Elemental sulfur aerosol-forming mechanism

Abstract

Elemental sulfur aerosols are ubiquitous in the atmospheres of Venus, ancient Earth, and Mars. There is now an evolving body of evidence suggesting that these aerosols have also played a role in the evolution of early life on Earth. However, the exact details of their formation mechanism remain an open question. The present theoretical calculations suggest a chemical mechanism that takes advantage of the interaction between sulfur oxides, SO n (n = 1, 2, 3) and hydrogen sulfide (nH 2 S), resulting in the efficient formation of a S n+1 particle. Interestingly, the SO n + nH 2 S → S n+1 + nH 2 O reactions occur via low-energy pathways under water or sulfuric acid catalysis. Once the S n+1 particles are formed, they may further nucleate to form larger polysulfur aerosols, thus providing a chemical framework for understanding the formation mechanism of S 0 aerosols in different environments. , SO 4 2− ) provide energy for different types of sulfur metabolisms in different environments. The sulfur cycle in the Archean atmosphere is also believed to have played a role in the early evolution of life on Earth (2-7). The emerging photochemical picture suggests that reduced elemental sulfur (S 0 ) and sulfate (SO 4 ) are the dominant sulfur species in the Archean (2-9). However, the latest isotope signatures of microscopic sulfides in marine sulfate deposits indicate that the ultimate source for this metabolic sulfur cycling was atmospherically derived S 0 (10). One of the most important sources of sulfur into the atmosphere is from volcanoes, and the most abundant sulfur gases are SO 2 and H 2 S. The photochemistry of these gases in the atmosphere yields elemental sulfur, sulfur particles, sulfuric acid, and oceanic sulfate. Scheme 1 illustrates the chemical processes suggested to be important in the photochemical oxidation of volcanic sulfur species in the early atmosphere of Earth.The S 0 aerosols are not only involved in the Archean life, but are also implicated in other environments (1,(11)(12)(13)(14)(15)(16)(17)(18)(19). For example, polysulfur (S x = S 2→8 ) aerosols are thought to exist in clouds of Venus and their role as the unknown UV absorber in its lower atmosphere has been discussed in the literature (14). The S 8 particles are also observed in the marine troposphere (15). Finally, the role of S 8 aerosols in explaining the early climate of Mars atmosphere has also been debated (16).Despite being of broad appeal, the formation mechanism of S 0 aerosols remains an open question. The photolysis of SO 2 and SO by UV light with λ < 220 nm has generally been invoked to explain the mass-independent fractionation (MIF) of isotope effects in the sulfur cycle during the Archean (2-9). However, the contribution of other mass-independent chemical reactions to this geologic record remains unclear. To fully understand the sulfur cycle, it is necessary to identify all sources of sulfur compounds and account for all species which can occur in the atmosphere.Results and Discussion SO n (n = 1, 2, 3) + nH 2 S Potential...

Five-S-isotope evidence of two distinct mass-independent sulfur isotope effects and implications for the modern and Archean atmospheres

“…Although the derived values should be quantitatively validated by laboratory experiments, the theoretical studies ( 31 , 32 ) reemphasize the potential role of the symmetry effect in S-MIF, which has been noted since early studies ( 33 ) but was generally not considered in SO 2 photolytic reactions, because of the trace amount of gaseous S 2 , S 3 , and S 4 at ambient temperatures ( 34 ) and the difficulty of experimental investigation ( 31 , 32 ). We argue that the elemental sulfur recombination reaction is strictly a thermal reaction, because elemental sulfur vapor (vapor pressure at 717.8 K: 1 atm) contains sulfur allotropes with two to eight sulfur atoms ( 35 ). In combustion processes, both S and S 2 are important short-lived intermediates ( 36 ).…”

“…Venting and polymerization of gaseous S 2 is ubiquitous on Io (one of Galilean moons), the most volcanically active body in the solar system ( 51 ). At terrestrial volcanic vents and fumaroles, elemental sulfur deposits formed in high-temperature sulfur-bearing gases are commonly found ( 35 , 52 ). Therefore, it is likely that basic reactions responsible for nonphotochemical S-MIF processes in combustion [probably recombination reactions ( 31 , 32 )], which are characterized by large 36 S anomalies, may occur in terrestrial volcanoes throughout Earth’s history.…”

Atmospheric sulfur isotopic anomalies recorded at Mt. Everest across the Anthropocene

“…4) implies that basic reactions responsible for sulfur MIF in combustion as discussed previously may also occur in the Paleoarchean. Although biomass burning is not possible, it is plausible, and cannot be ruled out, that recombination reactions of elemental sulfur (33) may be significant in the active Paleoarchean volcanism because formation of elemental sulfur related to volcanism are commonly observed on both Earth (60) and extraterrestrial bodies such as Io (61). Therefore, the negative 33 S anomalies in Paleoarchean barites produced in the volcano plume could in fact come from a strictly thermal reaction instead of photolytic reactions, which may yield different Δ 33 S-δ 34 S patterns.…”

Venus Atmospheric Thermal Structure and Radiative Balance

“…Many terrestrial microorganisms rely on Fe-S chemistry and some could have evolved independently on Venus when it had liquid water on the surface). Sulfur based aerosols are believed to have played a role in the evolution of early life on Earth (Farquhar et al 2000;Lyons 2008) and Kumar and Francisco (2014) propose a non-photochemical process for elemental sulfur aerosols which may occur on Venus. The question of the absorbers of sunlight in the Venus cloud layer is still very much unsettled.…”