Pandemic restrictions have reduced global concentrations of nitrogen dioxide (NO2) in the atmosphere almost 20 percent compared to the hypothesis of a 2020 without Covid-19.
It is the result of computer models used by NASA researchers, which were presented at the 2020 International Conference on High Performance Computing, Networking, Storage and Analytics.
Nitrogen dioxide is an air pollutant that is produced primarily 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 restrictions were going to have an impact on air quality,” lead author Christoph Keller said in a statement from the USRA (Universities Space Research Association) at NASA’s Goddard Space Flight Center. 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.
“It was also soon clear that it was going to be difficult to quantify how much of that change is related to the blocking measures, compared to general seasonality or variability in pollution,” he added.
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 could not be predicted in human behavior as the new coronavirus, and the regulations trying to control it, spread rapidly. Rather than try to reprogram their model with this unexpected event, Keller and his colleagues explained Covid-19 to make the model ignore the pandemic completely.
Model simulation and machine learning analysis were carried out at NASA’s Climate Simulation Center. His ‘normal situation’ 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 is 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 reductions in nitrogen dioxide 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, in China, it was the first municipality to report a Covid-19 outbreak. It was also the first to show a reduction in nitrogen dioxide emissions, 60% less than the expected simulated values. A 60% decrease in Milan and a 45% decrease in New York followed shortly, when local restrictions took effect.
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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