Elemental -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 are ubiquitous in the atmospheres of , reaction mechanism that may possibly convert the SOn + nH2S ancient Earth, and . There is now an evolving body of evidence (n = 1, 2, 3) chemistries into the S8 aerosol in the phase suggesting that these aerosols have also played a role in the evolution (Scheme S1). It is the of these processes, and of early on Earth. However, the exact details of their formation their by and sulfuric , 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 involving SOn,S8, and H2S as the potential S n = teraction between sulfur , SOn ( 1, 2, 3) and 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 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 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 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 (SO4) are the dominant competition from the SO + O2 → SO2 + Oinatmosphere,sug- – 1 sulfur species in the Archean (2 9). However, the latest gesting that the SO + H2Sismorelikelytohappenlocallywherethe signatures of microscopic in marine sulfate deposits indicate of sulfur 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 . 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 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 .

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 . 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 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 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 pathway for H2S2O, which 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 (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 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. The reaction of S2OwithH2Sandthe eventual decomposition of H2S3OintoS3 + 2H2O are significantly 1 1 influenced under catalysis. Although the S3 formation via the SO2 + the SO and SO2 reactions. The effective barrier for the SO re- 1 + action is 7.3 kcal/mol lower than that for the SO reaction. The 2H2S reaction is 11.4 kcal/mol less exothermic than that in the SO 2 2H S reaction, it is still expected to be the favored pathway because exothermicity of the H S O formation is dramatically higher than 2 2 2 it bypasses the high-barrier dehydrogenation step, which significantly that for the H S O formation via the SO + H S reaction. The 2 2 2 2 2 lowers the energetics of the overall reaction. The effect of catalysis higher reactivity of 1SO may provide an alternate mechanistic on the S -forming pathway from the SO + 3H S reaction is also explanation as to why SO is rarely observed in the troposphere. 4 3 2 appreciable; all of the transition states are submerged below the Once H2S2O is formed, it can either dehydrate to S2 or de- separated reactants and the overall reaction occurs in a barrierless EARTH, ATMOSPHERIC,

hydrogenate to S2O that can subsequently react with H2S, resulting in AND PLANETARY SCIENCES 1 manner (Fig. 1). Moreover, the complexed S4 particle, which is hy- the formation of S3. Thus, the overall SO + H2Sreactionleadstothe + drogen-bonded with catalyst, H2OorH2SO4, is at least 18.0 kcal/mol formation of a S2 or S3 particle. Analogously, the SO2 2H2Sand ∼ + more stable than SO3 and 7.0 kcal/mol more stable than free S4 SO3 3H2S(Fig. S4) chemistries produce S3 or S4 and S4 or S5 and catalyst. Among all of the sulfur gases considered, the sulfur particles, respectively. The SO3 reaction is predicted to be more fa- particle formation via the SO3 reactions is the most favorable. Fig. 2 vorable than the SO2 one. These mechanistic outcomes point toward summarizes the optimum path for the Sn+1 formation from the SOn + a more generalized interaction between a sulfur oxide, SOn and H2S, nH2S reaction under H2SO4 catalysis. This qualitative profile reveals which can be qualitatively summarized in the form of a reaction, SO + n an interesting reactivity pattern; for a given interaction between SOn n/(n+1)H S → S + + S + + n/(n+1)H O. The formation of S + 2 n 1 n 2 2 n 1 and nH2S, there are n high-energy dehydrogenation channels that particle is predicted to be energetically more viable because it only may or may not open up depending upon reaction conditions. involves low-barrier dehydration whereas the Sn+2 pathway must also go through high-barrier dehydrogenation in addition to low-barrier Redefining the Formation Mechanism of S0 Aerosols. The S0 aerosols dehydration. Thus, it is reasonable to suggest that the SOn + nH2S → in the anoxic atmosphere of Archean are suggested to be produced + n = Sn+1 nH2O is the most efficient reaction except for 1, where the by the UV photolysis of SO2 at λ< 220 nm (2–9). However, the Sn+2 formation may also become feasible under certain conditions. present results suggest a nonphotochemical mechanism for the 0 formation of these S aerosols, in which the Sn+1 particle is formed Reactivity Under Catalysis. Although the formation of a Sn+1 particle from the interaction of H O- or H SO -bound H SwithSO (Fig. + 2 2 4 2 n via the SOn nH2S reaction is more favorable, it still involves an 3). This indicates that once S2,S3,orS4 is formed, it could initiate a appreciable thermal barrier that seems insurmountable under at- self-reaction that would build larger S0 particles. The mechanistic mospheric conditions. However, recent studies (21–24) suggest beauty of this aerosol-forming chemical process is that it does not that there are certain species in the atmospheres of Earth and require the conventional gas-phase three-body sulfur atom re- Venus that may be able to catalyze these chemistries to such an combination, S + S, required for forming S2 (5, 26, 27) or the UV extent that these processes become accessible. For example, H2O photon-induced reaction between S and SH to form S2 and H (28). is the most dominant species in the troposphere and has been It should be noted that several S2–4 allotropes generally have very

