The oxygen isotopic composition (16O, 17O, and 18O) of sulfate formed from different oxidative reactions has been investigated. In the aqueous phase, sulfur oxidation by H2O2, O3, and O2, catalyzed ...by Fe(III) and Mn(II) were studied. In the gas phase we have investigated the only relevant reaction for the atmosphere: SO2+OH and its chain termination reaction SO3+H2O. The results show that none of these reactions, gas or aqueous phase, produce a mass‐independent oxygen isotopic composition in sulfate. Since H2O2 and O3 are known to possess a mass‐independent isotopic signature, we have investigated the possible transfer of this anomaly to sulfate. It appears that both these oxidant species transfer their anomaly. Isotopic analysis shows that two oxygen atoms from H2O2 are found in the product H2SO4. This result is in accord with previous work. For O3 we found that only one of the original ozone oxygen transfers to the product sulfate. These isotopic results contradict the free radical reaction mechanism proposed by Penkett et al. 1979 but agree with the nonfree radical mechanism suggested by Erickson et al. 1977. Therefore, it appears that only aqueous phase oxidation produces a mass‐independent oxygen isotopic composition in sulfate. This finding is a response of the origin of the mass‐independent oxygen isotopic composition of atmospheric and mineral deposits of sulfate on Earth Bao et al., 2000; Lee, 1997. Furthermore, this finding allows us to quantify the relative proportion of sulfate production by OH (gas phase formation) and by H2O2 and O3 (aqueous phase formation). The results can be used to test atmospheric chemical/transport models.
Water-soluble sulfate salts extracted from six CM chondrites have oxygen isotope compositions that are consistent with an extraterrestrial origin. The Δ
17O of sulfate are correlated with previously ...reported whole rock δ
18O and with an index of meteorite alteration, and may display a correlation with the date of the fall. The enrichments and depletions for Δ
17O of water-soluble sulfate from the CM chondrites relative to the terrestrial mass dependent fractionation line are consistent with sulfate formation in a rock dominated asteroidal environment, and from aqueous fluids that had undergone relatively low amounts of oxygen isotope exchange and little reaction with anhydrous components of the meteorites. It is unresolved how the oxidation of sulfide to sulfate can be reconciled with the inferred low oxidation state during the extraterrestrial alteration process. Oxygen isotope data for two CI chondrites, Orgueil and Ivuna, as well as the ungrouped C2 chondrite Essebi are indistinguishable from sulfate of terrestrial origin and may be terrestrial weathering products, consistent with previous assertions. Our oxygen isotope data, however, can not rule out a preterrestrial origin either.
Nitrous oxide in the earth's atmosphere contributes to catalytic stratospheric ozone destruction and is also a greenhouse gas component. A precise budgetary accounting of N$_2$O sources has remained ...elusive, and there is an apparent lack of source identification. One source of N$_2$O is as a by-product in the manufacture of nylon, specifically in the preparation of adipic acid. Characterization of the reaction N$_2$O stoichiometry and its isotopic composition with a simulated industrial adipic acid synthesis indicates that because of high rates of global adipic acid production, this N$_2$O may account for ∼10 percent of the increase observed for atmospheric N$_2$O.
The oxygen (δ
18O, δ
17O) and carbon (δ
13C, FMOD
14C-the fraction of modern
14C) isotopic compositions of carbonate were measured for a set of paired Antarctic CM chondrites (EET 96006, EET 96016, ...EET 96017, and EET 96019). While the oxygen isotopic compositions do not plot on the terrestrial fractionation line and indicate that a component of the carbonate minerals has an extraterrestrial origin, they also do not fall on the array defined for carbonates by CM falls and are thus consistent with the presence of a terrestrial carbonate component. The δ
13C and FMOD
14C measurements of carbonate suggest the presence of at least two carbon sources: carbonate derived from atmospheric CO
2 that is inferred to have been produced as a result of silicate weathering reactions and carbonate derived from another carbon source that is either old or non-atmospheric. The relationships between oxygen and carbon isotope data provide additional constraints on the weathering process, and allow the possibility that rock-dominated weathering of the meteorite caused the oxygen isotopic composition of Antarctic water added to the meteorite to evolve away from the terrestrial mass-fractionation array, leading to formation of low temperature terrestrial alteration products that do not lie on the terrestrial fractionation line.
Photodissociation of CO2 by ultraviolet light (λ = 185 nm) generates CO and O2, which are unusually enriched (more than 100‰) in 17O. The dissociation takes place through a spin forbidden process ...during transition from a singlet to a triplet state, the latter lying on a repulsive potential energy surface. The 17O isotopic enrichment is a primary process associated with this transition and could be due to near resonant spin‐orbit coupling of the low energy vibrational levels of the 16O12C17O molecule in the singlet state with those of the triplet state near the zone of transition. In contrast, photodissociation at shorter wavelengths (λ < 160 nm) involves no spin violation and produces CO and O2 which are fractionated in a conventional mass dependent fashion. The proposed explanation is further supported using 13C enriched CO2; in this case the products are enriched in both heavy isotopes but about 100‰ more in 18O. The 17O enrichment in CO and O2 generated by CO2 photolysis in a range of UV wavelengths may be a useful tracer in delineating processes in the atmospheres of Earth and Mars.
