The total closures due to the spread of the Corona virus at the beginning of last year have caused a decrease in air pollution all over the world.
Despite the decrease in levels of other pollutants, Beijing and northern China have recorded a significant increase in ground-based ozone levels, which may have harmful health effects.
Benefit or detriment?
Ozone - its chemical formula O3 - is found naturally in the stratosphere.
Despite the vital function of the upper ozone layer to protect us from ultraviolet rays, ozone may also be found in the lower Earth's "troposphere", known as tropospheric ozone.
In contrast to oxygen (O2), ozone is a highly reactive toxic gas with harmful health effects.
Ground ozone interacts with mixed pollutants in the air such as hydrocarbons and nitrogen oxides, as well as with volatile organic compounds in the atmosphere produced from fossil fuels, and produces a large group of highly reactive chemical fissures, which in turn start a series of reactions with the components of the atmosphere.
Nitrogen oxides - also from fossil fuels - play a role in increasing these chain reactions.
It is known that higher ground ozone levels are associated with increased levels of volatile organic compounds as well as nitrogen oxides.
The upper ozone layer protects the earth from harmful rays of the sun (Wikipedia - A Loess Nektae)
Incomprehensible increase
Despite the cessation of the usual forms of daily life in northern China in early 2020, which was accompanied by a decrease in nitrogen oxide emissions by 60% to 70%, ozone levels remained high.
This is not surprising in a region that witnessed an increase in ozone pollution 5 years ago.
However, this increase was in the summer months, and was not extended from late winter to spring as was seen last year.
So the new player, "Covid-19", may help us understand the reasons for this increase.
Recently, a related study was published by scientists from Harvard John A. Paulson School of Engineering and Applied Sciences and Nanjing University of Information Science & Technology.
The study, published in the Proceedings of the National Academy of Sciences, on March 9, attributed the increase in ozone levels in the winter and spring months to nitrogen oxides and volatile organic compounds.
Despite the complete lockdown, ground-based ozone levels have remained high (Pixabay)
Two-sided damage
According to the press release published by Harvard University, Daniel Jacob, co-author of the study, says, "The closure gave us an unintended opportunity to find out the causes of the sudden increase in ozone despite the decrease in emissions."
In a previous study, the researchers found that tiny particles suspended in the air - which have a radius of about 2.5 micrometers - act like a sponge that absorbs the chemical cracks responsible for worsening ground levels of ozone.
The researchers also concluded that the Chinese government's policies to reduce these suspended particles have led to an increase in pollution caused by the high level of ground ozone, especially in major cities.
In the current study, the team concluded that nitrogen oxides play a similar role during the winter season, by absorbing the active cracks and preventing them from forming ground ozone.
Hence, a decrease in nitrogen oxide levels - whether gradually as a result of pollution reduction policies, or suddenly as a result of closure - will increase the active cracks that can interact with VOCs, and produce more active fissures again that increase ground ozone levels.
Controlling volatile organic compounds will reduce the terrestrial ozone season (pixabay)
Therefore, the current study indicates the importance of further research needed to understand the sources and types of VOCs in order to regulate their emissions.
This is confirmed by Hong Liao, a senior researcher for the study at Nanjing University, saying that "controlling the emissions of volatile organic compounds will reduce the increase in the ground ozone season, which may bear fruit on pollution, public health and crop production."