Direct Formation of Sulfuric Acid in the Atmosphere Without Considering SO2


In the atmosphere, gaseous sulfuric acid can form particles that influence the physical properties of clouds.

In the atmosphere, gaseous sulfuric acid can form particles that influence the physical properties of clouds. Thus, the formation of sulfuric acid in the gas phase directly affects the radiative forcing and Earth’s climate. In addition to the known formation from sulfur dioxide, researchers at the Leibniz Institute for Tropospheric Research (TROPOS) have now been able to demonstrate through experiments that there is another formation pathway that has been speculated about for decades. Sulfuric acid in the atmosphere can also be formed directly by the oxidation of organic sulfur compounds. This new production pathway can be responsible for up to half of the gaseous sulfuric acid formation over the oceans and is thus of high importance for climate projections - especially over the oceans of the Southern Hemisphere, as the researchers write in the journal Nature Communications.

Organic sulfur compounds are mainly emitted from biogenic sources and contribute significantly to Earth's sulfur cycle. The sulfur cycle is important for Earth's climate because the oxidation products of organic sulfur compounds, such as sulfuric acid (H2SO4) and methanesulfonic acid (MSA, CH3SO3H), initiate new particle formation. Sulfate particles scatter incoming solar radiation efficiently and affect the formation of cloud condensation nuclei (CCN). Number and size of CCN significantly change the microphysical and radiative properties as well as the lifetime of clouds. Hence, knowledge of how H2SO4 is formed in the atmosphere is mandatory to understand fundamental processes in the climate system.

On global scale, with an annual emission rate of about 30 million tons of sulfur, the most important organic sulfur compound is dimethyl sulfide (DMS, CH3SCH3) followed by methyl mercaptan (MeSH, CH3SH) and, to a lesser extent, dimethyl disulfide (DMDS, CH3SSCH3). Numerous laboratory experiments and model simulations were performed at TROPOS to investigate the atmospheric oxidation pathways of these compounds. "To our knowledge, there is so far no experimental evidence for the direct formation of H2SO4 in the gas phase, except via SO2 oxidation by OH radicals or Criegee intermediates, although this has long been speculated about in the literature and even such pathways are implemented in models," explains Dr. Torsten Berndt of TROPOS.

Read more at Leibniz Institute for Tropospheric Research (TROPOS)

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