Photochemical and thermochemical pathways to S2 and polysulfur formation in the atmosphere of Venus.

Polysulfur species have been proposed to be the unknown near-UV absorber in the atmosphere of Venus. Recent work argues that photolysis of one of the (SO)2 isomers, cis-OSSO, directly yields S2 with a branching ratio of about 10%. If correct, this pathway dominates polysulfur formation by several orders of magnitude, and by addition reactions yields significant quantities of S3, S4, and S8. We report here the results of high-level ab-initio quantum-chemistry computations that demonstrate that S2 is not a product in cis-OSSO photolysis. Instead, we establish a novel mechanism in which S2 is formed in a two-step process. Firstly, the intermediate S2O is produced by the coupling between the S and Cl atmospheric chemistries (in particular, SO reaction with ClS) and in a lesser extension by O-abstraction reactions from cis-OSSO. Secondly, S2O reacts with SO. This modified chemistry yields S2 and subsequent polysulfur abundances comparable to the photolytic cis-OSSO mechanism through a more plausible pathway. Ab initio quantification of the photodissociations at play fills a critical data void in current atmospheric models of Venus.

Oxygen isotopic heterogeneity in the early Solar System inherited from the protosolar molecular cloud.

The Sun is 16O-enriched (Δ17O = -28.4 ± 3.6‰) relative to the terrestrial planets, asteroids, and chondrules (-7‰ < Δ17O < 3‰). Ca,Al-rich inclusions (CAIs), the oldest Solar System solids, approach the Sun's Δ17O. Ultraviolet CO self-shielding resulting in formation of 16O-rich CO and 17,18O-enriched water is the currently favored mechanism invoked to explain the observed range of Δ17O. However, the location of CO self-shielding (molecular cloud or protoplanetary disk) remains unknown. Here we show that CAIs with predominantly low (26Al/27Al)0, <5 × 10-6, exhibit a large inter-CAI range of Δ17O, from -40‰ to -5‰. In contrast, CAIs with the canonical (26Al/27Al)0 of ~5 × 10-5 from unmetamorphosed carbonaceous chondrites have a limited range of Δ17O, -24 ± 2‰. Because CAIs with low (26Al/27Al)0 are thought to have predated the canonical CAIs and formed within first 10,000-20,000 years of the Solar System evolution, these observations suggest oxygen isotopic heterogeneity in the early solar system was inherited from the protosolar molecular cloud.

A light carbon isotope composition for the Sun.

Measurements by the Genesis mission have shown that solar wind oxygen is depleted in the rare isotopes, 17O and 18O, by approximately 80 and 100‰, respectively, relative to Earth's oceans, with inferred photospheric values of about -60‰ for both isotopes. Direct astronomical measurements of CO absorption lines in the solar photosphere have previously yielded a wide range of O isotope ratios. Here, we reanalyze the line strengths for high-temperature rovibrational transitions in photospheric CO from ATMOS FTS data, and obtain an 18O depletion of δ18O = -50 ± 11‰ (1σ). From the same analysis we find a carbon isotope ratio of δ13C = -48 ± 7‰ (1σ) for the photosphere. This implies that the primary reservoirs of carbon on the terrestrial planets are enriched in 13C relative to the bulk material from which the solar system formed, possibly as a result of CO self-shielding or inheritance from the parent cloud.

Early inner solar system origin for anomalous sulfur isotopes in differentiated protoplanets.

Achondrite meteorites have anomalous enrichments in (33)S, relative to chondrites, which have been attributed to photochemistry in the solar nebula. However, the putative photochemical reactions remain elusive, and predicted accompanying (33)S depletions have not previously been found, which could indicate an erroneous assumption regarding the origins of the (33)S anomalies, or of the bulk solar system S-isotope composition. Here, we report well-resolved anomalous (33)S depletions in IIIF iron meteorites (<-0.02 per mil), and (33)S enrichments in other magmatic iron meteorite groups. The (33)S depletions support the idea that differentiated planetesimals inherited sulfur that was photochemically derived from gases in the early inner solar system (<∼2 AU), and that bulk inner solar system S-isotope composition was chondritic (consistent with IAB iron meteorites, Earth, Moon, and Mars). The range of mass-independent sulfur isotope compositions may reflect spatial or temporal changes influenced by photochemical processes. A tentative correlation between S isotopes and Hf-W core segregation ages suggests that the two systems may be influenced by common factors, such as nebular location and volatile content.

