Levels of two major air pollutants have been reduced significantly since lockdowns began in response to the COVID-19 pandemic, but a secondary pollutant—ground-level ozone—has increased in China, according to new research.

Two new studies in AGU’s journal Geophysical Research Letters find nitrogen dioxide pollution over northern China, Western Europe and the US decreased by as much as 60% in early 2020 as compared to the same time last year. Nitrogen dioxide is a highly reactive gas produced during combustion that has many harmful effects on the lungs. The gas typically enters the atmosphere through emissions from vehicles, power plants, and industrial activities.

TROPOMI NO₂ columns over China (a) before and (b-e) after the lockdowns (shown by padlocks) due to COVID-19. For comparison, columns over the same time periods are shown for 2019. The week of Chinese New Year holiday is indicated by the red lantern shown inset panels (b) and (h). The New Year holiday covers 4-10 February in 2019, and 24-30 January in 2020 (exceptionally extended to 2 February because of COVID-19). Partial loosening of the restrictions is suggested by the smaller padlock in panel (e). Grey areas indicate no valid data. Bauwens et al.

In addition to nitrogen dioxide, one of the new studies finds PM_2.5 has decreased by 35% in northern China. Particulate matter is composed of solid particles and liquid droplets that are small enough to penetrate deep into the lungs and cause damage.

The two new papers are part of an ongoing special collection of research in AGU journals related to the current pandemic.

Such a significant drop in emissions is unprecedented since air quality monitoring from satellites began in the 1990s, said Jenny Stavrakou, an atmospheric scientist at the Royal Belgian Institute for Space Aeronomy in Brussels and co-author of one of the papers. The only other comparable events are short-term reductions in China’s emissions due to strict regulations during events like the 2008 Beijing Olympics.

The improvements in air quality will likely be temporary, but the findings give scientists a glimpse into what air quality could be like in the future as emissions regulations become more stringent, according to the researchers.

Maybe this unintended experiment could be used to understand better the emission regulations. It is some positive news among a very tragic situation.

—Jenny Stavrakou

However, the drop in nitrogen dioxide pollution has caused an increase in surface ozone levels in China, according to one of the new studies. Ozone is a secondary pollutant formed when sunlight and high temperature catalyze chemical reactions in the lower atmosphere. Ozone is harmful to humans at ground-level, causing pulmonary and heart disease.

In highly polluted areas, particularly in winter, surface ozone can be destroyed by nitrogen oxides, so ozone levels can increase when nitrogen dioxide pollution goes down. As a result, although air quality has largely improved in many regions, surface ozone can still be a problem, according to Guy Brasseur, an atmospheric scientist at the Max Planck Institute for Meteorology in Hamburg, Germany, and lead author of one of the new studies.

It means that by just reducing the [nitrogen dioxide] and the particles, you won’t solve the ozone problem.

—Guy Brasseur

Stavrakou and her colleagues used satellite measurements of air quality to estimate the changes in nitrogen dioxide pollution over the major epicenters of the outbreak: China, South Korea, Italy, Spain, France, Germany, Iran and the United States.

They found that nitrogen dioxide pollution decreased by an average of 40% over Chinese cities and by 20% to 38% over Western Europe and the United States during the 2020 lockdown, as compared to the same time in 2019.

However, the study found nitrogen dioxide pollution did not decrease over Iran, one of the earliest and hardest-hit countries. The authors suspect this is because complete lockdowns weren’t in place until late March and before that, stay-at-home orders were largely ignored. The authors did see a dip in emissions during the Iranian New Year holiday after March 20, but this dip is observed during the celebration every year.

The second study looked at air quality changes in northern China where the virus was first reported and where lockdowns have been most strict.

Left Panel: Evolution of the mean concentration of PM_2.5, NO₂, O₃ , CO and SO₂ [all in µg m^-3 except in mg m^-3 for CO] recorded by the monitoring stations in the urban area of Wuhan from 1 January 2019 to 28 February 2019. The vertical red line corresponds to the beginning of the Spring Festival on 5 February 2019. The horizontal dash lines indicate the averages of the quantities before and after this date.
Right Panel: same as on the left panel, but for the period 1 January 2020 to 29 February 2020. The red vertical line indicates the day (23 January 2010) on which the lockdown of Wuhan was implemented by the Chinese authorities. The horizontal dash lines show the mean of the represented quantities before and after this date. The temperature data are from the Copernicus Climate Change Service (2017). Shi and Brasseur.

Brasseur analyzed levels of nitrogen dioxide and several other types of air pollution measured by 800 ground-level air quality monitoring stations in northern China.

Brasseur and his colleague found particulate matter pollution decreased by an average of 35% and nitrogen dioxide decreased by an average of 60% after the lockdowns began on 23 January.

However, they found the average surface ozone concentration increased by a factor of 1.5-2 over the same time period. At ground level, ozone forms from complex reactions involving nitrogen dioxide and volatile organic compounds (VOCs), gases emitted by a variety of household and industrial products, but ozone levels can also be affected by weather conditions and other factors.

Resources

• Bauwens, M., Compernolle, S., Stavrakou, T., Müller, J.‐F., van Gent, J., Eskes, H., et al. (2020) “Impact of coronavirus outbreak on NO₂ pollution assessed using TROPOMI and OMI observations.” Geophysical Research Letters, 47, e2020GL087978. doi: 10.1029/2020GL087978

• Xiaoqin Shi and Guy P. Brasseur (2020) “The Response in Air Quality to the Reduction of Chinese Economic Activities during the COVID-19 Outbreak” Geophysical Research Letters