Elemental sulfur aerosol-forming mechanism Manoj Kumara and Joseph S. Franciscoa,1 aDepartment of Chemistry, University of Nebraska-Lincoln, Lincoln, NE 68588 Contributed by Joseph S. Francisco, December 21, 2016 (sent for review November 13, 2016; reviewed by James Lyons and Hua-Gen Yu) Elemental sulfur aerosols are ubiquitous in the atmospheres of Venus, reaction mechanism that may possibly convert the SOn + nH2S ancient Earth, and Mars. There is now an evolving body of evidence (n = 1, 2, 3) chemistries into the S8 aerosol in the gas phase suggesting that these aerosols have also played a role in the evolution (Scheme S1). It is the thermodynamics of these processes, and of early life on Earth. However, the exact details of their formation their catalysis by water and sulfuric acid, that we investigate here. mechanism remain an open question. The present theoretical calcula- This mechanism may not only help in better understanding the tions suggest a chemical mechanism that takes advantage of the in- role of sulfur cycle involving SOn,S8, and H2S as the potential S n = teraction between sulfur oxides, SOn ( 1, 2, 3) and hydrogen sulfide MIF carrier from the atmosphere to the ocean surface, but may 0 (nH2S), resulting in the efficient formation of a Sn+1 particle. Interest- also provide deeper insight into the formation mechanism of S + → + ingly, the SOn nH2S Sn+1 nH2O reactions occur via low-energy aerosols in various other environments. pathways under water or sulfuric acid catalysis. Once the S + particles n 1 We first explored the uncatalyzed gas-phase reactions of SOn with are formed, they may further nucleate to form larger polysulfur aero- nH2S using quantum-chemical calculations at the coupled cluster sols, thus providing a chemical framework for understanding the for- single and double substitution method with a perturbative treatment 0 mation mechanism of S aerosols in different environments. of triple excitations [CCSD(T)]/aug-cc-pVTZ//M06-2X/aug-cc-pVTZ level of theory. We considered both singlet and triplet states for SO. sulfur aerosols | catalysis | planetary environment | nonphotochemical | Although the triplet ground state of SO is more stable than its singlet hydrogen sulfide 3 state, the calculations suggest that the SO + H2Sreactionleadsto the formation of HS and HOS radicals, and is endothermic by 1 ulfur chemistry is a ubiquitous component in the atmo- 33.5 kcal/mol (Fig. S1). By contrast, the SO + H2Sreactionishighly Sspheres of Venus, early Earth, and Mars (1). The different exothermic (Fig. S2). The relative energies of the computed transi- − − 0 2− 2− 2− 1 forms of sulfur (e.g., S2 ,S ,S,S2O3 ,SO3 ,SO4 ) provide tion-state structures and minima for the uncatalyzed SO + H2Sre- energy for different types of sulfur metabolisms in different en- action are shown in Fig. S2. The possible source of 1SO is either the vironments. The sulfur cycle in the Archean atmosphere is also photolysis of SO2 at λ < 220 nm or the partial oxidation of H2S. believed to have played a role in the early evolution of life on There have also been reports that 1SO could be ejected directly from – 1 Earth (2 7). The emerging photochemical picture suggests that the volcanic vent (20). However, the SO + H2Sreactionwouldface 0 1 reduced elemental sulfur (S ) and sulfate (SO4) are the dominant competition from the SO + O2 → SO2 + Oinatmosphere,sug- – 1 sulfur species in the Archean (2 9). However, the latest isotope gesting that the SO + H2Sismorelikelytohappenlocallywherethe signatures of microscopic sulfides in marine sulfate deposits indicate concentration of sulfur gases is expected to be high. 1 that the ultimate source for this metabolic sulfur cycling was at- The SO + H2S reaction results in the stepwise formation of 0 mospherically derived S (10). One of the most important sources of H2S2O, which involves a barrier of 23.2 kcal/mol and has an sulfur into the atmosphere is from volcanoes, and the most abun- exothermicity of 30.1 kcal/mol. The comparative analysis of the 1 dant sulfur gases are SO2 and H2S. The photochemistry of these potential energy surfaces for the SO and SO2 (Fig. S3)reactions gases in the atmosphere yields elemental sulfur, sulfur particles, reveals that the 1SO reaction is relatively more favorable. Al- sulfuric acid, and oceanic sulfate. Scheme 1 illustrates the chemical though the uncatalyzed SO2 + H2S reaction has been previously processes suggested to be important in the photochemical oxidation calculated (13), we reexamined the reaction here in greater detail of volcanic sulfur species in the early atmosphere of Earth. at the same level of theory to facilitate the comparison between The S0 aerosols are not only involved in the Archean life, but – are also implicated in other environments (1, 11 19). For example, Significance polysulfur (Sx = S2→8) aerosols are thought to exist in clouds of Venus and their role as the unknown UV absorber in its lower The elemental sulfur aerosols are an important constituent in the atmosphere has been discussed in the literature (14). The S8 atmospheres of Earth, Mars, and Venus. There is now evidence particles are also observed in the marine troposphere (15). Finally, suggesting that these aerosols have also played a role in the the role of S8 aerosols in explaining the early climate of Mars evolution of early life on Earth. Traditionally, the photolysis of atmosphere has also been debated (16). 