For the first time, an entirely new class of super-reactive chemical compounds has been found under atmospheric conditions. Scientists at the University of Copenhagen, in close collaboration with international colleagues, have documented the formation of so-called trioxides – an extremely oxidizing chemical compound likely to affect both human health and the global climate.
Hydrogen peroxide is a commonly known chemical compound. As all peroxides have two oxygen atoms bonded together, 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.
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There has been speculation in recent years about whether trioxides – chemical compounds with three oxygen atoms bonded together and therefore even more reactive than peroxides – are also found in the atmosphere. But until now, this has never been unequivocally proven.
“This is what we got now,” says Professor Henrik Grum Kjærgaard from the Department of Chemistry at the University of Copenhagen. Kjærgaard is the senior author of the study, published May 26, 2022, in the prestigious journal Science.
He continues, “The types of compounds we’ve discovered are unique in their structure. And, because they are extremely oxidizing, they probably bring a series of effects that we have yet to discover.”
Hydrotrioxides (ROOOH), as they are known, are a completely new class of chemical compounds. Researchers at the University of Copenhagen (UCPH), along with colleagues at the Leibniz Institute for Tropospheric Research (TROPOS) and the California Institute of Technology (Caltech), have demonstrated that these compounds are formed 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 usually 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 are formed during the atmospheric decomposition of several known and widely emitted substances, including isoprene and dimethylsulfide.
“It is quite significant that we can now show, through direct observation, that these compounds do form in the atmosphere, that they are surprisingly stable, and that they are formed from almost all chemical compounds. All speculation must now be put to one side,” says Jing Chen, a doctoral student in the Department of Chemistry and second author of the study.
Hydrotrioxides are formed in a reaction between two types of radicals (see illustration below). The researchers expect nearly all chemical compounds to form hydrotrioxides in the atmosphere and estimate that their lifespan ranges from minutes to hours. This makes them stable enough to react with many other atmospheric compounds.
Presumably absorbed in aerosols
The research team also has the trioxides under strong suspicion of being able to penetrate tiny airborne particles known as aerosols, which pose a health risk and can lead to respiratory and cardiovascular disease.
“They will likely enter aerosols, where they will form new compounds with new effects. It is easy to imagine that new substances are formed in aerosols that are harmful if inhaled. But more research is needed to address these potential health effects,” says Henrik Grum Kjærgaard.
While aerosols also have an impact on 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 affect how many aerosols are produced.
“As sunlight is reflected and absorbed by aerosols, this affects the Earth’s thermal balance – that is, the proportion of sunlight that Earth absorbs and sends back into space. When aerosols absorb substances, they grow and contribute to cloud formation, which also affects Earth’s climate,” says the co-author and PhD. student, Eva R. Kjærgaard.
The effect of the compound needs to be further studied
The researchers hope the discovery of hydrotrioxides will help us learn more about the effect of the chemicals we emit.
“Most human activity leads to the emission of chemicals into the atmosphere. Therefore, knowledge of the reactions that determine atmospheric chemistry is important if we want to predict how our actions will affect the atmosphere in the future,” says co-author and postdoc Kristan H. Møller.
However, neither he nor Henrik Grum Kjærgaard is concerned 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 are formed and live for a certain period of time means that it is possible to study their effect in a more targeted way and answer if they become dangerous,” says Henrik Grum Kjærgaard.
“The discovery suggests that there may be many other things in the air that we don’t yet know about. In fact, the air around us is a huge tangle of complex chemical reactions. As researchers, we need to keep an open mind if we want to improve the search for solutions,” concludes Jing Chen.
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