470 research outputs found
Knowledge-based probabilistic representations of branching ratios in chemical networks
AbstractWe present the use of Nested Dirichlet distributions to represent uncertain branching ratios in chemical networks. The interest is twofold: (1) to preserve the structure of experimental data by imposing sum-to-one representations; and (2) to be able to introduce totally unknown subsets of branching ratios (missing data). These points are central to sound uncertainty propagation and sensitivity analysis in complex chemical networks
Characterization of aromaticity in analogues of titan's atmospheric aerosols with two-step laser desorption ionization mass spectrometry
The role of polycyclic aromatic hydrocarbons (PAH) and Nitrogen containing
PAH (PANH) as intermediates of aerosol production in the atmosphere of Titan
has been a subject of controversy for a long time. An analysis of the
atmospheric emission band observed by the Visible and Infrared Mapping
Spectrometer (VIMS) at 3.28 micrometer suggests the presence of neutral
polycyclic aromatic species in the upper atmosphere of Titan. These molecules
are seen as the counter part of negative and positive aromatics ions suspected
by the Plasma Spectrometer onboard the Cassini spacecraft, but the low
resolution of the instrument hinders any molecular speciation.
In this work we investigate the specific aromatic content of Titan's
atmospheric aerosols through laboratory simulations. We report here the
selective detection of aromatic compounds in tholins, Titan's aerosol
analogues, produced with a capacitively coupled plasma in a N2:CH4 95:5 gas
mixture. For this purpose, Two-Step Laser Desorption Ionization Time-of-Flight
Mass Spectrometry (L2DI-TOF-MS) technique is used to analyze the so produced
analogues. This analytical technique is based on the ionization of molecules by
Resonance Enhanced Multi-Photon Ionization (REMPI) using a {\lambda}=248 nm
wavelength laser which is selective for aromatic species. This allows for the
selective identification of compounds having at least one aromatic ring. Our
experiments show that tholins contain a trace amount of small PAHs with one to
three aromatic rings. Nitrogen containing PAHs (PANHs) are also detected as
constituents of tholins. Molecules relevant to astrobiology are detected as is
the case of the substituted DNA base adenine
Nitrogen isotopic fractionation during abiotic synthesis of organic solid particles
The formation of organic compounds is generally assumed to result from
abiotic processes in the Solar System, with the exception of biogenic organics
on Earth. Nitrogen-bearing organics are of particular interest, notably for
prebiotic perspectives but also for overall comprehension of organic formation
in the young solar system and in planetary atmospheres. We have investigated
abiotic synthesis of organics upon plasma discharge, with special attention to
N isotope fractionation. Organic aerosols were synthesized from N2-CH4 and
N2-CO gaseous mixtures using low-pressure plasma discharge experiments, aimed
at simulating chemistry occurring in Titan s atmosphere and in the protosolar
nebula, respectively. Nitrogen is efficiently incorporated into the synthesized
solids, independently of the oxidation degree, of the N2 content of the
starting gas mixture, and of the nitrogen speciation in the aerosols. The
aerosols are depleted in 15N by 15-25 permil relative to the initial N2 gas,
whatever the experimental setup is. Such an isotopic fractionation is
attributed to mass-dependent kinetic effect(s). Nitrogen isotope fractionation
upon electric discharge cannot account for the large N isotope variations
observed among solar system objects and reservoirs. Extreme N isotope
signatures in the solar system are more likely the result of self-shielding
during N2 photodissociation, exotic effect during photodissociation of N2
and/or low temperature ion-molecule isotope exchange. Kinetic N isotope
fractionation may play a significant role in the Titan s atmosphere. We also
suggest that the low delta15N values of Archaean organic matter are partly the
result of abiotic synthesis of organics that occurred at that time
Simulating Titan's upper atmosphere and its photochemistry in the vacuum ultra-violet (VUV)
International audienceTitan, the largest moon of Saturn, has a dense atmosphere whose upper layers are mainly composed of methane (CH4) and molecular nitrogen (N2). The Cassini mission revealed that the interaction between those molecules and the solar VUV radiation, as well as the electrons from Saturn’s magnetosphere, leads to a complex chemistry above an altitude of 800km.Cassini instruments such as INMS or CAPS revealed that this naturally ionized environment contains heavy organic molecules like benzene (C6H6) even at altitudes higher than 900 k
