Sulfurous acid H2SO3 - and it does exist

Leipzig. Once again, the atmosphere amazes us with its diverse chemical processes. For the first time, researchers at the Leibniz Institute for Tropospheric Research (TROPOS) in Leipzig have demonstrated the existence of sulfurous acid (H₂SO₃) under atmospheric conditions in the gas phase. The results were published in the journal Angewandte Chemie.

In contrast to the well-known sulfuric acid (H₂SO₄), sulfurous acid (H₂SO₃) is considered as compound that is difficult or impossible to access (produce). Textbooks suggest the possible formation of H₂SO₃ in aqueous sulfur dioxide (SO₂) solution, although its existence in isolated form is considered impossible. However, despite great efforts using various spectroscopic methods, the experimental detection of H₂SO₃ in aqueous SO₂ solution has so far been unsuccessful. Only the corresponding bases bisulphite HSO₃^- and sulphite SO₃^2- were detectable.

The only experimental detection of H₂SO₃ to date was achieved by Helmut Schwarz's team at TU Berlin in 1988 using in-situ generation in a mass spectrometer. 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 tonnes annually.

The possible reaction pathway to H₂SO₃ starting from the DMS was investigated experimentally in the laboratory at TROPOS in Leipzig. The formation of H₂SO₃ in the gas phase was clearly demonstrated in flow reactors for atmospheric conditions. Under the experimental conditions, the sulfurous acid remained stable for half a minute regardless of the humidity. 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. "It was very impressive to see the clear H₂SO₃ signals in the spectrometer for a compound that had been assumed to be possibly "non-existent"," says Dr Torsten Berndt from TROPOS, who came up with the idea and carried out the experiments.

The new reaction pathway was then implemented in a global chemistry-climate model. 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. The new results can contribute to a better understanding of the atmospheric sulfur cycle," add the scientists responsible for global modelling, Dr Andreas Tilgner and Dr Erik Hoffmann.

As with many research findings, many new interesting questions arise here too: Once formed in the gas phase, sulfurous acid appears to have at least a certain stability. However, the lifetime with regard to the reaction with trace gases in the atmosphere is still completely unclear. The reaction with water vapour has also not yet been satisfactorily clarified. "Much more research is needed in further optimised experiments in order to sufficiently clarify the significance of H₂SO₃," adds Dr Torsten Berndt.

The detection of H₂SO₃ is another example of the discovery of new reaction pathways and the experimental proof of compounds that were previously only theoretically proposed or difficult to access. This is made possible by the interplay of optimised reaction control combined with highly sensitive detection methods. For example, a mass spectrometer with a detection limit of 10⁴ molecules of a product per cubic centimetre at atmospheric pressure was used in this study, i.e. it is possible to detect a specific molecule in a mixture of 10¹⁵ molecules (1 quadrillion molecules). Ever-improving methods will allow an even deeper insight into reaction processes and thus contribute to an even better understanding of atmospheric chemistry and all other areas of chemistry.