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  • Other Sources  (14)
  • 2015-2019  (14)
  • 1
    Publication Date: 2018-11-27
    Description: The neodymium isotope proxy has become a valuable tool for the reconstruction of past ocean water mass provenance and mixing. For its accurate application, knowledge about the origin and preservation of Nd in sedimentary archives is crucial. Recently, concerns have emerged regarding the applicability of neodymium isotopes as a conservative palaeo water mass tracer, given potential Nd fluxes from sediments into bottom waters (Abbott et al., 2015a) and inferred relabelling of ocean waters by settling detrital material (Roberts and Piotrowski, 2015). Consequently, a decoupling of water mass provenance and proxy variations may arise. We investigate the mobility of Nd around extreme detrital sedimentation events such as glacial ice rafting pulses and turbidite deposition in the Northeast Atlantic. The constructed records from sediment leachates span extreme Nd isotope variations including volcanic (εNd ∼ 0) and Laurentian (εNd ∼ −27) sources. We find that Nd was released into pore waters from reactive detritus inside some detrital layers during early diagenesis, thereby overprinting any archived bottom water Nd signature and precluding the reconstruction of past water mass provenance during the affected time intervals. However, we do not observe any definite indication of diffusive vertical migration of Nd into adjacent layers. Furthermore, bottom water Nd isotope signatures were not modified to a measurable degree by any potential benthic flux of Nd during the deposition of these detrital sediment layers. Consequently, the Nd isotope composition of the pelagic glacial Northeast Atlantic water masses were resilient to such episodic large detrital fluxes. Apart from extreme local sedimentation events, we confirm the presence of detritally overprinted deep waters north of 47°N during the peak glacial from comparison of Northeast Atlantic depth transects. We furthermore suggest that the sensitivity of deep waters to this overprinting effect increased during periods of reduced Atlantic Meridional Overturning Circulation and elevated ice rafting. Overall, our study demonstrates that a thorough evaluation of the proportion of Nd originating from physical water mass advection versus in situ chemical inputs is crucial for the reliable application of Nd isotopes as a water mass tracer.
    Type: Article , PeerReviewed
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  • 2
    Publication Date: 2017-01-05
    Type: Conference or Workshop Item , NonPeerReviewed
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  • 3
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    In:  [Poster] In: International Conference on Paleoceanography 2016, 28.08.-02.09.2016, Utrecht, Netherlands .
    Publication Date: 2017-01-05
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  • 4
    Publication Date: 2018-05-16
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  • 5
    Publication Date: 2019-04-02
    Description: Highlights • Icelandic input of radiogenic Nd essentially limited to coastal waters • Offshore bottom water Nd isotope signatures consistent with conservative mixing of intermediate and deep water masses • Decreased bottom water Nd concentrations likely reflect removal by particle scavenging Radiogenic neodymium (Nd) isotopes have been widely used as a proxy for tracing present and past water masses and ocean circulation, yet relatively few data exist for seawater from the important deep water formation area around Iceland. We have analyzed the dissolved seawater Nd isotope compositions (expressed as ƐNd) of 71 seawater samples, as well as Nd concentrations [Nd] of 38 seawater samples, collected at full water column profiles from 18 stations in the shelf area off the southern coast of Iceland. The goal of this work was to determine to what extent weathering inputs from Icelandic basalts, which are characterized by a distinctly radiogenic ƐNd signature within the North Atlantic, contribute to the Nd isotope and concentration signatures of water masses in the northern Iceland Basin. Radiogenic ƐNd values of up to −3.5 and elevated concentrations of up to 21 pmol/kg compared to nearby open ocean sites were found in surface waters at shallow sites closest to shore and to river mouths of Iceland. This documents partial dissolution of highly radiogenic basaltic particles, which are transported northwards by the coastal currents. A comparable signal is not observed, however, in offshore surface waters likely as a result of the advection of surface currents mainly directed onshore, thus isolating these sites from Icelandic weathering contributions. The dominance of Subpolar Mode Waters and Intermediate Water unaffected by Icelandic contributions in the offshore study area is supported by unradiogenic ƐNd signatures between −15 and −12. In agreement with hydrographic data, highly radiogenic bottom waters at one site on the Iceland-Faroe Ridge (ƐNd = −7.5) reveal the presence of almost pure Iceland Scotland Overflow Water (ISOW) near its formation site further to the east. In bottom waters of all deeper offshore sites, the combination of depleted Nd concentrations and similar ƐNd values (averaging at ≃−11.75 for the R/V Poseidon data and ≃−11 for the R/V Thalassa data) confirms the rapid entrainment of Atlantic mid-depth and deep waters into the overflow waters, which is accompanied by near bottom Nd removal via particle scavenging. Overall, our findings demonstrate that at present, apart from the radiogenic isotope signature of ISOW itself, the direct contribution of radiogenic Nd originating from weathering of Iceland basalts to the water column of the Iceland Basin is limited. This supports the reliable application of ƐNd values to trace changes in the mixing of open North Atlantic water masses (including ISOW).
