NO2 over China, US, Europe dips

Reduction in NO2 is important for India because NO2 emissions lead to secondary particulate matter formation.

Nitrogen dioxide (NO2) pollution over northern China, Western Europe and the US decreased by 60% in early 2020 till April as compared to the same period last year, according to two papers published in the American Geophysical Union’s (AGU) journal.

In northern China, average NO2 concentrations reduced by 60% and particulate matter (PM) pollution reduced by 35% since January 23 during lockdown, which is the most significant drop in emissions since air quality monitoring from satellites began in the 1990s. (Reuters)

“The reductions in India are almost similar to what the AGU papers are suggesting. Our analysis will take some time to be released because we are also trying to simulate lockdown conditions to understand exactly ceasing of which economic activity led to how much reduction. We are also focusing on industrial areas and areas where thermal power plants are located,” said Sagnik Dey, associate professor at the Centre for Atmospheric Sciences, IIT Delhi, whose team, like AGU scientists, is tracking satellite data from Tropospheric Monitoring Instrument (TROPOMI).

A Central Pollution Control Board official said they had seen similar reduction in NO2 concentrations from March 25 to April 15, and assessments till May are being made. Reduction in NO2 is important for India because NO2 emissions lead to secondary particulate matter formation.

In northern China, average NO2 concentrations reduced by 60% and particulate matter (PM) pollution reduced by 35% since January 23 during lockdown, which is the most significant drop in emissions since air quality monitoring from satellites began in the 1990s. The AGU papers indicate that curbs on traffic and industrial pollution can significantly improve air quality.

But one of the papers found that the average surface ozone (O3) concentration over China increased by a factor of 1.5-2 during lockdown, which may be as a result of NO2 levels dropping. In highly polluted areas, O3 can be destroyed by NO2, which brings ozone levels down, the study said. “Without analyzing supporting data, it is difficult to comment on the exact reason. One hypothesis is that more sunlight is available to accelerate ozone cycle reactions during lockdown due to a large reduction in particulate matter,” said Dey.

O3 is a highly reactive gas that can cause respiratory and heart disease even at relatively lower concentrations. It is a secondary pollutant formed when sunlight and high temperature catalyse chemical reactions in the lower atmosphere.

NO2 pollution reduced by an average of 40% over Chinese cities and by 20%- 38% over Western Europe and the US during the 2020 lockdown, as compared to the same time in 2019. The main source of NO2 emissions is fuel combustion by vehicles and industries. But NO2 pollution did not decrease over Iran, one of the hardest-hit countries. The authors suspect this is because complete lockdowns weren’t in place until late March and before that restrictions may have been ignored.

The lockdown can give scientists and policymakers clues on emissions regulations in future, the authors said. “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, an atmospheric scientist at the Royal Belgian Institute for Space Aeronomy in Brussels and co-author of one of the papers, said in a statement in Science Daily on May 11.

Model studies will be needed to separate the effects of the enforced lockdown from other contributing factors like meteorology. The authors also recommended further studies to evaluate the effect of temporary lockdowns on climate.