Interaction dust-plasma in Titan's ionosphere: feedbacks on the gas phase composition
Titan's organic aerosols are formed in the ionosphere, a layer ionized by
solar VUV photons and energetic particles from the magnetosphere of Saturn,
forming a natural N2-CH4-H2 plasma. Previous works showed some chemical
evolution processes: VUV photons slightly alter the aerosols nitrile bands,
hydrogen atoms tend to hydrogenate their surface and carbon-containing species
participate to the growth of the aerosols. This work investigates the effect of
the other plasma species, namely the N2-H2 derived ions, radicals and excited
states. Industrial plasmas often use N2-H2 discharges to form ammonia-based
fertilizers, for metal nitriding, and to erode organic surfaces. Consequently,
these are likely to affect Titan's organic aerosols. We therefore developed the
THETIS experiment to study the interactions between analogues of Titan's
aerosols (tholins) and the erosive N2-H2 plasma species found in Titan's
ionosphere. Following a first paper on the evolution of the solid phase by
Scanning Electron Microscopy and IR transmission spectroscopy (Chatain et al.,
Icarus, 2020), this paper focuses on evolution of the gas phase composition, by
neutral and ion mass spectrometry. Newly formed HCN, NH3-CN and C2N2 are
extracted from the tholins as well as some other carbon-containing species and
their derived ions. On the other hand, the production of ammonia strongly
decreases, probably because the H, NH and N radicals are rather used for the
production of HCN at the surface of tholins. Heterogeneous processes are
suggested: chemical processes induced by radicals at the surface would modify
and weaken the tholin structure, while ion sputtering would desorb small
molecules and highly unsaturated ions. The effect of plasma erosion on aerosols
in Titan's ionosphere could therefore lead to the formation of CN bonds in the
aerosol structure and the production of HCN or R-CN species in the gas phase.Comment: This paper has been accepted in Icarus (February 2023). The current
version in arXiv is the submitted versio
Chemistry under EUV Irradiation of H-CO-N Gas Mixtures: Implications for Photochemistry in the Outer CSE of Evolved Stars
{CircumStellar Envelopes (CSEs) of stars are complex chemical objects for
which theoretical models encounter difficulties in elaborating a comprehensive
overview of the occurring chemical processes. Along with photodissociation,
ion-neutral reactions and dissociative recombination might play an important
role in controlling molecular growth in outer CSEs. The aim of this work is to
provide experimental insights into pathways of photochemistry-driven molecular
growth within outer CSEs to draw a more complete picture of the chemical
processes occurring within these molecule-rich environments. A simplified CSE
environment was therefore reproduced in the laboratory through gas-phase
experiments exposing relevant gas mixtures to an Extreme UltraViolet (EUV)
photon source. This photochemical reactor should ultimately allow us to
investigate chemical processes and their resulting products occurring under
conditions akin to outer CSEs. We used a recently developed EUV lamp coupled to
the APSIS photochemical cell to irradiate CSE relevant gas mixtures of H,
CO and N, at one wavelength, 73.6 nm. The detection and identification of
chemical species in the photochemical reactor was achieved through in-situ mass
spectrometry analysis of neutral and cationic molecules. We find that exposing
CO-N-H gas mixtures to EUV photons at 73.6 nm induces photochemical
reactions that yield the formation of complex, neutral and ionic species. Our
work shows that NH can be formed through photochemistry along with
highly oxygenated ion molecules like HCO in CSE environments. We also
observe neutral N-rich organic species including triazole and aromatic
molecules. These results confirm the suitability of our experimental setting to
investigate photochemical reactions and provide fundamental insights into the
mechanisms of molecular growth in the outer CSEs
Characterization of a DC glow discharge in N2-H2 with electrical measurements and neutral and ion mass spectrometry
The addition of small amounts of H2 were investigated in a DC glow discharge
in N2, at low pressure (~1 mbar) and low power (0.05 to 0.2 W.cm-3). We
quantified the electric field, the electron density, the ammonia production and
the formation of positive ions for amounts of H2 varying between 0 and 5%,
pressure values between 0.5 and 4 mbar, and currents between 10 and 40 mA. The
addition of less than 1% H2 has a strong effect on the N2 plasma discharges.