    Type: Article , PeerReviewed
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  • 6
    Publication Date: 2019-07-13
    Description: Glyoxal (CHOCHO) is produced in the atmosphere by the oxidation of volatile organic compounds(VOCs). Like formaldehyde (HCHO), another VOC oxidation product, it is measurable from space by solar backscatter. Isoprene emitted by vegetation is the dominant source of CHOCHO and HCHO in most of the world. We use aircraft observations of CHOCHO and HCHO from the Southeast Nexus (SENEX) campaign over the southeast US in summer 2013 to better understand the CHOCHO time-dependent yield from isoprene oxidation, its dependence on nitrogen oxides (NO (sub x) triple bonded to NO plus NO2), the behavior of the CHOCHO-HCHO relationship, the quality of Ozone Monitoring Instrument (OMI) CHOCHO satellite observations, and the implications for using CHOCHO observations from space as constraints on isoprene emissions. We simulate the SENEX and OMI observations with the Goddard Earth Observing System chemical transport model (GEOSChem) featuring a new chemical mechanism for CHOCHO formation from isoprene. The mechanism includes prompt CHOCHO formation under low-NO (sub x) conditions following the isomerization of the isoprene peroxy radical (ISOPO2).The SENEX observations provide support for this prompt CHOCHO formation pathway, and are generally consistent with the GEOS-Chem mechanism. Boundary layer CHOCHO and HCHO are strongly correlated in the observations and the model, with some departure under low-NO (sub x) conditions due to prompt CHOCHO formation. SENEX vertical profiles indicate a free-tropospheric CHOCHO background that is absent from the model. The OMI CHOCHO data provide some support for this free-tropospheric background and show southeast US enhancements consistent with the isoprene source but a factor of 2 too low. Part of this OMI bias is due to excessive surface reflectivities assumed in the retrieval. The OMI CHOCHO and HCHO seasonal data over the southeast US are tightly correlated and provide redundant proxies of isoprene emissions. Higher temporal resolution in future geostationary satellite observations may enable detection of the prompt CHOCHO production under low-NO (sub x) conditions apparent in the SENEX data.
    Keywords: Environment Pollution; Geophysics
    Type: GSFC-E-DAA-TN47279 , Atmospheric Chemistry and Physics (e-ISSN 1680-7324); 17; 14; 8725-8738
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  • 7
    Publication Date: 2019-07-13
    Description: Recent laboratory experiments have shown that a first generation isoprene oxidation product, ISOPOOH, can decompose to methyl vinyl ketone (MVK) and methacrolein (MACR) on instrument surfaces, leading to overestimates of MVK and MACR concentrations. Formaldehyde (HCHO) was suggested as a decomposition co-product, raising concern that in situ HCHO measurements may also be affected by an ISOPOOH interference. The HCHO measurement artifact from ISOPOOH for the NASA In Situ Airborne Formaldehyde instrument (ISAF) was investigated for the two major ISOPOOH isomers, (1,2)-ISOPOOH and (4,3)-ISOPOOH, under dry and humid conditions. The dry conversion of ISOPOOH to HCHO was 3+/-2% and 6+/-4% for (1,2)-ISOPOOH and (4,3)-ISOPOOH, respectively. Under humid (RH= 40-60%) conditions, conversion to HCHO was 6+/-4% for (1,2)-ISOPOOH and 10+/-5% for (4,3)-ISOPOOH. The measurement artifact caused by conversion of ISOPOOH to HCHO in the ISAF instrument was estimated for data obtained on the 2013 September 6 flight of the Studies of Emissions and Atmospheric Composition, Clouds and Climate Coupling by Regional Surveys (SEAC4RS) campaign. Prompt ISOPOOH conversion to HCHO was the source for 〈4% of the observed HCHO, including in the high-isoprene boundary layer. Time-delayed conversion, where previous exposure to ISOPOOH affects measured HCHO later in flight, was conservatively estimated to be 〈 10% of observed HCHO and is significant only when high ISOPOOH sampling periods immediately precede periods of low HCHO.