Kumar and Francisco PNAS | January 31, 2017 | vol. 114 | no. 5 | 865 Downloaded by guest on October 2, 2021 Fig. 1. Calculated reaction profiles for the gas-phase reactions of SOn (n = 1, 2, 3) with H2S, (black), H2S–H2O (magenta) and H2S–H2SO4 (green), respectively. − Relative energies (kcal mol 1) of minima and transition-state structures are calculated at the CCSD(T)/aug-cc-pVTZ//M06-2X/aug-cc-pVTZ level of theory. Note that the SO reactions have been calculated at the uCCSD(T)/aug-cc-pVTZ//uM06-2X/aug-cc-pVTZ level.

low vapor pressures in planetary atmospheres (7) However, the significant fraction of H2SO4 may exist as a van der Waals complex consideration of the kinetics of condensation may make these gas- with H2S, and thus may catalyze the SO3 + H2S chemistry by invoking •• phase sulfur self-reactions important under certain conditions. a bimolecular reaction between SO3 and H2SO4 H2S. •• This mechanism has broad implications. For example, in Venus The CCSD(T) calculated binding energy of H2SO4 H2S complex •• clouds, the exact source of polysulfur particle, which absorbs UV is 5.3 kcal/mol. The equilibrium constants for the H2SO4 H2S light, is unknown (14). The present calculations suggest that the Sn+1 complex calculated at various temperatures are collected in Table 1. •• formation is the most favorable under H2SO4 catalysis. The Sn+1 The SO3 + H2SO4 H2Sreactionoccursinalow-energymannerand particle may then nucleate into the polysulfur particle. Sulfur species is strongly exothermic. Note that the SO2 mixingratiointhemiddle are abundantly available in the Venus atmosphere. The estimated atmosphere is the highest among the sulfur gases (17, 18) implying ± ± •• concentration of SO2 lies in the range of 180 50 ppm (25) and 130 that the SO2 + H2SO4 H2Sreactionwouldalsomakeanimpor- 35 ppm (29), whereas the H2S concentration has been predicted tant contribution toward the overall elemental sulfur production via ± ∼ to be 80 40 ppm (30). H2SO4 and H2O are present in 5- and SOn + nH2S reaction. These conclusions are consistent with the fact ∼30-ppm amounts, respectively (31), which lends support to such that the sulfur cycles in the middle atmosphere are fast. Although a mechanism. In a recently studied kinetic model of Venus chemistry chlorosulfane chemistry has been previously proposed to explain the (1), the SO3 mixing ratio in the 40–45-km altitude inversely correlates elemental sulfur aerosols (30), the present results suggest a direct with the sulfur particle mixing ratio. The reaction of SO3 with 3H2S connection between the chemistries of sulfur gases and elemental to form S4 may explain this inverse correlation. The SO3 + H2Sre- sulfur particles. action involves a relatively smaller barrier than that of SO2 + H2S. In In a recent Venus model by Zhang et al. (18), a strong anti- 35–45-km altitude, the H2SO4 mixing ratio is ∼5 ppm, implying that a correlation between the sulfur gases (SO3 and SO) and the elemental

866 | www.pnas.org/cgi/doi/10.1073/pnas.1620870114 Kumar and Francisco Downloaded by guest on October 2, 2021 Table 1. Calculated equilibrium constants for the complexes of hydrogen sulfide (H2S) with water (H2O), thiosulfurous acid (H2S2O2), sulfuric acid (H2SO4), and carbonic acid (H2CO3) at various temperatures 3• −1 , Keq,cm molecule

