In the Earth’s atmosphere, highly reactive oxygen compounds play an important role, as they contribute, inter alia, to for greenhouse gas reduction and self-cleaning. Now scientists have discovered a completely new class of such oxygen compounds in the atmosphere. These hydroxides 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 particles can form in the atmosphere each year. However, their impact on climate and health is still unclear.
The lower part of the Earth’s atmosphere determines not only our weather and climate, but countless chemical reactions also take place in it. They form reactive oxygen compounds, such as the hydroxyl radical (OH), which bind greenhouse gases and organic molecules and contribute to the self-cleaning of the gaseous envelope. These reactions also involve several hundred million tons of hydrocarbons each year, which are released in natural processes from forests or come from anthropogenic sources. This leads to a wide variety of oxidation processes which until now have only been partially understood.
Three oxygen atoms in a row
Chemists have long suspected that hydroxides could also be formed as an intermediate in the atmosphere when hydrocarbons react with oxygen and hydroxyl radicals and other reactive oxygen compounds. They consist of a hydrocarbon residue to which is attached a group of three consecutive oxygen and one hydrogen atoms (R-OOOH). These hydroxides 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 could also arise under natural conditions.
Torsten Berndt from the Leibniz Institute for Tropospheric Research (TROPOS) in Leipzig and his colleagues have now investigated this. In their research, they used a so-called free-flow tube to study the hydroxide formation in normal ambient air with a highly sensitive mass spectrometer. Chemists were able to prove that hydroxides can be formed under atmospheric conditions. Analyzes have shown that these molecules are formed both by the reaction of 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 made of precursors abundant in the atmosphere,” says senior author Henrik Kjærgaard of the University of Copenhagen. “The molecules we discovered have a unique structure.” Hydroxides are relatively short lived and remain stable for 20 minutes to about two hours before being degraded again in chemical reactions. Quantum chemistry calculations and model simulations have shown that about ten million tons of hydrogen hydroxide are produced in the atmosphere each year from isoprene alone. Given their viability, the concentration of isoprene hydroxides could be ten billion molecules per cubic centimeter of air, Bernd and his colleagues report.
The impact of hydrogen hydroxide on climate and health is still unclear. “Since they are very oxidizing, they likely have a whole host of effects that we now need to investigate,” says Kjærgaard. Scientists suspect, for example, that hydroxides dissolve in aerosols and trigger reactions there. This could influence the climate effect of suspended droplets, for example by changing cloud formation. Another factor is the potential health effects of these highly reactive molecules. “It’s easy to imagine that aerosol reactions create new substances that are harmful when inhaled,” says Kjærgaard. “More 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