Surface plasmons are collective oscillations of electrons in metals or semiconductors that enable confinement and control of electromagnetic energy at subwavelength scales. Rapid progress in ...plasmonics has largely relied on advances in device nano-fabrication, whereas less attention has been paid to the tunable properties of plasmonic media. One such medium--graphene--is amenable to convenient tuning of its electronic and optical properties by varying the applied voltage. Here, using infrared nano-imaging, we show that common graphene/SiO(2)/Si back-gated structures support propagating surface plasmons. The wavelength of graphene plasmons is of the order of 200 nanometres at technologically relevant infrared frequencies, and they can propagate several times this distance. We have succeeded in altering both the amplitude and the wavelength of these plasmons by varying the gate voltage. Using plasmon interferometry, we investigated losses in graphene by exploring real-space profiles of plasmon standing waves formed between the tip of our nano-probe and the edges of the samples. Plasmon dissipation quantified through this analysis is linked to the exotic electrodynamics of graphene. Standard plasmonic figures of merit of our tunable graphene devices surpass those of common metal-based structures.
The "Snowball Earths" were cataclysmic events during the late Neoproterozoic's Cryogenian period (720-635 Ma) in which most, if not all, of Earth’s surface was covered in ice. Paleoenvironmental ...reconstructions of these events utilize isotopic systems, such as Δ
17
O and barium isotopes of barites. Other isotopic systems, such as zinc (Zn), can reflect seawater composition or environmental conditions (e.g., temperature changes) and biological productivity. We report here a multi-isotopic C, O, and Zn data set for carbonates deposited immediately after the Marinoan glaciation (635 Ma) from the Otavi Group in northern Namibia. In this study, we chemically separated calcite and non-calcitic carbonate phases, finding isotopically distinct carbon and oxygen isotopes. These could reflect changes in the source seawater composition and conditions during carbonate formation. Our key finding is largescale Zn isotopic variations over the oldest parts of the distal foreslope cap carbonate sections. The magnitude of variation is larger than any found throughout post-snowball cap carbonates to date, and in a far shorter sequence. This shows a heretofore undiscovered difficulty for Zn isotopic interpretations. The primary Zn sources are likely to be aeolian or alluvial, associated with the massive deglaciation related run-off from the thawing continent and a greater exposed surface for atmospheric aerosol entrainment. The samples with the lightest Zn isotopic compositions (δ
66
Zn < 0.3 ‰) potentially reflect hydrothermally sourced Zn dominating the carbonates’ Zn budget. This finding is likely unique to the oldest carbonates, when the meltwater lid was thinnest and surface waters most prone to upwelling of hydrothermally dominated Snowball Earth brine. On the other hand, local variations could be related to bioproductivity affecting the Zn isotopic composition of the seawater. Similarly, fluctuations in sea-level could bring the depositional site below and above a redoxcline, causing isotopic variations. These variations in Zn isotope ratios preclude the estimation of a global Zn isotopic signature, potentially indicating localized resumption of export production.
The oxygen isotopic composition (Δ17O) of non‐seasalt sulfate (NSS) aerosol was measured in samples from Alert, Canada over one year (July 1999–June 2000) and used to quantify the S(IV) oxidants. ...Measurements of Δ17O in NSS are used to evaluate the relative contributions of O3, H2O2, and OH oxidation leading to the formation of SO42− compared to a model of Feichter et al. (1996). The isotopic values suggest that there is a twofold overestimate of ozone oxidation in the model during winter. The isotopic composition is consistent with 10 to 18% contribution from a non‐photochemical oxidation pathway, likely Fe3+/Mn2+‐catalyzed O2 oxidation, during the dark Arctic winter. Isotopic evidence also invokes a 3 to 10% contribution of a mass dependent oxidant during springtime Arctic ozone depletion events.
Isotopic ($\delta^{17}$O and $\delta^{18}$O) measurements of stratospheric and mesospheric carbon dioxide (CO$_2$) and oxygen (O$_2$), along with trace species concentrations (N$_2$O, CO, and ...CO$_2$), were made in samples collected from a rocket-borne cryogenic whole air sampler. A large mass-independent isotopic anomaly was observed in CO$_2$, which may in part derive from photochemical coupling to ozone (O$_3$). The data also require an additional isotopic fractionation process, which is presently unidentified. Mesospheric O$_2$ isotope ratios differed from those in the troposphere and stratosphere. The cause of this isotopic variation in O$_2$ is presently unknown. The inability to account for these observations represents a fundamental gap in the understanding of the O$_2$ chemistry in the stratosphere and mesosphere.
The recent discovery of an anomalous enrichment in 17O isotope in atmospheric sulfate has opened a new way to investigate the oxidation pathways of sulfur in the atmosphere. From laboratory ...investigations, it has been suggested that the wet oxidation of sulfur in rain droplets was responsible for the excess 17O. In order to confirm this theory, sulfur and oxygen isotope ratios of different primary sulfates produced during fossil fuel combustion have been investigated and are reported. None of these samples exhibits any anomalous oxygen or sulfur isotopic content, as compared to urban sulfate aerosols. These results, in agreement with the laboratory investigations, reinforce the idea of an aqueous origin for the oxygen‐17 anomaly found in tropospheric sulfates.
PDF