Oxygen isotope anomaly observed in water vapor from Alert, Canada and the implication for the stratosphere.

To identify the possible anomalous oxygen isotope signature in stratospheric water predicted by model studies, 25 water vapor samples were collected in 2003-2005 at Alert station, Canada (82°30'N), where there is downward transport of stratospheric air to the polar troposphere, and were analyzed for δ(17)O and δ(18)O relative to Chicago local precipitation (CLP). The latter was chosen as a reference because the relatively large evaporative moisture source should erase any possible oxygen isotope anomaly from the stratosphere. A mass-dependent fractionation coefficient for meteoric waters, λMDF(H2O) = 0.529 ± 0.003 [2σ standard error (SE)], was determined from 27 CLP samples collected in 2003-2005. An oxygen isotopic anomaly of Δ(17)O = 76 ± 16 ppm (2σ SE) was found in water vapor samples from Alert relative to CLP. We propose that the positive oxygen isotope anomalies observed at Alert originated from stratospheric ozone, were transferred to water in the stratosphere, and subsequently mixed with tropospheric water at high latitudes as the stratospheric air descended into the troposphere. On the basis of this ground signal, the average Δ(17)O in stratospheric water vapor predicted by a steady-state box model is ∼40‰. Seven ice core samples (1930-1991) from Dasuopu glacier (Himalayas, China) and Standard Light Antarctic Precipitation did not show an obvious oxygen isotope anomaly, and Vienna Standard Mean Ocean Water exhibited a negative Δ(17)O relative to CLP. Six Alert snow samples collected in March 2011 and measured at Laboratoire des Sciences du Climat et de l'Environnement, Gif sur Yvette, France, had (17)Oexcess of 45 ± 5 ppm (2σ SE) relative to Vienna Standard Mean Ocean Water.

High-resolution study of 13C16O A-X(v' = 0-9) bands using the VUV-FTS at SOLEIL: revised term values.

We present high-resolution absorption spectral measurements of the A(1)Π-X(1)Σ(+) band system of (13)C(16)O. These were recorded with the VUV Fourier transform spectrometer (VUV-FTS) installed on the DESIRS beamline at the SOLEIL synchrotron. This work includes revised term values that extend to higher J' values than previous measurements for most v' levels and lower J' values for v' = 0. We confirm previously observed perturbations of the rotational levels in greater detail and present evidence for new perturbations. The accuracy in the wavelength determination and term values is on average within 0.01 cm(-1), improving upon previous measurements.

Unique Hg stable isotope signatures of compact fluorescent lamp-sourced Hg.

The recent widespread adoption of compact fluorescent lamps (CFL) has increased their importance as a source of environmental Hg. Stable isotope analysis can identify the sources of environmental Hg, but the isotopic composition of Hg from CFL is not yet known. Results from analyses of CFL with a range of hours of use show that the Hg they contain is isotopically fractionated in a unique pattern during normal CFL operation. This fractionation is large by comparison to other known fractionating processes for Hg and has a distinctive, mass-independent signature, such that CFL Hg could be uniquely identified from other sources. The fractionation process described here may also explain anomalous fractionation of Hg isotopes in precipitation.

The kinetics study of the S + S2 → S3 reaction by the chaperone mechanism.

The recombination of S atoms has been found to be stepwise from the smallest unit, the elemental S atom, to the most abundant molecule S(8). The reaction between S + S(2) → S(3) has not been reported either experimentally or by theory, but may be a key intermediate step in the formation of sulfur aerosols in low-O(2) atmospheres. In this work, the kinetics of this reaction is reported with Ar gas used as the chaperone molecule in the production of S(3) via two complex intermediates: SAr + S(2) and S(2)Ar + S. Quasi-classical and classical trajectory methods are used. The rate constant of the S + S(2) + Ar → S(3) + Ar reaction is determined to be 2.66 × 10(-33) cm(6) mol(-1) s(-1) at 298.15 K. The temperature dependence of the reaction is found to be 2.67 × 10(-33) exp[143.56(1∕T-1∕298.15)]. The second-order rate constant of S + S(2) → S(3) is 6.47 × 10(-14) cm(3) molecule(-1) s(-1) at 298.15 K and the Arrhenius-type rate constant is calculated to be 6.25 × 10(-14) exp[450.15(1∕T-1∕298.15)] cm(3) molecule(-1) s(-1). This work provides a rate coefficient for a key intermediate species in studies of sulfur formation in the modern Venus atmosphere and the primitive Earth atmosphere, for which assumed model rate coefficients have spanned nearly 4 orders of magnitude. Although a symmetry-induced mass-independent isotope effect is not expected for a chaperone mechanism, the present work is an important step toward evaluating whether mass-independence is expected for thiozone formation as is observed for ozone formation.