•• •• •• •• Temperature, K H2O H2S(ΔE = −1.36) H2S2O2 H2S(ΔE = −5.32) H2CO3 H2S(ΔE = −5.07) H2SO4 H2S(ΔE = −6.69)

200 4.35 × 10−22 1.73 × 10−20 6.21 × 10−20 2.79 × 10−18 210 3.71 × 10−22 8.99 × 10−21 3.42 × 10−20 1.26 × 10−18 − − − − 220 3.23 × 10 22 4.97 × 10 21 1.99 × 10 20 6.17 × 10 19 − − − − 230 2.85 × 10 22 2.90 × 10 21 1.23 × 10 20 3.22 × 10 19 240 2.56 × 10−22 1.78 × 10−21 7.87 × 10−21 1.78 × 10−19 − − − − 250 2.32 × 10 22 1.14 × 10 21 5.25 × 10 21 1.03 × 10 19 − − − − 260 2.13 × 10 22 7.54 × 10 22 3.63 × 10 21 6.28 × 10 20 270 1.98 × 10−22 5.17 × 10−22 2.58 × 10−21 3.97 × 10−20 280 1.85 × 10−22 3.65 × 10−22 1.89 × 10−21 2.60 × 10−20 − − − − 290 1.74 × 10 22 2.64 × 10 22 1.42 × 10 21 1.76 × 10 20 − − − − 298.15 1.67 × 10 22 2.07 × 10 22 1.14 × 10 21 1.31 × 10 20 300 1.65 × 10−22 1.96 × 10−22 1.09 × 10−21 1.23 × 10−20

CCSD(T)/aug-cc-pVTZ//M06-2X/aug-cc-pVTZ calculated binding energies of the H2S complexes are given in parentheses.

sulfur aerosol profiles below 65 km has also been observed. The calculations on the reactions of SO2 and SO3 with H2X(X= O, and •• thermal reactions of SO3 and SO with H2SO4 H2S clearly explain S) suggest that the bimolecular reactions of sulfur gases with H2S this anticorrelation. These thermal reactions not only provide useful involve smaller barriers than the analogous reactions involving H2O mechanistic insights into an important sink of sulfur gases below (Fig. 4). This indicates that the SO3 + H2S reaction under H2SO4 90 km in the Venus atmosphere, but may also help in understanding catalysis may occur even at higher altitude, leading to the formation the mixing profiles of SO and SO2 at higher altitudes (1, 17, 18, 32– of elemental sulfur aerosol. 36). At 96-km Venus atmosphere, the rates of the SO3 hydration In the Venus photochemistry model analyzed by Mills (37), the SO and the SO3 photolysis are found to be comparable (17), which is and S2O sulfur gases, in addition to SO2, are present in significant suggestiveofthefactthatnearlyhalfofthesulfurinH2SO4 goes into amounts in the middle atmosphere, which makes the low-barrier •• •• SO3 and produces the inversion layers of SO2 and SO. However, our SO + H2SO4 H2SandS2O + H2SO4 H2S chemistries important for the S2-andS3-formation mechanisms. A recently observed correla- tion between SO2 depletion and enhancement in UV absorber also endorses this nonphotochemical mechanism (33). This mechanism also opens up a chemical channel for the formation of S8 aerosols in theMarsatmosphere,whichhavebeenshowntoplayaroleinMars’ early climate (16). EARTH, ATMOSPHERIC, AND PLANETARY SCIENCES

Fig. 2. Reaction scheme network showing the optimum path for the sulfuric Fig. 3. Reaction scheme showing the optimum path for the formation of ele-

acid-assisted formation of elemental Sn+1 aerosols from the SOn (n = 1, 2, 3) + mental S8 aerosols from the SOn (n = 1, 2, 3) + nH2S reaction. The green con- nH2S reaction. The blue and green connections represent the favorable path- nections represent the probable pathways whereas the red connections ways whereas the red connections represent high-energy less likely pathways. represent less likely pathways.