Hydrogen quenches the (higher) vibrational levels of N2 and some of its highly
energetic metastable states. This leads to the increase of the discharge
electric field and consequently of the average electron energy. As a result,
higher quantities of radical and excited species are suspected to be produced.
The addition of hydrogen also leads to the formation of new species. In
particular, ammonia and hydrogen-bearing ions have been observed: N2H+ and NH4+
being the major ones, and also H3+, NH+, NH2+, NH3+, N3H+ and N3H3+. The
comparison to a radiofrequency capacitively coupled plasma (RF CCP) discharge
in similar experimental conditions shows that both discharges led to similar
observations. The study of N2-H2 discharges in the laboratory in the adequate
ionization conditions then gives some insights on which plasma species made of
nitrogen and hydrogen could be present in the ionosphere of Titan. Here, we
identified some protonated ions, which are reactive species that could
participate to the erosion of organic aerosols on Titan.Comment: Paper accepted in Plasma Sources Science and Technology in March
2023. The current version on arXiv is the submitted versio
Gaseous chemistry for a Titan's atmospheric plasma experimental simulation
We present the first study of gaseous composition monitoring for the PAMPRE experiment, which simulates Titan's atmospheric chemistry by radio-frequency N 2-CH 4 plasma. Methane consumption is quantified for various N 2-CH 4 gas mixtures. Moreover in situ mass spectrometry (MS) and ex-situ gas chromatography coupled with mass spectrometry (GC-MS) analyses reveal a large dominance of nitrile species in the gas phase chemistry
N2-H2 capacitively coupled radio-frequency discharges at low pressure: II. Modeling results: The relevance of plasma-surface interaction
In this work, we present the results of simulations carried out for N2-H2 capacitively coupled radio-frequency discharges, running at low pressure (0.3-0.9 mbar), low power (5-20 W), and for amounts of H2 up to 5%. Simulations are performed using a hybrid code that couples a two-dimensional time-dependent fluid module, describing the dynamics of the charged particles in the discharge, to a zero-dimensional kinetic module, that solves the Boltzmann equation and describes the production and destruction of neutral species. The model accounts for the production of several vibrationally and electronic excited states, and contains a detailed surface chemistry that includes recombination processes and the production of NH x molecules. The results obtained highlight the relevance of the interactions between plasma and surface, given the role of the secondary electron emission in the electrical parameters of the discharge and the critical importance of the surface production of ammonia to the neutral and ionic chemistry of the discharge.The Portuguese Foundation sponsored this research for Science and Technology (FCT) in the framework of the
Strategic Funding UID/FIS/04650/2019
Seasonal riverine inputs may affect diet and mercury bioaccumulation in Arctic coastal zooplankton
Climate change driven increases in permafrost thaw and terrestrial runoff are expected to facilitate the mobilization and transport of mercury (Hg) from catchment soils to coastal areas in the Arctic, potentially increasing Hg exposure of marine food webs. The main aim of this study was to determine the impacts of seasonal riverine inputs on land-ocean Hg transport, zooplankton diet and Hg bioaccumulation in an Arctic estuary (Adventfjorden, Svalbard). The Adventelva River was a source of dissolved and particulate Hg to Adventfjorden, especially in June and July during the river's main discharge period. Stable isotope and fatty acid analyses suggest that zooplankton diet varied seasonally with diatoms dominating during the spring phytoplankton bloom in May and with increasing contributions of dinoflagellates in the summer months. In addition, there was evidence of increased terrestrial carbon utilization by zooplankton in June and July, when terrestrial particles contributed substantially to the particulate organic matter pool. Total (TotHg) and methyl Hg (MeHg) concentrations in zooplankton increased from April to August related to increased exposure to riverine inputs, and to shifts in zooplankton diet and community structure. Longer and warmer summer seasons will probably increase riverine runoff and thus Hg exposure to Arctic zooplankton.Seasonal riverine inputs may affect diet and mercury bioaccumulation in Arctic coastal zooplanktonpublishedVersio
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