COVID-19 Lockdowns Cut Pollution, but Not All of It – “It Was Kind of a Surprise”

February 2019 – February 2020

Early on in the COVID-19 pandemic, satellite observations and human experience showed that the world’s air was getting cleaner. But new research shows that not all pollutants have been taken out of circulation during societal lockdowns. In particular, the concentration of tiny air pollution particles, known as PM2.5, didn’t change that much because natural variability in weather patterns dominated, largely obscuring the reduction from human activity.

“Intuitively, you would think that if there were a major lockdown situation we would see dramatic changes, but we didn’t,” said Melanie Hammer, a visiting researcher at Washington University in St. Louis and leader of the study. “It was a bit of a surprise that the effects on PM2.5 were modest.”

PM2.5 describes particles, produced by both human activities and natural processes, that are smaller than 2.5 micrometers, or about 30 times smaller than the width of a human hair. PM2.5 is small enough to linger in the atmosphere and, if inhaled, is associated with an increased risk of heart attack, cancer, asthma and a host of other human health effects. “We were most interested in looking at changes in PM2.5 because it is the leading environmental risk factor for premature death worldwide,” Hammer said.

Combining NASA spacecraft data with ground-based monitoring and an innovative computer modeling system, the scientists mapped PM2.5 levels in China, Europe and North America during the early months of the pandemic. They found that seasonal differences in PM2.5 between recent years were mainly caused by the natural variability of meteorology, not by pandemic lockdowns. Some of the meteorological effects included changes in the sources and intensity of seasonal dust storms, the way pollutants react to sunlight in the atmosphere, the mixing and transfer of heat through weather fronts, and the removal of pollutants from the atmosphere by falling rain and snow. .

An example is shown in the map above, which compares PM2.5 levels in China in February 2020 to February 2019. Note that while pollution levels dropped significantly in some of the most industrialized parts of China, they were actually higher in China. near the desert regions of China. The pollution mapping effort involved data from NASA’s Terra and Aqua satellites, as well as meteorological modeling from the NASA Global Modeling and Assimilation Office. The study was published in June 2021 in the journal Science Advances.

PM2.5 is one of the most complicated pollutants to study because its particle size, composition and toxicity vary widely depending on the source and environmental conditions. For example, it is known that some PM2.5 pollution comes from the reaction of another pollutant – nitrogen dioxide (NO2) – with other chemicals in the atmosphere. NO2 is an important by-product of the combustion of fossil fuels by motor vehicles and industrial activities. In early 2020, NASA and other scientific agencies discovered a significant drop in NO2 pollution during COVID-19 lockdowns, and some people assumed this would mean a dramatic decrease in all pollution.

However, the two pollutants do not have a linear relationship. Half as much nitrogen dioxide in the atmosphere does not necessarily lead to half as much PM2.5 production. Hammer and colleagues decided to investigate whether the lockdowns led to a decrease in particulate matter pollution. “Tackling PM2.5 is a very complex issue,” Hammer said, “and you have to consider the multiple sources, not just the fact that there are fewer people on the road.”

February 2019 – February 2020. Left detail. Correct details.

To ensure a comprehensive analysis, the team focused on regions with extensive ground monitoring systems and compared monthly estimates of PM2.5 from January to April in 2018, 2019 and 2020. When they compared PM2.5 concentrations during the lockdown. months in North America or Europe, they found no clear signals. The main differences related to the closure were found in China, particularly over the North China Plain, where pollution levels are typically high and the most severe closures have been concentrated. But even that signal was a bit muddled.

To decipher whether the lockdown was responsible for the change in China and other small countries in Europe and North America, the team ran several “sensitivity simulations” using the GEOS-Chem chemical transport model. They simulated a scenario in which anthropogenic emissions of nitrogen dioxide and other pollutants were held constant and meteorological variability was solely responsible for year-over-year differences. They also ran simulations in which they reduced emissions from motor vehicles and other anthropogenic sources, following the lockdowns. They found that the simulation that included both meteorological and transport effects most closely matched the real situation, with natural effects accounting for the most differences. One of those results is shown in the map above.

Hammer suspects that the change in PM2.5 levels over the North China Plain was more apparent due to the region’s higher pollution levels during non-COVID times. The new insights also highlight a pertinent point that is not intuitive from the 2020 observations: Average PM2.5 levels have been declining steadily for years in North America and Europe, and pollution concentrations that are already low are more difficult to change.

“The big story here is actually the global characterization of air quality, especially in places where there are no surface monitors,” said Ralph Kahn, co-author and atmospheric scientist at NASA’s Goddard Space Flight Center. “The satellites provide an important part of this, the models provide an important part and the ground measurements also make an important contribution.”

Reference: “Effects of COVID-19 lockdowns on particulate matter concentrations” by Melanie S. Hammer, Aaron van Donkelaar, Randall V. Martin, Erin E. McDuffie, Alexei Lyapustin, Andrew M. Sayer, N. Christina Hsu, Robert C Levy, Michael J. Garay, Olga V. Kalashnikova, and Ralph A. Kahn, June 23, 2021, Science Advances.
DOI: 10.1126 / sciaadv.abg7670

NASA Earth Observatory images by Lauren Dauphin, using data from Hammer, Melanie, et al. (2021). Story by Brandie Jefferson, Washington University in St. Louis, and Roberto Molar Candanosa, NASA’s Earth Science News Team, with Mike Carlowicz.