Kumar and Francisco PNAS | January 31, 2017 | vol. 114 | no. 5 | 867 Downloaded by guest on October 2, 2021 (38), water is the most abundant gas species, followed by dioxide (CO2) and sulfur gases, SO2 and H2S. As a result, an ap- preciable fraction of H2O may exist in a complexed form with H2S. This may alter the energetics of the overall elemental sulfur forma- tion by invoking the bimolecular reaction between SO2 and •• H2O H2S. Our results indicate that the barrier for the rate- •• determining step of the reaction between SO2 and H2O H2Sis lowered by ∼40% compared with the uncatalyzed SO2 + H2Sre- action. Alternatively, H2O may add across either CO2 to form - bonic acid (H2CO3)orSO2 to form thiosulfurous acid (H2S2O2), which may subsequently influence the SO2 + H2Sreaction.Both H2CO3 (–C=O, OH) and H2S2O2 (–S=O, OH) form strong com- plexes with H2S (Table 1) and possess mandatory functionalities to allow the SO2 + H2S reaction to occur under acid catalysis. Consid- ering that these proposed chemical processes are all H2S-based, and no elemental sulfur formation has been seen in the absence of H2S (40), the results could help in better understanding the of the magmatic–hydrothermal systems. In summary, we have used electronic structure calculations to suggest a nonphotochemical mechanism for the formation of elemental sulfur aerosols in planetary atmospheres. The mech- anistic beauty of this proposal is that the reactions of sulfur ox- ides and hydrogen sulfide under water or sulfuric acid catalysis provide low-energy pathways for the formation of S2–S4 parti- cles. Interestingly, the uncatalyzed reactions of sulfur oxides and hydrogen sulfide result in the intermediates that are functionally similar to sulfuric acid, which points to the fact that these sulfur chemistries could be autocatalyzed. Methods

The SOn (n = 1, 2, 3) + nH2S reactions in the gas phase have been examined in the absence and presence of H2OandH2SO4 catalysts. The uncatalyzed reactions have been briefly explored whereas the effect of catalysis on the most probable

reactions has been examined in detail. In particular, the effect of H2OorH2SO4 catalysis on the H2S addition reactions and the subsequent H2O elimination re- actions have been explored. The effect of catalysis on the dehydrogenation reac- tions has not been examined because these reactions involve very high barriers and are less likely to be important in atmosphere. The impact of catalysis was quantified

by calculating the reaction profiles for the bimolecular reactions between the H2O- or H2SO4-bound H2SandSOn. All of the reactions have been calculated assuming the singlet ground state except for the SO reactions, which have been explored for both the singlet and triplet states. The singlet SO reactions have been examined because the spectroscopic signatures of the direct singlet SO ejection from the volcanic vent have been detected (20). All calculations were performed with Gaussian 09 (41). All geometries were optimized using the -functional theory Fig. 4. CCSD(T)/aug-cc-pVTZ//M06-2X/aug-cc-pVTZ calculated zero-point– method, M06-2X (42), and the augmented correlation-consistent basis set, aug-cc- corrected reaction profiles for the reactions of sulfur dioxide (Top) and sulfur pVTZ (43). Because the open-shell 1SO is more stable than the closed-shell one, the trioxide (Bottom) with , H X(X= O, and S). The energies 2 calculations involving 1SO have been done using the uM06-2X/aug-cc-pVTZ level of are given in kcal/mol units. theory. The energetics were further improved by performing single-point calcula- tions at the CCSD(T) (44) and the aug-cc-pVTZ basis set. This level of theory has been found to provide an accurate description of hydrogen atom transfer-based Elemental sulfur deposits are also common at volcanic vents chemistries in the recent past (23, 24). All stationary points were characterized by and where high-temperature discharge gas is supersat- frequency calculations and reported energies include zero-point energy correc- urated in sulfur (38) as a result of equilibrium chemical reactions tions (unscaled) from the method used for geometry optimization. involving a variety of magmatic sulfur gases. The reaction between SO2 and H2S has been used to explain these sulfur sediments (39). ACKNOWLEDGMENTS. We are grateful to the Holland Computing Center, It is important to note that in Kawah Ijen discharges University of Nebraska-Lincoln for computational support of this work.

