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Mass-Independent SulFate IsoTopes

Periodic Reporting for period 1 - MISFIT (Mass-Independent SulFate IsoTopes)

Reporting period: 2016-04-01 to 2018-03-31

The discovery of sulfur and oxygen isotope mass independent fractionation (S- and O- MIF) provides a direct link between atmospheric composition and signals in the rock and ice core records and a new metric for the investigation of early Earth as well as past and present day atmospheric chemistry. While the origin of O-MIF and its transfer to other oxygen-bearing compounds (e.g. H2O2, NO3-) are relatively well understood, recently controversy has emerged regarding the fundamental step lying at the origin of the S-MIF. Even the consensus that S-MIF is triggered by exposure of SO2 to UV radiation is now under debate. We should thus acknowledge that after passing the first wave of euphoria following the discovery of S-MIF in Archean and volcanic samples, the interpretation of the S-MIF does not rest on a solid mechanistic foundation. The confusion on the origin of S-MIF is particularly detrimental as the occurrence of S-MIF in both Archean rock samples and modern volcanic sulfate in polar ice offers the opportunity to examine the dependence of the MIF-signals in relation with the evolution of Earth’s atmosphere and surface environment. As such, recently NASA has placed the resolution of the origin of the S-MIF as one of the top priorities for its astrobiology program, recognizing the importance of MIF in solving the epic question of the origin of life and its interaction with the planetary environment.

In this project, we aim to determine the processes responsible for S-MIF and assess the implications for the distribution of S-MIF in atmospheric sulfate, and to build a quantitative understanding of atmospheric MIF processes including its origin and transfer using O- and S-MIF model simulations, ensuring proper extraction of information embedded in sulfate MIF data.

In order to achieve the objectives, we carried out a new set of chamber experiments on SO2-related production of S-MIF considering environmental conditions that are as close as possible to those of the stratosphere from where S-MIF in modern sulfate samples were observed. New protocol to process sulfate samples for S-MIF analysis was developed, and O- and S-MIF analysis were conducted on sulfate samples produced from chamber experiments. Preliminary analysis on the isotopic data suggests the role of SO2 photo-excitation in creating the observed S-MIF, with implications for the effects of SO2 isotopologues absorption cross section difference, self-shielding and intersystem crossing on the systematics of S-MIF. Further work need to be done to explore quantitatively the relative contributions of the above-mentioned processes to the observed S-MIF. Overall, the isotopic results, combined with chamber experimental parameters, will be used to constrain the origin of S-MIF, and to develop S-MIF and O-MIF isotope chemistry schemes during the formation of atmospheric sulfate. The later will be coupled and incorporated into a global chemistry-transport model (i.e. GEOS-chem) to test the proposed origin and global distribution of S-MIF following natural processes, i.e. volcanic eruptions. This work is in progress.
1) We have tested a new fluorination method which converts H2S gas to SF6, the latter is necessary for precise analysis of all four sulphur isotopes (32S, 33S, 34S and 36S) in order to determine S-MIF. This new method used CoF3 as the fluorination agent, which avoids the use of extremely dangerous gas F2 and thus is safer. Unfortunately, the reactivity of CoF3 and H2S turned to be very erratic, and even became inactive by time. We have also tested the possibility of fluorinating Ag2S to SF6 using use CoF3.

2) We have developed a new protocol that converts sulphate to sulphide, which is the first step of precise analysis of all four sulphur isotopes (32S, 33S, 34S and 36S) in order to determine S-MIF. The new protocol is easy to handle and allows to process multiple samples at a time, and is demonstrated with good reproducibility in terms of H2S yield and for further isotope analysis. It is thus a good alternative to the manual conventional method which suffers from a cumbersome distillation apparatus system, long reaction time and large volume of the reducing solution. The new protocol is especially useful for samples with limited amount of sulphate available.

3) We have also compiled S-MIF data of home-made S-33 enriched sulphate standards measured by 5 different laboratories cross the world. The standards were made by the host and distributed to the 5 laboratories, in order to compare the precision of the S-MIF analysis by different laboratories, and to seek the possibility to provide the first S-MIF standard reference material. An manuscript on this topic is drafted.

4) We have done 3 campaigns (totally 5 weeks) of lab experiments using the stainless-steel chamber (CESAM) in Laboratoire Interuniversitaire des Systèmes Atmosphériques (LISA) to simulate atmospheric sulphate production under designed conditions. These experiments were used to study O-MIF and S-MIF of sulphate produced under atmospheric conditions with different UV exposure, relative humility and with or without O3 present. In total, we have collected 91 sulphate samples for O-MIF and S-MIF analysis and 6 nitrate samples for O-MIF analysis in order to quantify the magnitude of O-MIF in O3 which is ultimate source of O-MIF.

5) We have completed the oxygen isotopic composition analysis of all samples at IGE, Grenoble, as well as sulphur isotopic composition analysis of all sulphate samples at IPGP, Paris. The S-MIF results, specifically, the systematics of δ34S, ∆33S and ∆36S, comparing with previous published data in the literature, suggesting that the S-MIF in stratospheric sulphate after explosive volcanic eruptions probably originates from the absorption cross section difference between 240-340 nm of different SO2 isotopologues. At high SO2 concentrations (>1 ppm), or in the air without O2, the effects of self-shielding or intersystem crossing may arise, respectively.
Through the project, the IGE (former LGGE) isotope lab now has a reduction line which has multiple advantages compared to the conventional cumbersome distillation apparatus system, and the new reduction line significantly improve the capability of sample preparation for quadruple sulphur isotope analysis. In addition, we have improved the pre-existing O-MIF of sulphate analysis line at IGE to make it more stable and now we are trying to modify it to lower the sample size requirement.

Through this project, the fellow has earned experiences on vacuum lines, atmospheric chamber experiments, and fluorination line for S-MIF analysis. Benefit from this project, the fellow has obtained a new professional job as a professor at the University of Science and Technology of China under the support of Chinese “1000-young talent” program. The fellow is now with the host in plan of continuing the project as well as building new collaborations to further explore the potential of S-MIF and O-MIF in addressing the mystery of rapid aerosol pollution break-out in China where rapid growth of atmospheric sulfate is one of the major contributors. For the later, knowledges or information regarding the origin and transformation of O-MIF and S-MIF in atmospheric sulfate learned from the outcomes of the chamber experiments and modeling approaches will be highly useful.
Reduction line at IGE to convert sulfate to sulfide
Atmospheric simulation chamber in Paris used to simulate the formation of sulfuric acid aerosols