SO<sub>2</sub> photolysis as a source for sulfur mass-independent isotope signatures in stratospehric aerosols

Abstract

Abstract. Signatures of sulfur isotope mass-independent fractionation (S-MIF) have been observed in stratospheric sulfate aerosols deposited in polar ice. The S-MIF signatures are thought to be associated with stratospheric photochemistry following stratospheric volcanic eruptions, but the exact mechanism responsible for the production and preservation of these signatures is debated. In order to identify the origin and the mechanism of preservation for these signatures, a series of laboratory photochemical experiments were carried out to investigate the effect of temperature and added O 2 on the S-MIF produced by two absorption band systems of SO 2 : photolysis in the 190 to 220 nm region and photoexcitation in the 250 to 350 nm region. The SO 2 photolysis (SO 2 + hν → SO+O) experiments showed S-MIF signals with large 34 S/ 32 S fractionations, which increases with decreasing temperature. The overall S-MIF pattern observed for photolysis experiments, including high 34 S/ 32 S fractionations, positive mass-independent anomalies in 33 S, and negative anomalies in 36 S, is consistent with a major contribution from optical isotopologue screening effects and data for stratospheric sulfate aerosols. In contrast, SO 2 photoexcitation produced products with positive S-MIF anomalies in both 33 S and 36 S, which is different from stratospheric sulfate aerosols. SO 2 photolysis in the presence of O 2 produced SO 3 with S-MIF signals, suggesting the transfer of the S-MIF anomalies from SO to SO 3 by the SO + O 2 + M → SO 3 + M reaction. This is supported with energy calculations of stationary points on the SO 3 potential energy surfaces, which indicate that this reaction occurs slowly on a single adiabatic surface, but that it can occur more rapidly through intersystem crossing. Based on our experimental results, we estimate a termolecular rate constant on the order of 10 −37 cm 6 molecule −2 s −1 . This rate can explain the preservation of mass independent isotope signatures in stratospheric sulfate aerosols and provides a minor, but important, oxidation pathway for stratospheric SO 2 . The production and preservation of S-MIF signals requires a high SO 2 column density to allow for optical isotopologue screening effects to occur and to generate a large enough signature that it can be preserved. In addition, the SO 2 plume must reach an altitude of around 20 to 25 km, where SO 2 photolysis becomes a dominant process. These experiments are the first step towards understanding the origin of the sulfur isotope anomalies in stratospheric sulfate aerosols.

Use of Isotope Effects To Understand the Present and Past of the Atmosphere and Climate and Track the Origin of Life

“…X 1 A 1 ): SO 2 will be excited to at riplet state.T he first absorption band is difficult to investigate because there was no photocell window or laboratory light source available.Aseries of synchrotron experiments applied to varying molecules (CO,N 2 ,a nd H 2 S) [60-62, 64, 65, 144] open new areas for studying photochemically-induce isotope effects in the VUV range and investigation of SO 2 is currently underway. As shown in existing experiments,f or not only SO 2 [142,148] but also CO [61] and N 2 , [65] intersystem crossing is governed by many parameters such as column density and temperature.Their effects are difficult to predict because the exited state potential energy surfaces of varying isotopomers and corresponding vibrational/rotational energy distribution are impossible to accurately and precisely calculate at arequisite level. Therefore,e xisting studies were focused on the second and third absorption bands (i.e., photo-dissociation and photo-excitation bands,r espectively).…”

