Chemists at the Leibniz Institute for Tropospheric Research have provided experimental evidence that sulfurous acid (H2SO3), once formed in the gas phase, is kinetically stable enough to allow its characterization and subsequent reactions.
In the gas phase, if once formed, sulfurous acid shows a certain kinetic stability with an estimated lifetime of at least one second for atmospheric water vapor conditions. Image credit: Berndt et al., doi: 10.1002/anie.202405572.
Sulfurous acid has the formula H2SO3 and the molecular weight of 82.075 g/mol.
Also known as sulfuric (IV) acid and thionic acid, this molecule represents a hard-to-reach acid that has never been observed in aqueous solution.
However, sulfurous acid has been detected in the gas phase in 1988 by the dissociative ionization of diethyl sulfite.
“The only experimental detection of sulfurous acid to date was achieved by Helmut Schwarz’s team at TU Berlin in 1988 using in-situ generation in a mass spectrometer,” said Dr. Torsten Berndt and his colleagues at the Leibniz Institute for Tropospheric Research.
“An extremely short lifetime under vacuum conditions in the range of 10 microseconds and more was estimated.”
“Theoretical calculations suggested the formation of H2SO3 as a possible reaction product of the gas-phase reaction of OH radicals, which are formed in the troposphere primarily from ozone and water molecules in the presence of UV radiation, with dimethyl sulfide (DMS).”
“DMS is mainly produced by biological processes in the sea and is the largest biogenic sulfur source for the atmosphere, producing around 30 million tons annually.”
The study authors experimentally investigated the possible reaction pathway to H2SO3 starting from the DMS.
The formation of H2SO3 in the gas phase was clearly demonstrated in flow reactors under atmospheric conditions.
“Under the experimental conditions, the sulfurous acid remained stable for half a minute regardless of the humidity,” the researchers said.
“Longer residence times could not yet be investigated with the existing experimental setup.”
“Therefore, H2SO3 could also exist sufficiently long enough in the atmosphere and have an influence on the chemical processes.”
“The observed yield was even somewhat greater than theoretically assumed.”
The associated model simulations showed that around 8 million tons of H2SO3 are formed globally every year.
“This pathway produces about 200 times more mass of H2SO3 than the direct formation of sulfuric acid (H2SO4) from dimethyl sulfide in the atmosphere,” said Dr. Andreas Tilgner and Dr. Erik Hoffmann, also from the Leibniz Institute for Tropospheric Research.
“The new results can contribute to a better understanding of the atmospheric sulfur cycle.”
The team’s paper was published in the journal Angewandte Chemie.
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Torsten Berndt et al. 2024. Gas-Phase Formation of Sulfurous Acid (H2SO3) in the Atmosphere. Angewandte Chemie 63 (30): e202405572; doi: 10.1002/anie.202405572
Source : Breaking Science News