1. Krasnopolsky VA (2011) on Venus, Earth, and Mars: Main 6. Ono S, et al. (2003) New insights into Archean sulfur cycle from mass-independent features and comparison. Planet Space Sci 59:952–964. sulfur isotope records from the Hamersley Basin, Australia. Earth Planet Sci Lett 2. Farquhar J, Bao H, Thiemens M (2000) Atmospheric influence of Earth’s earliest sulfur 213:15–30. cycle. Science 289(5480):756–759. 7. Lyons JR (2008) An estimate of the equilibrium speciation of sulfur vapor over solid 3. Farquhar J, Wing BA (2003) Multiple sulfur and the evolution of the atmo- sulfur and implications for planetary atmospheres. J Sulfur Chem 29:269–279. sphere. Earth Planet Sci Lett 213:1–13. 8. Kamber BS, Whitehouse M (2007) Micro-scale sulphur isotope evidence for sulphur 4. Farquhar J, Savarino J, Airieau S, Thiemens MH (2001) Observation of wavelength- cycling in the late Archean shallow ocean. Geobiology 5:5.

sensitive mass-independent sulfur isotope effects during SO2 photolysis: Implications 9. Halevy I (2013) Production, preservation, and biological processing of mass-independent for the early atmosphere. J Geophys Res 106:32829–32839. sulfur isotope fractionation in the Archean surface environment. Proc Natl Acad Sci USA 5. Pavlov AA, Kasting JF (2002) Mass-independent fractionation of sulfur isotopes in 110(44):17644–17649. Archean sediments: Strong evidence for an anoxic Archean atmosphere. 10. Philippot P, et al. (2007) Early Archaean microorganisms preferred elemental sulfur, 2(1):27–41. not sulfate. Science 317(5844):1534–1537.

868 | www.pnas.org/cgi/doi/10.1073/pnas.1620870114 Kumar and Francisco Downloaded by guest on October 2, 2021 11. Glarborg P, Kubel D, Dam-Johansen K, Chiang H-M, Bozzelli JW (1996) Impact of SO2 29. Golovin YM, Moshkin BE, Ekonomov AP (1981) Aerosol component properties as and NO on CO oxidation under post-flame conditions. Int J Chem Kinet 28:773–790. measured by the Venera 11 and 12 spectrophotometer. Cosm Res 19:295–302.

12. Sendt K, Jazbec M, Haynes BS (2002) Chemical kinetic modeling of the H/S system: H2S 30. Mukhin LM, et al. (1983) Gas chromatographic analysis of the Venus atmospheric thermolysis and H2 sulfidation. Proc Combust Inst 29:2439–2446. chemical composition by the Vanera 13 and 14 descent probes. Cosm Res 21:168–172.

13. Sendt K, Haynes BS (2005) Role of the direct reaction H2S + SO2 in the homogeneous 31. Krasnopolsky VA, Pollack JB (1994) H2O-H2SO4 system in Venus’ clouds and OCS, CO, Claus reaction. J Phys Chem A 109(36):8180–8186. and H2SO4 profiles in Venus’ troposphere. Icarus 109(1):58–78. 14. Toon OB, Turco RP, Pollack JB (1982) The ultraviolet absorber on Venus-amorphous 32. Mills FP, Allen M (2007) A review of selected issues concerning the chemistry in Venus’ sulfur. Icarus 51:358–373. middle atmosphere. Planet Space Sci 55:1729–1740. 15. Atlas E (1991) Observation of possible elemental sulfur in the marine atmosphere and 33. Marcq E, Belyaev D, Bertaux J-L, Fedorova A, Montmessin F (2011b) Long-term speculation on its origin. Atmos Environ 25A:2701–2705. monitoring SO2 above the clouds of Venus using SPICAV-UV in nadir mode. EGU Gen 16. Tian F, et al. (2010) Photochemical and climate consequences of sulfur outgassing on Assem 13:EGU2011–EGU2003.

early Mars. Earth Planet Sci Lett 295:412–418. 34. Marcq E, et al. (2011a) An investigation of the SO2 content of the Venusian meso- 17. Krasnopolsky VA (2007) Chemical kinetic model for the lower . sphere using SPICAV-UV in nadir mode. Icarus 211:58–69. Icarus 191:25–37. 35. Sandor BJ, Clancy RT, Moriarty-Schieven G, Mills FP (2010) Sulfur chemistry in the

18. Zhang X, Liang MC, Mills FP, Belyaev DA, Yung YL (2012) Sulfur chemistry in the Venus mesosphere from SO2 and SO microwave spectra. Icarus 208:49–60. middle atmosphere of Venus. Icarus 217:714–739. 36. Krasnopolsky VA (2010b) Spatially-resolved high-resolution of Venus 2.