“…[136] Tw om ajor discoveries in 2000 (the sulfur isotope mass independent composition in Martian meteorites [76] and Archean rocks [96] )r equired an additional mechanistic explanation because S 2 F 10 is not relevant and concentrations of CS 2 in the Martian and terrestrial atmospheres are too low to explain the observed effect. Thef irst series of SO 2 and H 2 S photochemistry experiments was conducted by Farquhar et al, [76,97] and confirmed that as ulfur isotope mass independent effect can be found in these photolytic reactions and qualitatively resemble observations in nature.A ll multiple sulfur isotope measurements in extraterrestrial and terrestrial samples (e.g., meteorites,rocks,ice cores,aerosols) in the first decade of the 21 st century were interpreted based on similar photochemistry.B ecause existing theoretical knowledge of sulfur isotope effect was not totally satisfied to explain observations,e xperimental investigations accelerated and broadened in the second decade of the 21 st century:T he cross sections of anumber of isotopomers ( 32 SO 2 , 33 SO 2 , 34 SO 2 , 36 SO 2 ,O C 32 S, OC 33 S, OC 34 S, O 13 CS) [137][138][139][140] were firstly determined in as eries of experiments (joint efforts by many scientists in Japan and Denmark:D anielache,H attori, Johnson, Schmidt, Ueno,a nd Yoshida) for advancing theoretical studies;U eno and colleagues expanded their experiments to SO 2 photolysis; [141] Farquhar and colleagues also revisited their early SO 2 photolysis experiment results by carrying out additional experiments; [142,143] H 2 Sp hotolysis experiments were extended to the VUV range by Chakraborty et al; [144] another series of photochemistry experiments (with am ajor focus on SO 2 [145][146][147][148] and expanded to COS [149] and phenacylphenylsulfone [150] )w as carried out by Ono and colleagues as well. Ar eview of detailed experimental results and theoretical interpretation was recently provided by Ono.…”

“…X 1 A 1 ): SO 2 will be dissociated to SO and O, and SO can be recombined with another SO to form S and SO 2 or dissociated to Sa nd O; (iii)Between % 250 and % 330 nm (A 1 A 2 -B 1 B 1 ! Theformer one may not be the dominant factor because calculation using experimentally determined cross section cannot reproduce the experimental data, [138,139,147,148] although there remain uncertainties such as errors in cross section measurements and potentially slight changes of lamp spectrum due to other gases in experiment chambers.Given that small variations in minor isotope ratios are being interpreted, conventional errors in cross sections produce major isotope uncertainties.T he latter one (i.e., perturbation) has been observed in N 2 and CO molecules and discussed in this Minireview.G iven the rich excited states of SO 2 and overlapping of their excited state potential energy surfaces,i ntersystem crossing perturbations arise with consequences for isotope effects.T he CS 2 photolysis, [136] SO 2 photo-excitation, [147] and H 2 SVUV photolysis experiments [144] have suggested such effects in interpreting the data. X 1 A 1 ): SO 2 will be excited to at riplet state.T he first absorption band is difficult to investigate because there was no photocell window or laboratory light source available.Aseries of synchrotron experiments applied to varying molecules (CO,N 2 ,a nd H 2 S) [60-62, 64, 65, 144] open new areas for studying photochemically-induce isotope effects in the VUV range and investigation of SO 2 is currently underway.…”

Total Pressure Dependence of Sulfur Mass‐Independent Fractionation by SO2Photolysis

“…The photolysis rate of SO 2 is much faster than the photoexcitation of SO 2 , which could also produce large MIF (Endo et al, 2016;Whitehill et al, 2015;Whitehill & Ono, 2012;Whitehill et al, 2013). The photolysis rate of SO 2 is much faster than the photoexcitation of SO 2 , which could also produce large MIF (Endo et al, 2016;Whitehill et al, 2015;Whitehill & Ono, 2012;Whitehill et al, 2013).…”

Recombination reactions as a possible mechanism of mass-independent fractionation of sulfur isotopes in the Archean atmosphere of Earth

“…However, the actual chemical process, or processes, responsible for generation of significant enrichments of sulfur-bearing species in heavy isotopes of sulfur, and leading to mass-independent fractionation, still remains unidentified. Some mass-independent fractionation was observed experimentally in SO 2 photolysis (10-12) and in nonadiabatic dynamics of SO 2 photodissociation (13,14). Another important chemical pathway, specific to a low-oxygen atmosphere (7,8,(15)(16)(17), is a chain of recombination reactions that start from recombination of photolytically produced sulfur atoms (S + S → S 2 ), go through formation of larger sulfur allotropes (S + S 2 → S 3 , S 2 + S 2 → S 4 , and S + S 3 → S 4 ), and end up at the elemental sulfur (S 4 + S 4 → S 8 ) that can be deposited, preserved, and could contribute to S-MIF in the Archean atmosphere.…”