvements should be made. When it comes to air quality, 2020 has served as a laboratory for how policies oriented at behavioral change and natural resource management can impact the air that citizens around the world breathe on a daily basis. 

Publications addressing the impacts of COVID-19 lockdowns on air quality

This image displays the number of publications that address the impacts of COVID-19 on air quality. There is a pronounced spike in the number of publications in the period of March 2020 to May 2020 at the height of global COVID-19 lockdowns, demonstrating the view that this period of time was seen as a global air quality experiment with public and media attention put on the visible differences in air quality. (Image source: COVID-19 Air Quality Data Collection)

The unique economic and behavioral conditions that came with COVID-induced lockdowns led to unprecedented improvements in air quality in many locations around the world. Because the initial shutdown of business, travel, and other economic activity accounted for an unprecedented decrease in global activity, air pollution rapidly decreased in a remarkably visible way, with many individuals even noting visibly clearer skies. A study looking at 34 countries around the world found an average 60% reduction in NO2 and 31% reduction in fine particulate matter (PM2.5) compared to levels found in previous years during the same period and weather conditions. 

In Europe, the European Environment Agency (EEA) published their “Air quality in Europe — 2020 report”, showing reductions of up to 60% for certain air pollutants in many European countries since lockdowns were implemented in the spring of 2020. Meanwhile in the United States, cities across the country saw air pollution reach unprecedented highs and lows in the span of a single year due to the unique economic conditions associated with COVID-19 and a record-breaking wildfire season in the West. 

In economically developing regions, such as India and some countries in Southeast Asia, air quality improvements were also realized, though many of the initial improvements in air quality due to COVID-19 lockdowns rebounded by the end of 2020 when industry and transport ramped back up. Air quality improvements in this region also varied more widely depending on existing urban conditions, such as population density and existing pollution levels.

Lockdowns in India have created visible differences in air quality as images of the same location are compared between November 2018 and April 2020

The image above displays the visible disparities in air quality seen before and during the COVID-19 lockdown periods at the same location in India. (Image source: Reuters)

Before the pandemic: how air quality trends differ around the world

Even before the onset of the global COVID-19 pandemic, air quality trends and pollutant levels were far from uniform around the world, differing widely depending on pollution sources, government regulation, urbanization, population density, and a variety of other factors. 

In the United States, air pollutants regulated under the National Ambient Air Quality Standards (NAAQS) have decreased steadily since the 1990s under the Clean Air Act and its later amendments. At the same time, GDP, population, and vehicle miles traveled have increased. This trend presents evidence that air quality improvement does not have to come at the cost of economic growth.

Declining national air pollutant concentration averages from 1990-2018

Under the Clean Air Act regulation in the United States, national air pollutants have decreased over the past few decades, and all are now under the NAAQS standards. This graph displays the level of pollutant concentration averages. Pollutant emissions totals have also decreased over the same period. (Image source: EPA)

In other regions, such as Central and Southern Asia and Sub-Saharan Africa, populations continue to be exposed to high levels of air pollution despite efforts to improve air quality. According to the World Health Organization (WHO), less than 8% of people in Asia breathe what is classified as “clean air”. In South Asia in particular, annual pollution levels are anywhere from 5-10 times greater than WHO’s allowed pollutant levels. 

The Eastern Mediterranean, along with South Asia and the Western Pacific, has seen urban air pollution levels increase by over 5% in two-thirds of their cities from 2008 to 2013.

In Africa, urban air pollution data is more sparse, making it more difficult to get a full picture of the state of air quality. Existing data on particulate matter reflects levels above the global median. Efforts to increase data coverage and availability of air pollution data help to fill in these data gaps. To learn more about the potential to use hybrid sensor networks in economically developing countries to create more worldwide air quality data equity, read our blog here.

Despite global PM2.5 levels dropping [mostly in North America and Europe], “no such improvements are seen in other regions where the proportion has remained virtually constant and extremely high (e.g., greater than 99% in Central, Southern, Eastern and South-Eastern Asia Sustainable Development Goal (SDG) regions”

— G. Shaddick et al.

In Asia, geographical factors may be part of the reason for higher air pollution levels. Many countries are landlocked or situated close to mountain ranges which allow for less pollutant dispersion. Human factors—such as rapid industrialization, heavy reliance on biomass and coal, and less strict regulation on industrial emissions—also contribute to poor air quality.

Now that we’ve looked at global air quality trends prior to the onset of COVID-19, let’s explore some of the research conducted in 2020 and examine how the global pandemic influenced air quality around the world. 

