Dimethyl sulfide is the sulfur compound most commonly emitted naturally into the atmosphere. It has a strong influence on the climate through the formation of natural particles and clouds. The new findings of the international research team show that important steps in the Earth's sulphur cycle have not yet been correctly understood, as they call into question the previously assumed pathways of formation for sulphur dioxide, methanesulphonic acid and carbonyl sulphide starting from dimethyl sulphide.
In the laboratory studies, a free-jet flow system was used at TROPOS in Leipzig, which allows the investigation of oxidation reactions under atmospheric conditions without disturbing wall effects. The products of the reactions were measured with state-of-the-art mass spectrometers using different ionization methods. The investigations on the degradation process of dimethyl sulfide showed that this predominantly proceeds by a two-step radical isomerization process, in which hydroperoxymethyl thioformate (HOOCH2SCHO, HPMTF) is formed as a stable intermediate product as well as hydroxyl radicals. There has been theoretical speculation about this reaction pathway for four years now, but the German-Austrian-Finnish team has only now been able to prove it. “The interaction of optimal reaction conditions and highly sensitive detection methods such as the PTR3-TOF developed at the University of Innsbruck allows us to look almost directly into a reaction system,” reports Torsten Berndt from TROPOS, who led the investigations. The new reaction pathway is significantly faster than the traditional bimolecular radical reactions with nitrogen monoxide, hydroperoxy and peroxy radicals. “Further investigations on the degradation of the intermediate HOOCH2SCHO will hopefully give us clarity about the formation channels, especially of sulfur dioxide and carbonyl sulphide”, Berndt adds on further investigations.
Greatest uncertainty in climate models
Dimethyl sulfide is a sulfur-containing organic gas that occurs almost everywhere: the degradation product of bacteria, for example, is part of human bad breath. On the other hand, the large quantities of dimethyl sulfide that are produced and outgassed during decomposition processes in the ocean are important for the climate: Estimated 10 to 35 million metric tons from the seawater are released into the atmosphere every year. Dimethyl sulfide is thus the largest natural source of sulfur for the atmosphere. As a result of its reaction with hydroxyl radicals, sulfuric acid is formed starting from sulfur dioxide and methanesulfonic acid, which play a major role in the formation of natural particles and clouds over the oceans. Carbonyl sulfide is also important, as its low reactivity in the atmosphere allows it to be transported into the stratosphere, where it contributes to the formation of sulfuric acid aerosols and thus to the cooling of the Earth's atmosphere.
“The new findings about the degradation pathways of dimethyl sulfide help to improve the knowledge about the formation of natural aerosols”, says Armin Hansel from the Department of Ion Physics and Applied Physics at the University of Innsbruck. „The contribution of aerosols and the resulting clouds is still the greatest uncertainty in climate models.” In contrast to greenhouse gases such as carbon dioxide, cloud formation processes are much more complex and difficult to model.
The studies were supported by the European Research Council, the European Union and the Finnish Academy, among others.