Planet Earth

New type of extremely reactive substance discovered in the atmosphere

The atmosphere of planet Earth

A whole new class of super-reactive chemical compounds, the trioxides, have been discovered under atmospheric conditions.

For the first time, an entirely new class of super-reactive chemical compounds has been discovered under atmospheric conditions. Scientists from the University of Copenhagen, in close collaboration with international colleagues, have documented the formation of so-called trioxides – a highly oxidizing chemical compound that likely affects both human health and our global climate.

Hydrogen peroxide is a commonly known chemical compound. Because all peroxides have two oxygen atoms attached to each other, they are highly reactive and often combustible and explosive. They are used for everything from whitening teeth and hair to cleaning wounds and even as rocket fuel. However, peroxides are also found in the air around us.

There has been speculation in recent years about whether trioxides – chemical compounds with three oxygen atoms bonded to each other, and therefore even more reactive than peroxides – are also found in the atmosphere. But until now, this had never been proven unequivocally.

“That’s what we have now achieved,” says Professor Henrik Grum Kjærgaard, from the Department of Chemistry at the University of Copenhagen. Kjærgaard is the lead author of the study, published May 26, 2022, in the prestigious journal, Science.

Henrik Grum Kjaergaard

Professor Henrik Grum Kjærgaard in the laboratory. Credit: University of Copenhagen

He keeps on:

“The types of compounds we discovered are unique in their structure. And, because they are extremely oxidative, they most likely bring about a whole host of effects we haven’t discovered yet.

Hydrotrioxides (ROOOH), as they are called, are an entirely new class of chemical compounds. Researchers from the University of Copenhagen (UCPH), along with colleagues from the Leibniz Institute for Tropospheric Research (TROPOS) and the California Institute of Technology (Caltech), have demonstrated that these compounds form under atmospheric conditions.

Extremely reactive substance in the atmosphere

Reaction: ROO + OH → ROOOH (oxygen atoms in red). When chemical compounds are oxidized in the atmosphere, they often react with OH radicals, usually forming a new radical. When this radical reacts with oxygen, it forms a third radical called peroxide (ROO), which in turn can react with the OH radical, forming hydrotrioxides (ROOOH). Credit: University of Copenhagen

The researchers also showed that hydrotrioxides form during the atmospheric breakdown of several known and widely emitted substances, including isoprene and dimethyl sulfide.

“It is quite significant that we can now show, through direct observation, that these compounds actually form in the atmosphere, that they are surprisingly stable, and that they are formed from almost any chemical compound. All speculation must now be put to rest,” says Jing Chen, a doctoral student in the Department of Chemistry and second author of the study.

just how much

  • Isoprene is one of the most frequently emitted organic compounds into the atmosphere. The study shows that approximately 1% of all the isoprene released is transformed into hydrotrioxides.
  • Researchers estimate ROOOH concentrations in the atmosphere to be around 10 million per cm3. By comparison, OH radicals, one of the most important oxidants in the atmosphere, are found in about 1 million radicals per cm3.

Hydrotrioxides are formed during a reaction between two types of radicals (see illustration below). Researchers expect almost all chemical compounds to form hydrotrioxides in the atmosphere and estimate their lifetimes to range from minutes to hours. This makes them stable enough to react with many other atmospheric compounds.

Likely absorbed in aerosols

The research team also strongly suspects that trioxides can penetrate tiny airborne particles, called aerosols, which pose a health risk and can lead to respiratory and cardiovascular disease.

“They will most likely enter aerosols, where they will form new compounds with new effects. It is easy to imagine new substances forming in the aerosols which are harmful if inhaled. But further investigation is needed to address these potential health effects,” says Henrik Grum Kjærgaard.

Although aerosols also impact climate, they are one of the most difficult things to describe in climate models. And according to the researchers, there is a high probability that hydrotrioxides will have an impact on the number of aerosols produced.

Free-jet flow experiment at TROPOS

Laboratory setup of the free-jet flow experiment at TROPOS in Leipzig, with this direct evidence was provided for the first time that the formation of hydrotrioxides (ROOOH) also takes place under atmospheric conditions from the reaction peroxy radicals (RO2) with hydroxyl radicals (OH). Credit: Tilo Arnhold, TROPOS

“As sunlight is both reflected and absorbed by aerosols, this affects the Earth’s heat balance, that is, the ratio of sunlight that the Earth absorbs and reflects back into the space. When aerosols absorb substances, they expand and contribute to cloud formation, which also affects the Earth’s climate,” says co-author and PhD student, Eva R. Kjærgaard.

The effect of the compound needs further investigation

Researchers hope the discovery of hydrotrioxides will help us learn more about the effect of the chemicals we emit.

“Most human activities release chemicals into the atmosphere. Thus, knowledge of the reactions that determine atmospheric chemistry is important if we are to be able to predict how our actions will affect the atmosphere in the future,” says co-author and postdoc, Kristan H. Møller.

Hydrotrioxide experiments at TROPOS

Until now there was only speculation about hydrotrioxides (ROOOH), that these organic compounds with the unusual group OOOH would exist. In laboratory experiments at TROPOS in Leipzig, their formation during the oxidation of important hydrocarbons, such as isoprene and alpha-pinene, could now be clearly demonstrated. Credit: Tilo Arnhold, TROPOS

However, neither he nor Henrik Grum Kjærgaard are worried about the new discovery:

“These compounds have always existed – we just didn’t know about them. But the fact that we now have evidence that the compounds form and live for a period of time means that it is possible to study their effect in a more targeted way and to react if they turn out to be dangerous,” says Henrik Grum. Kjaergaard.

“The discovery suggests that there could be many other things in the air that we don’t yet know about. Indeed, the air around us is a huge tangle of complex chemical reactions. As researchers, we have to keep an open mind if we want to improve in finding solutions,” Jing Chen concludes.

Reference: “Hydrotrioxide (ROOOH) Formation in the Atmosphere” by Torsten Berndt, Jing Chen, Eva R. Kjærgaard, Kristian H. Møller, Andreas Tilgner, Erik H. Hoffmann, Hartmut Herrmann, John D. Crounse, Paul O Wennberg and Henrik G. Kjaergaard, May 26, 2022, Science.
DOI: 10.1126/science.abn6012

About the study

  • While the theories behind the new research findings were developed in Copenhagen, the experiments were conducted using mass spectrometry, partly at the Leibniz Institute for Tropospheric Research (TROPOS) in Germany, and partly in part at the California Institute of Technology (Caltech) in the United States. .
  • Although higher concentrations must be used in many experiments, these experiments are performed in an environment almost identical to the atmosphere, which makes the results very reliable and comparable to the atmosphere. The measurement of hydrotrioxides has been made possible through the use of incredibly sensitive measuring instruments.
  • The study was conducted by: Torsten Berndt, Andreas Tilgner, Erik H. Hoffmann and Hartmut Hermann of the Leibniz Institute for Tropospheric Research (TROPOS); Jing Chen, Eva R. Kjærgaard, Kristian H. Møller and Henrik Grum Kjærgaard from the Department of Chemistry, University of Copenhagen; and John D. Crounse and Paul O. Wennberg at Caltech.

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