Air pollution has unprecedentedly decreased worldwide with fewer vehicles on the streets and industrial activity shut down, particularly during the last two months due to imposed COVID-19 lockdowns. Nitrogen dioxide, a major air pollutant produced from fossil fuel combustion, was lower by 60 percent across the globe in early 2020 compared to the corresponding time period last year.
Nitrogen oxides are highly reactive, forming acid rain and other air pollutants. Exposure to the gas leads to asthma and respiratory illnesses. According to two studies recently published in the American Geophysical Union’s journal, Geophysical Research Letters, NO2 levels came down in northern China, Western Europe and the U.S.
“Maybe this unintended experiment could be used to better understand the emission regulations. It is some positive news among a very tragic situation,” Jenny Stavrakou, an atmospheric scientist at the Royal Belgian Institute for Space Aeronomy in Brussels and co-author of one of the studies, said, as qouted by the news release.
Researchers used measurements taken from two satellites, TROPOMI onboard Sentinel‐5 Precursor and OMI on Aura, of nitrogen dioxide levels in countries the pandemic hit the hardest. The United States, China, South Korea, Italy, Spain, France, Germany and Iran were the major epicenters examined for pollution levels. In comparison to 2019, nitrogen dioxide levels have decreased over the United States and Western Europe by 20 to 38 percent.
This was not the case in Iran, which did not enforce lockdowns and home quarantining when the virus first hit. A drop in emission levels was observed during the Iranian New Year holiday post March 20, however this happens every year. But nitrogen dioxide levels did not decrease over Iran amidst the beginning of the COVID-19 pandemic.
Surface Ozone Increases In China
In China, a 40 percent decrease in nitrogen dioxide was noted. Additionally, 35 percent decrease in particulate matter was observed in the aforementioned studies. This is good news since PM 2.5 droplets are known to cause lung damage, which is why it is such a harmful air pollutant. The last time pollution levels decreased in China was the 2008 Beijing Olympics when the country had strict regulations in place.
This positive occurrence in China is accompanied by a side effect: formation of ozone. Heat and sunlight cause reactions between nitrogen oxides and volatile compounds to form ozone at the ground level, which is unlike stratospheric ozone that protects the earth from radiation.
How did this happen? Generally, surface ozone is destroyed by nitrogen oxides in the winter. “It means that by just reducing the [nitrogen dioxide] and the particles, you won’t solve the ozone problem,” Guy Brasseur, an atmospheric scientist at the Max Planck Institute for Meteorology in Hamburg, Germany, and lead author of one of the new studies, said.
In the study conducted by Brasseur and his colleague Xiaoqin Shi, 800 ground-level air quality monitoring stations in Northern China indicated that PM 2.5 came down by 35 percent and NO2 came down by 60 percent after the lockdowns started on January 23. During the same time period, surface ozone levels increased by 1.5 to 2 points.
However, post the lockdown in China, not just ozone, but PM2.5, NO2 and SO2 levels have risen drastically, even more than last year, as per a new study by the Helsinki-based Centre for Research on Energy and Clean Air (CREA).