Pandemic restrictions have reduced global NO2 concentrations by nearly 20 percent, a study shows

Reviewed by Emily Henderson, B.Sc.November 18, 2020

Since the start of the COVID-19 pandemic, space and ground-based observations have shown that the 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 arrests, compared to what would have occurred in a pandemic-free 2020.

Using computer models 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 for High Performance Computing, Networking, Storage and Analytics.

Nitrogen dioxide is an air pollutant produced mainly by the combustion of fossil fuels used by industry and transportation, both of which have been significantly reduced during the height of the pandemic to prevent the spread of the novel coronavirus.

“We all knew that the blocks would have an impact on air quality,” lead author Christoph Keller said with 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 (GMAO), which uses high-tech computer models to monitor ocean and atmosphere chemistry and predict future climate scenarios.

He says, “it was also soon clear that it would be difficult to quantify how much of that change is related to blocking measures, relative to general seasonality or pollution variability.”

No two years are exactly alike. Normal variations in weather and atmospheric circulation change the composition and chemistry of the Earth’s atmosphere. Comparing 2020 nitrogen dioxide concentrations with data from 2019 or 2018 alone would not take into account the differences from one year to the next.

However, because NASA’s model projections account for these natural variations, scientists can use them to analyze how much of the 2020 atmospheric composition change was caused by COVID-19 containment measures.

Even with the models, it was not possible to predict sudden and drastic changes in human behavior as the new coronavirus – and the regulations attempting to control it – spread rapidly. Instead of trying to reprogram their model with this unexpected event, Keller and his colleagues took COVID-19 into account by making the model ignore the pandemic altogether.

Model simulation and machine learning analysis took place at the NASA Center for Climate Simulation. His “business as usual” scenario showed a 2020 version of the alternate reality, one that did not undergo unexpected changes in human behavior caused by the pandemic.

From there it is simple subtraction. The difference between the simulated values ​​from the model 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 – that transmit hourly measurements of atmospheric composition in near real time. At the city level, 50 of the 61 cities analyzed show nitrogen dioxide reductions between 20-50%.

In a way, I was surprised at how much he fell. Many countries have already done a great job of lowering their nitrogen dioxide concentrations over the past few decades thanks to clean air regulations, but what our results clearly show is that there is still a significant contribution driven by human behavior. “.

Christoph Keller, lead author of the study, Universities Space Research Association (USRA), NASA / Goddard Space Flight Center

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, which was 60% lower than the simulated predicted values. This was followed shortly by a decrease of 60% in Milan and 45% in New York, with the entry into force of local restrictions.

“Sometimes you could even see the decrease in nitrogen dioxide before official policies went into effect,” co-author Emma Knowland with USRA told Goddard’s GMAO.

“People were probably reducing transit because there was talk of the COVID-19 threat even before we were actually told to shut down.” Once the restrictions were relaxed, the nitrogen dioxide decreases decreased, but remained below predicted “as usual” values.

Keller compared his estimates of nitrogen dioxide decreases with the reported economic numbers, or gross domestic products, of the nations included in the study. According to Keller, they lined up surprisingly well.

“We would expect them to be related in some way because nitrogen dioxide is so closely related to economic activities, such as people traveling and factories in operation,” he said. “It looks like our data captures it very well.”