0 sulfur gases by UV light is thought to be the main mechanism for Despite being of broad appeal, the formation mechanism of S the formation of sulfur particles in these atmospheres. But, in aerosols remains an open question. The photolysis of SO2 and SO the theoretical calculations reported here, we propose a non- λ < by UV light with 220 nm has generally been invoked to explain photochemical mechanism for the formation of elemental sulfur the mass-independent fractionation (MIF) of isotope effects in the aerosols that takes advantage of the interaction between sulfur – sulfur cycle during the Archean (2 9). However, the contribution of oxides and hydrogen sulfide under water or sulfuric acid other mass-independent chemical reactions to this geologic record catalysis. These results provide a chemical framework for remains unclear. To fully understand the sulfur cycle, it is necessary understanding the formation mechanism of S0 aerosols in to identify all sources of sulfur compounds and account for all planetary atmospheres. species which can occur in the atmosphere. Author contributions: M.K. and J.S.F. designed research; M.K. performed research; M.K. Results and Discussion analyzed data; and M.K. wrote the paper. SOn (n = 1, 2, 3) + nH2S Potential Energy Surface. As can be seen Reviewers: J.L., Arizona State University; and H.-G.Y., Brookhaven National Laboratory. from Scheme 1, which summarizes the current state of sulfur The authors declare no conflict of interest. chemistry in the atmosphere, there is a gap in our understanding 1To whom correspondence should be addressed. Email: [email protected]. of the connection between sulfur oxide chemistry and sulfur This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10. aerosol formation. Herein, we describe a nonphotochemical 1073/pnas.1620870114/-/DCSupplemental. 864–869 | PNAS | January 31, 2017 | vol. 114 | no. 5 www.pnas.org/cgi/doi/10.1073/pnas.1620870114 Downloaded by guest on October 2, 2021 shown to catalyze hydrogen atom transfer (HAT)-based addition reactions (21). Sulfuric acid (H2SO4) is an important constituent in the Venus atmosphere (25) and has been predicted to be one of the most efficient catalysts available for the HAT-based reactions (22–24). Building upon these recent developments, we next ex- amined the SOn + nH2S reaction, which also involves an HAT reaction, in the presence of H2OandH2SO4. 1 The formation of H2S2Ofromthe SO + H2Sreactionandits subsequent dehydration to S2 becomes facile under H2OorH2SO4 catalysis (Fig. 1). H2SO4 turns out to be a better catalyst than water because of its ability to stabilize reactants and products by forming sterically more favorable double hydrogen-bonding interactions. The alternate decomposition pathway for H2S2O, which leads to S2O, is also significantly impacted under catalysis. However, the barriers for the S2O-forming decomposition under catalysis are relatively higher than the dehydration one, suggesting that the probability of this de- composition pathway in water-rich surfaces or acidic environments 1 may be quite low. H2S2Ointhe SO + H2Sreactionisformedwith an excess energy of 30.1 kcal/mol, which may play a role in making a S2O-based S3 channel accessible under catalytic conditions. On the other hand, we only examined the SO2 + 2H2S → S3 + 2H2O-forming pathway in the presence of a single H2OandH2SO4 molecule (Fig. 1). This is because the S4-particle-forming pathways are mediated by very high-lying transition states (Fig. S3)andarenot expected to become accessible even under H2OorH2SO4 catalysis. The overall SO2 + 2H2S → S3 + 2H2O reaction is calculated to be 5.7 kcal/mol exothermic. The uncatalyzed H2S2O2 formation involves an effective barrier of 30.5 kcal/mol. Under H2OandH2SO4 catalysis, the reaction barrier is appreciably lowered to 14.6 and 14.7 kcal/mol, respectively. The subsequent dehydration of H2S2O2, which produces S2O, has a barrier of 28.4 kcal/mol and an exothermicity of 2.5 kcal/mol Scheme 1. Sulfur photochemistry in an anoxic early atmosphere. Sulfur is that are significantly impacted under catalysis. H2SO4 produces emitted to the atmosphere from volcanoes as sulfur dioxide and hydrogen more catalytic effect than water in this case; the dehydration barriers sulfide, and is removed by rainout of soluble gases and by formation and for the H2SO4-andH2O-catalyzed reactions are lowered to 12.1 and deposition of sulfate and elemental sulfur particles. 15.8 kcal/mol, respectively.