Comment on "Experimental test of self-shielding in vacuum ultraviolet photodissociation of CO".

Chakraborty et al. (Reports, 5 September 2008, p. 1328) demonstrated very large, wavelength-dependent mass-independent isotopic effects during carbon monoxide (CO) photodissociation and argued that self-shielding in CO was not responsible. We suggest that variations in band oscillator strengths and linewidths among CO isotopologs are responsible for most of the wavelength dependence observed and that the reported experiments confirm the importance of self-shielding during CO photodissociation.

Identification of the reactive cis,mer isomer of [Ir(CO)2I3Me]-: relation to the mechanism of iridium-catalyzed methanol carbonylation.

Thermal dissociation of CO from cis,fac-[Ir(CO)(2)I(3)Me](-) (1a) gives the iodide-bridged dimer [{Ir(CO)I(2)(mu-I)Me}(2)](2-), which was characterized crystallographically as its Ph(4)As(+) salt. This dimer reacts with CO at ambient temperature to give the acetyl complex trans,mer-[Ir(CO)(2)I(3)(COMe)](-). An intermediate in this reaction is the previously unobserved cis,mer-[Ir(CO)(2)I(3)Me](-) (1b), which was characterized by IR and NMR spectroscopy. Carbonylation of 1b is much faster (t(1/2) approximately 1 min at 25 degrees C) than for the cis,fac isomer 1a and also faster than the neutral tricarbonyl [Ir(CO)(3)I(2)Me]. The observations show that the relative positioning of carbonyl ligands influences reactivity more than than their number, and that CO insertion is particularly accelerated when a CO ligand is placed trans to the migrating methyl group. DFT calculations indicate that the Ir-CO bond trans to methyl contracts significantly as 1b approaches the transition state for methyl migration, facilitating stronger pi-backbonding from Ir to the spectator CO ligand, which stabilizes the transition state. The results confirm the recently proposed CO-loss mechanism for the photochemical carbonylation of 1a and suggest that the cis,mer isomer may play a minor role in the catalytic cycle for methanol carbonylation.

An ab initio study of the low-lying electronic states of S3.

Accurate calculations of the low-lying singlet and triplet electronic states of thiozone, S(3), have been carried out using large multireference configuration interaction wave functions. Cuts of the full potential energy surfaces along the stretching and bending coordinates have been presented, together with the vertical excitation spectra. The strong experimentally observed absorption around 395 nm is assigned to the 1 (1)B(2) state, which correlates to ground state products. Absorption at wavelengths shorter than 260 nm is predicted to lead to singlet excited state products, S(2) (a (1)Delta(g))+S((1)D). The spectroscopic properties of the X (3)Sigma(g) (-), a (1)Delta(g), and b (1)Sigma(g) (+) electronic states of the S(2) radical have also been accurately characterized in this work. The investigations of the low-lying electronic states were accompanied by accurate ground state coupled cluster calculations of the thermochemistry of both S(2) and S(3) using large correlation consistent basis sets with corrections for core-valence correlation, scalar relativity, and atomic spin-orbit effects. Resulting values for D(0)(S(2)+S) and SigmaD(0) for S(3) are predicted to be 61.3 and 162.7 kcal/mol, respectively, with conservative uncertainties of +/-1 kcal/mol. Analogous calculations predict the C(2v)-D(3h) (open-cyclic) isomerization energy of S(3) to be 4.4+/-0.5 kcal/mol.

High-level ab initio studies of the structure, vibrational spectra, and energetics of S3.

Observation of mass-dependent and non-mass-dependent sulfur isotope fractionations in elemental sulfur is providing new insight into the nature of the sulfur cycle in the atmosphere. Interpretation of the experimental isotope data requires estimation of the energetics for the reaction S+S2-->S3 (isoelectronic with O+O2-->O3). Key molecular properties of the S3 potential-energy surface, such as vibrational frequencies and isotopic shifts, are presented that can be used to assess the mass-dependent fractionation effect. Ab initio results are compared to the available experimental results for S2 to evaluate the reliability of the computational results for S3. The S-S bond dissociation energy for S3 is determined to be 60.9+/-1 kcal mol(-1).