Highly reactive oxygen compounds play an important role in the earth’s atmosphere, because they contribute, among other things, to the reduction of greenhouse gases and to self-cleaning. Now researchers have discovered a whole new class of such oxygen compounds in the atmosphere. These hydrotrioxides are characterized by a molecular group of three oxygen atoms and one hydrogen (R-OOOH) and are formed by the reaction of organic compounds with hydroxyl radicals (OH). Millions of tons of these molecules could be formed in the atmosphere every year. However, their effect on climate and health is still unclear.
The lower part of the earth’s atmosphere not only determines our weather and climate, countless chemical reactions also take place in it. They create reactive oxygen compounds such as the hydroxyl radical (OH), which bind greenhouse gases and organic molecules and contribute to the self-cleaning of the gas envelope. Several 100 million tons of hydrocarbons are also involved in these reactions each year, which are released by natural processes from forests or come from anthropogenic sources. This leads to a wide variety of oxidation processes, which have so far only been partially understood.
Three oxygen atoms in a row
Chemists have long suspected that hydrotrioxides could also be formed as an intermediate product in the atmosphere when hydrocarbons react with oxygen radicals and hydroxyl radicals and other reactive oxygen compounds. These consist of a hydrocarbon residue to which is attached a group of three consecutive oxygen atoms and one hydrogen (R-OOOH). These hydrotrioxides are even more reactive than peroxides and are produced and used in chemistry as oxidizing agents for alkenes. However, this requires organic solvents and strong cooling. However, it was previously unclear whether these molecules can also form under natural conditions.
Torsten Berndt from the Leibniz Institute for Tropospheric Research (TROPOS) in Leipzig and his colleagues have now investigated this. For their study, they used a so-called free-jet flow tube to investigate the formation of hydrotrioxide in normal ambient air using a highly sensitive mass spectrometer. The chemists were actually able to prove that hydrotrioxides can form under atmospheric conditions. The analyzes showed that these molecules are formed both in the reaction of the hydrocarbon isoprene with hydroxyl radicals and with other organic compounds such as trimethylamide.
Impact on climate and health still unknown
“It is really exciting to show the existence of a new general class of compounds formed from atmospherically abundant precursors,” says senior author Henrik Kjærgaard from the University of Copenhagen. “The molecules we discovered are unique in their structure.” The hydrotrioxides are relatively short-lived and remain stable for between 20 minutes and around two hours before being broken down again by chemical reactions. Quantum chemical calculations and model simulations showed that around ten million tons of hydrotrioxide are formed in the atmosphere every year from isoprene alone. If one takes into account their lifetime, the concentration of the hydrotrioxides formed from isoprene could be ten billion molecules per cubic centimeter of air, as Bernd and his colleagues report.
The effects of hydrotrioxide on climate and health are still unclear. “Because they are extremely oxidizing, they probably have a whole range of effects that we now have to investigate,” says Kjærgaard. Among other things, the scientists suspect that the hydrotrioxides dissolve in aerosols and trigger reactions there. This could influence the climate effect of the suspended droplets, for example through a change in cloud formation. Another factor is the potential health effects of these highly reactive molecules. “It is easy to imagine that the reactions in the aerosols create new substances that are harmful when inhaled,” says Kjærgaard. “Further research is also needed to assess these potential health consequences.”
Source: Torsten Berndt (Leibniz Institute for Tropospheric Research (TROPOS), Leipzig) et al., Science, doi: 10.1126/science.abn6012