    Keywords: Environment Pollution
    Type: GSFC-E-DAA-TN41602 , Atmospheric Measurement Techniques (ISSN 1867-1381) (e-ISSN 1867-8548); 9; 9; 4561-4568
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  • 8
    Publication Date: 2019-07-13
    Description: Isoprene oxidation schemes vary greatly among gas-phase chemical mechanisms, with potentially significant ramifications for air quality modeling and interpretation of satellite observations in biogenic-rich regions. In this study, in situ observations from the 2013 SENEX mission are combined with a constrained O-D photochemical box model to evaluate isoprene chemistry among five commonly used gas-phase chemical mechanisms: CBO5, CB6r2, MCMv3.2, MCMv3.3.1, and a recent version of GEOS-Chem. Mechanisms are evaluated and inter-compared with respect to formaldehyde (HCHO), a high-yield product of isoprene oxidation. Though underestimated by all considered mechanisms, observed HCHO mixing ratios are best reproduced by MCMv3.3.1 (normalized mean bias = -15%), followed by GEOS-Chem (-17%), MCMv3.2 (-25%), CB6r2 (-32%) and CB05 (-33%). Inter-comparison of HCHO production rates reveals that major restructuring of the isoprene oxidation scheme in the Carbon Bond mechanism increases HCHO production by only approx. 5% in CB6r2 relative to CBO5, while further refinement of the complex isoprene scheme in the Master Chemical Mechanism increases HCHO production by approx. 16% in MCMv3.3.1 relative to MCMv3.2. The GEOS-Chem mechanism provides a good approximation of the explicit isoprene chemistry in MCMv3.3.1 and generally reproduces the magnitude and source distribution of HCHO production rates. We analytically derive improvements to the isoprene scheme in CB6r2 and incorporate these changes into a new mechanism called CB6r2-UMD, which is designed to preserve computational efficiency. The CB6r2-UMD mechanism mimics production of HCHO in MCMv3.3.1 and demonstrates good agreement with observed mixing ratios from SENEX (-14%). Improved simulation of HCHO also impacts modeled ozone: at approx. 0.3 ppb NO, the ozone production rate increases approx. 3% between CB6r2 and CB6r2-UMD, and rises another approx. 4% when HCHO is constrained to match observations.
    Keywords: Environment Pollution
    Type: GSFC-E-DAA-TN47241 , Atmospheric Environment (ISSN 1352-2310); 164; 325-336
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  • 9
    Publication Date: 2019-07-13
    Description: VOCs (Volatile Organic Compounds) related to oil and gas extraction operations in the United States were measured by H3O (sup plus) chemical ionization time-of-flight mass spectrometry (H3O (sup plus) ToFCIMS/PTR-ToF-MS (Time of Flight Chemical Ionization Mass Spectrometry/Proton Transfer Reaction-Time of Flight-Mass Spectroscopy) from aircraft during the Shale Oil and Natural Gas Nexus (SONGNEX) campaign in March-April 2015. This work presents an overview of major VOC species measured in nine oil- and gas-producing regions, and a more detailed analysis of H3O (sup plus) ToF-CIMS measurements in the Permian Basin within Texas and New Mexico. Mass spectra are dominated by small photochemically produced oxygenates and compounds typically found in crude oil: aromatics, cyclic alkanes, and alkanes. Mixing ratios of aromatics were frequently as high as those measured downwind of large urban areas. In the Permian, the H3O (sup plus) ToF-CIMS measured a number of underexplored or previously unreported species, including aromatic and cycloalkane oxidation products, nitrogen heterocycles including pyrrole (C4H5N) and pyrroline (C4H7N), H2S, and a diamondoid (adamantane) or unusual monoterpene. We additionally assess the specificity of a number of ion masses resulting from H3O (sup plus) ion chemistry previously reported in the literature, including several new or alternate interpretations.
    Keywords: Environment Pollution
    Type: GSFC-E-DAA-TN47232 , Atmospheric Measurement Techniques (e-ISSN 1867-8548); 10; 8; 2941-2968
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  • 10
    Publication Date: 2019-07-13
    Description: We use a 0-D photochemical box model and a 3-D global chemistry-climate model, combined with observations from the NOAA Southeast Nexus (SENEX) aircraft campaign, to understand the sources and sinks of glyoxal over the Southeast United States. Box model simulations suggest a large difference in glyoxal production among three isoprene oxidation mechanisms (AM3ST, AM3B, and Master Chemical Mechanism (MCM) v3.3.1). These mechanisms are then implemented into a 3-D global chemistry-climate model. Comparison with field observations shows that the average vertical profile of glyoxal is best reproduced by AM3ST with an effective reactive uptake coefficient gamma(sub glyx) of 2 x 10(exp -3) and AM3B without heterogeneous loss of glyoxal. The two mechanisms lead to 0-0.8micrograms m(exp -3) secondary organic aerosol (SOA) from glyoxal in the boundary layer of the Southeast U.S. in summer. We consider this to be the lower limit for the contribution of glyoxal to SOA, as other sources of glyoxal other than isoprene are not included in our model. In addition, we find that AM3B shows better agreement on both formaldehyde and the correlation between glyoxal and formaldehyde (RGF[GLYX]/[HCHO]), resulting from the suppression of delta-isoprene peroxy radicals (delta-ISOPO2). We also find that MCM v3.3.1 may underestimate glyoxal production from isoprene oxidation, in part due to an underestimated yield from the reaction of isoprene epoxydiol (IEPOX) peroxy radicals with HO2. Our work highlights that the gas-phase production of glyoxal represents a large uncertainty in quantifying its contribution to SOA.
    Keywords: Environment Pollution
    Type: GSFC-E-DAA-TN41635 , Journal of Geophysical Research: Atmospheres (ISSN 2169-897X) (e-ISSN 2169-8996); 121; 16; 9849-9861
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