Air pollution reductions during lockdowns in Europe

     With lockdowns beginning in Europe in February 2020, air quality improvements became evident over the spring. The COVID-19 lockdown measures were found to contribute to the largest decreases in nitrogen oxides and carbon dioxide from fossil fuel emissions—over the course of January 1 to July 31, 2020, nitrogen oxides decreased by 13.1% and carbon dioxide fossil fuel emissions decreased by 10.3%. The largest reduction in these pollutants occurred during April, when many countries’ lockdowns were at their height of mobility restrictions for the entire month, with 34% and 26.8% reductions in nitrogen oxides and carbon dioxide, respectively.

     Because nitrogen dioxide is largely released by road transport and vehicle emissions, the decreased transportation activity occurring under COVID-19 lockdowns resulted in substantial nitrogen dioxide reductions. Nitrogen dioxide, or NO2, is considered a local pollutant, meaning pollution spikes can be seen directly near a source like a roadway where the pollutant originates. Scientists observed decreases in pollutants like NO2 directly in line with the dates that different countries began their respective lockdown, in contrast to non-local pollutants such as ozone, which did not decrease in direct relation to decreased transport or other activity.

Effect of COVID lockdown measures on air quality comparison between expected and actual NO2 concentrations (selected countries, April 2020)

The figure above compares nitrogen dioxide concentrations during lockdowns in selected European countries (April 2020) versus their expected concentrations during this time of year. Though the changes in NO2 varied from country to country in Europe, each country studied experienced a decrease, ranging from 20% to 61%. This improvement is marked by the decrease in transportation and vehicle usage. (Image source: European Environment Agency)

Particulate matter (PM) also decreased during the COVID-19 lockdowns in Europe, though this reduction was slightly less pronounced than that for NO2. While NO2 can be closely tied to road transport, PM comes from a greater variety of sources, including emissions from residential heating, agriculture, and industrial activities, none of which experienced as stark of a decline as transportation.

Effect of COVID lockdown measures on air quality comparison between expected and actual PM10 concentrations (selected countries, April 2020)

The figure above compares PM10 concentrations during lockdowns in selected European countries (April 2020) versus their expected concentrations during this time of year. Though the decreases are not as stark as those with NO2, the selected countries all exhibit a decrease in particulate matter concentration, ranging from a 9% to a 30% decrease. (Image source: European Environment Agency)

Reductions in CO2 and SO2 have also been noted during the COVID-19 lockdowns in Europe, particularly due to declines in the output of the energy sector.

How lockdowns in Asia have affected air pollution

Many regions of Asia experienced a comparable decrease in particulate matter pollution due to COVID-19 lockdowns as was observed in Europe, especially in urban environments. A study from July 2020 focusing on India notes a decrease in PM ranging from 10% in Mumbai to 54% in Delhi, which is comparable to PM decreases in other cities, such as a 42% decrease in Shanghai and a 60% decrease in Vienna. 

Research also demonstrates that the existing urban conditions in a particular city affect the extent to which air pollutants like PM decrease in accordance with lockdowns. Cities with denser populations and higher existing PM levels, such as Delhi, Seoul, and Wuhan, experienced more significant PM reductions.

However, many cities with improved air quality during initial lockdowns did not sustain these gains throughout the course of the year. When economic activity increased again, rebounds in transport and industry drove air pollution back up. For cities in Asia like Delhi and Seoul, this meant smaller annual pollution reductions than those initially observed during the early lockdown period. However, for other global cities such as Los Angeles, increases in economic activity following initial lockdowns, along with the devastating 2020 wildfire season on the west coast of the United States (more on this below), ultimately resulted in a 15% increase in PM.

Sulfur dioxide, or SO2, is another pollutant that decreased significantly due to COVID-19 lockdowns in Asia. A study from February 2021 found that in Islamabad, Pakistan, SO2 levels decreased from 120 µg/m3 to 18.86 µg/m3 during the lockdown, and these levels decreased from 9.90 µg/m3 to 6.0 µg/m3 in Delhi, India. This trend is similar to the decreases in NO2 seen across Europe, in which the pollutants reflect a more local, direct relationship to decreases in transportation, fossil fuel combustion, and other economic activities, including overall energy and electricity demand. 

The case of ozone and pollutant interaction

In contrast to other pollutants, many regions did not experience a decrease in ozone pollution—in fact, ozone slightly increased worldwide. This occurrence highlights the importance of examining the secondary reactions that take place in the atmosphere, as well as the importance of focusing on the trends of pollutants that individuals are actually breathing at the ground level. 