19. Kerber L, Forget F, Wordsworth R (2015) Sulfur in the early martian atmosphere re- Variations of HDO, OCS and SO2 at the tops. Icarus 209:314–322. visited: Experiments with a 3-D Global Climate Model. Icarus 261:133–148. 37. Mills FP (1998) I. Observations and photochemical modeling of the Venus middle 20. de Pater I, RoeP H, Graham JR, Strobel DF, Bernath P (2002) Detection of the forbidden atmosphere. II. Thermal infrared spectroscopy of and Callistro. PhD thesis 1 3 SO a Δ→ X rovibronic transition on I0 at 1.7 μm. Icarus 156:296–301. (California Institute of Technology, Pasadena, CA). 21. Vöhringer-Martinez E, et al. (2007) Water catalysis of a -molecule gas-phase 38. Delmelle P, Bernard A, Kusakabe M, Fischer TP, Takano B (2000) Geochemistry of the reaction. Science 315(5811):497–501. magmatic-hydrothermal system of Kawah Ijen , East Java, Indonesia. 22. Torrent-Sucarrat M, Francisco JS, Anglada JM (2012) Sulfuric acid as autocatalyst in J Volcanol Geotherm Res 97:31–53. the formation of sulfuric acid. J Am Chem Soc 134(51):20632–20644. 39. Rowe GL (1994) , hydrogen, and sulfur isotope systematics of the crater lake 23. Kumar M, Francisco JS (2015) Red light-induced decomposition of an organic peroxy system of Poas volcano, Costa Rica. Geochem J 28:263–287.

radical: A new source of the HO2 radical. Angew Chem Int Ed Engl 54(52):15711–15714. 40. Delmelle P, Bernard A (1994) Geochemistry, mineralogy, and chemical modeling of the acid 24. Kumar M, Sinha A, Francisco JS (2016) Role of double hydrogen atom transfer reac- crater lake of Kawah Ijen volcano, Indonesia. Geochim Cosmochim Acta 58:2445–2460. tions in atmospheric chemistry. Acc Chem Res 49(5):877–883. 41. Frisch MJ, et al. (2009) Gaussian 09, revision D.01 (Gaussian, Pittsburgh). 25. Hoffman JH, Hodges RR, Jr, Donahue TM, Mcelroy MB (1980) Composition of the Venus 42. Zhao Y, Truhlar DG (2008) The M06 suite of density functionals for main lower atmosphere from the Pioneer Venus mass spectrometer. J Geophys Res 85:7882–7890. thermochemistry, thermochemical kinetics, noncovalent interactions, excited states, 26. Peterson KA, Lyons JR, Francisco JS (2006) An ab initio study of the low-lying elec- and transition elements: Two new functionals and systematic testing of four M06- tronic states of S3. J Chem Phys 125(8):084314. class functionals and 12 other functionals. Theor Chem Acc 120:215–241. 27. Du S, et al. (2011) The kinetics study of the S + S2 → S3 reaction by the chaperone 43. Kendall RA, Dunning TH, Jr, Harrison RJ (1992) Electron affinities of the first-row mechanism. J Chem Phys 134(15):154508. revisited. Systematic basis sets and wave functions. J Chem Phys 96:6796–6806. 28. Woodall J, Agundez M, Markwick-Kemper AJ, Millar TJ (2007) The UMIST database 44. Noga J, Bartlett RJ (1987) The full CCSDT model for molecular electronic structure. for 2006. Astron Astrophys 466:1197–1204. J Chem Phys 86:7041–7050. EARTH, ATMOSPHERIC, AND PLANETARY SCIENCES

Kumar and Francisco PNAS | January 31, 2017 | vol. 114 | no. 5 | 869 Downloaded by guest on October 2, 2021