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Since the COVID-19 pandemic began, ground and space observations have shown that Earth’s atmosphere has seen significant reductions in some air pollutants.
However, the scientists wanted to know how much of that decline can be attributed to changes in human activity during pandemic-related shutdowns, compared to how much would have occurred in a 2020 without a pandemic.
Using computer modeling to generate a COVID-free 2020 for comparison, NASA researchers found that since February, pandemic restrictions have reduced global nitrogen dioxide concentrations by nearly 20%. The results were presented at the 2020 International Conference on High Performance Computing, Networking, Storage and Analytics.
Nitrogen dioxide is an air pollutant that is produced mainly by the combustion of fossil fuels used by industry and transportation, which were significantly reduced during the height of the pandemic to prevent the spread of the new coronavirus.
“We all knew the lockdowns were going to have an impact on air quality,” said lead author Christoph Keller of the Universities Space Research Association (USRA) at NASA’s Goddard Space Flight Center in Greenbelt, Maryland. Keller works in Goddard’s Global Modeling and Assimilation Office (CMMS), which uses high-tech computer models to help track ocean and atmosphere chemistry and forecast future climate scenarios. He says, “It was also soon clear that it was going to be difficult to quantify how much of that change is related to lockdown measures, versus overall seasonality or variability in pollution.”
The pandemic-related shutdowns have affected the way people act, so scientists began monitoring how that has affected the planet, specifically nitrogen dioxide emissions. How do COVID-19 contamination patterns influence NASA computer models? NASA’s GEOS atmospheric composition model shows us the answer. Credits: NASA Goddard Space Flight Center
No two years are exactly the same. Normal variations in climate and atmospheric circulation change the composition and chemistry of the Earth’s atmosphere. Comparing 2020 nitrogen dioxide concentrations to 2019 or 2018 data alone would not account for year-to-year differences. But, because NASA’s model projections take these natural variations into account, scientists can use them to analyze how much of the 2020 atmospheric composition change was caused by the COVID-19 containment measures.
Even with the models, sudden and drastic changes in human behavior could not be predicted as the new coronavirus, and the regulations that try to control it, spread rapidly. Rather than trying to reprogram their model with this unexpected event, Keller and his colleagues explained COVID-19 by making the model ignore the pandemic entirely.
Model simulation and machine learning analysis were carried out at NASA’s Climate Simulation Center. Its “business as usual” scenario showed an alternate reality version of 2020, one that did not experience any unexpected changes in human behavior brought on by the pandemic.
From there, it’s a simple subtraction. The difference between the simulated model values and the measured ground observations represents the change in emissions due to the pandemic response. The researchers received data from 46 countries, a total of 5,756 ground observation sites, transmitting hourly atmospheric composition measurements in near real time. At the city level, 50 of the 61 cities analyzed show nitrogen dioxide reductions between 20 and 50%.
“In a way, I was surprised at how much it fell,” Keller said. “Many countries have already done a very good job of reducing their nitrogen dioxide concentrations over the past decades due to clean air regulations, but what our results clearly show is that there is still a significant contribution driven by human behavior.” .
Wuhan, China, was the first municipality to report a COVID-19 outbreak. It was also the first to show a reduction in nitrogen dioxide emissions, 60% lower than expected simulated values. A 60% decrease in Milan and a 45% decrease in New York followed shortly, when local restrictions went into effect.
“Sometimes you could even see nitrogen dioxide decline before official policies were put in place,” USRA co-author Emma Knowland said in Goddard’s CMMS. “People were probably reducing their traffic because rumors about the COVID-19 threat were already happening before they told us to close.” Once the restrictions were relaxed, the decreases in nitrogen dioxide decreased, but remained below the expected values ”as usual”.
Keller compared his estimates of nitrogen dioxide declines with reported economic figures, that is, gross domestic product, of the nations included in the study. According to Keller, they lined up surprisingly well. “We would hope that they were related in some way because nitrogen dioxide is closely linked to economic activities, such as people traveling and factories running,” he said. “It seems our data captures this very well.”
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Research is ongoing and the GEOS model data used in this study are publicly available.
You can find more information about GEOS at: https://gmao.gsfc.nasa.gov/GEOS/
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