The process of ozone and pollutant interaction is a complicated one. High levels of NO2 in the air can actually prevent certain reactions which form ozone, so in urban areas with high NO2 levels ozone levels may be lower. Furthermore, NO2 can react with ozone itself when there is enough of the pollutant present, and thus remove some ozone from the air. Consequently, ozone has increased in cities like Beijing, where previously abundant NO2 levels were reduced during the pandemic.

The complex and nonlinear relationship between certain pollutants means that a decrease in one pollutant can yield varying results on others. For example, dropping NO2 levels in China—with as much as a 93% NO2 reduction in Wuhan—existed side-by-side with heightened particulate matter hotspots seen during the same period of lockdown in China. 

Record-breaking air pollution in the United States due to wildfire

While air quality improved in many areas of the world during 2020, the devastating wildfire season in the United States dramatically reversed this trend. Forced evacuations occurred throughout California, Oregon, Washington, and Colorado amidst the release of large amounts of particulate matter into the air. The region also sustained environmental and habitat damage as a result of the fires.

Changes in overall pollution levels observed from 2019-2020 in the United States

The image above displays the dramatic increases in air pollution levels seen in the western region of the United States as contrasted to other regions, such as the East Coast, where the air became cleaner from 2019 to 2020, likely due to the COVID-19 lockdown effects explored earlier. The air became as much as 329% more polluted in some areas. (Image source: The Washington Post)

Increasing air pollution due to rampant wildfires also directly affects the health of those breathing this air. 1 in 7 Americans has experienced at least one day of unhealthy air quality during the 2020 wildfire season, accounting for an increase of over 9 million individuals since 2018. 

More counties saw longer periods of ‘very unhealthy’ air in 2020 compared to previous years (2011-2020)

When comparing the western coast of the United States from 2011 to 2020, research shows that many counties experienced air classified as ‘very unhealthy’ for greater periods of time in 2020 compared to previous years. Public health impacts of lingering poor air quality especially affect children, older individuals, and those with lung conditions, though everyone in these regions is harmed by high levels of particulate matter released by wildfire smoke. (Image source: NPR)

To learn more about the destructive impacts of the 2020 wildfire season in the United States, you can check out our wildfire blog here.

Air pollution from wildfires in 2020 counteracted the clean air gains from COVID-19 lockdowns, resulting in a tumultuous year for air quality in the western United States. Despite a reported 13% decrease in fine particulate matter pollution from March to July 2020, particulate matter increased overall by almost 7% above 2019 levels, primarily because of wildfire smoke in California, Oregon, and Washington.

In September [2020], 24 of the world’s top 25 most polluted cities were in California and Oregon”

— IQAir World Air Quality Report

Air pollution from wildfires impacts air quality not only on days with exceptionally high fire activity but also year-round. Annual PM2.5 exposure exceeded WHO targets in 38% of cities in the U.S. in 2020, compared to 21% and 20% of cities in 2019 and 2018, respectively. High levels of PM2.5 are thought to stem from both prolonged and worsening wildfire seasons as well as environmental regulation rollback.

Wildfire smoke causes significant environmental and human damage—and not only in the region where the fire is taking place. Wildfire smoke plumes can reach up to 14 miles into the atmosphere and spread hundreds and thousands of miles to other geographic areas, bringing with it harmful particulate matter. 

Smoke from the wildfires in California and Oregon reached the East Coast of the U.S., creating a hazy sky in parts of Washington, D.C. and New York. Devastating wildfires occurring in Siberia in the summer of 2020 traveled across the Pacific Ocean, worsening air quality in places as far away as Alaska and Seattle, Washington. These Siberian wildfires set a record for the greatest amount of air pollution released in a single month for the last 18 years of record keeping.

Drastic, prolonged, and more frequent wildfires manifest as a result of increasing climate change. Check out our blog about the relationship between climate change and air quality to learn more.

2020: A global air quality experiment

Over the course of 2020, air quality reached unprecedented highs and lows across the globe. The positive impacts of COVID-19 lockdowns gave hope for a changing trend in air quality where hazy skies did not have to be the norm. However, once economic activity and industry increased after the first lockdowns, many countries saw their pollution levels returning to pre-lockdown levels. Other parts of the world, particularly the western United States, saw initial clear air gains disappear with a destructive wildfire season in the late summer and fall. 

Because of the many exceptional events in 2020, the year served as a laboratory for measuring how human activity and policy decisions directly affect our air. Policymakers and individuals alike can look to 2020—and the wealth of air quality research conducted during this period—to bring greater awareness of the deep impact humans and industry have on the air we breathe.