Kinetic fractionation

Résumé

Kinetic fractionation is an isotopic fractionation process that separates stable isotopes from each other by their mass during unidirectional processes. Biological processes are generally unidirectional and are very good examples of "kinetic" isotope reactions. All organisms preferentially use lighter isotopic species, because "energy costs" are lower, resulting in a significant fractionation between the substrate (heavier) and the biologically mediated product (lighter). As an example, photosynthesis preferentially takes up the light isotope of carbon 12C during assimilation of an atmospheric CO2 molecule. This kinetic isotope fractionation explains why plant material (and thus fossil fuels, which are derived from plants) is typically depleted in 13C by 25 per mil (2.5 per cent) relative to most inorganic carbon on Earth. A naturally occurring example of non-biological kinetic fractionation occurs during the evaporation of seawater to form clouds under conditions in which some part

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https://en.wikipedia.org/wiki/Kinetic_fractionation

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Flow velocities, residence times, and tracer breakthroughs at the lysimeter scale are affected by matrix properties and preferential flow. Despite their relevance to transport processes, however, the relative timing of preferential flow, and its link to transit times through the soil block, is still poorly described. Here we present and analyze tracer data from a 2.5m(3) vegetated lysimeter experiment where 18mm of isotopically labeled water was added as a pulse and then followed with a series of tracer-free controlled rainfall events for 5months. A solution of two fluorobenzoid acid tracers was also injected and tracked. Time series of soil water samples at three different depths and bottom drainage samples were collected and analyzed. Unlike past lysimeter experiments, a willow tree grown within the lysimeter exerted strong evapotranspiration fluxes. By comparative analysis of soil water and bottom drainage samples, we show the presence of both translatory and preferential flow features reflecting the interplay of slow vertical percolation and fast recharge through macropores. We found that water ponding and evaporating from the top of the lysimeter after irrigation prompted samples to be highly and irregularly fractionated. Comparative analyses of multitracer breakthroughs (adjusted by removing fractionation effects) showed that fluorobenzoid acid tracers reached the bottom of the lysimeter earlier than the isotopes, likely due to the effect of plant uptake. Our results underscore the essential role of models to interpret tracer behavior and, critically, the importance of future experiments aimed at measuring the ages of the water abstracted by vegetation.

2019

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Effect-directed identification of oxygen and sulfur heterocycles as major polycyclic aromatic cytochrome P4501A-Inducers in a contaminated sediment

Kristin Schirmer

Heterocyclic polyaromatic compounds, including dinaphthofurans, 2-(2-naphthalenyl)benzothiophene, methylated chrysene, and benz[a]anthracene, were identified and confirmed as major cytochrome P4501A (CYP1A)-inducing compounds in a contaminated sediment close to the industrial site of Bitterfeld (Germany). Identification was achieved by the application of an effect-directed fractionation and analysis approach. This approach comprised the combination of a rainbow trout liver cell line (RTL-W1) bioassay to select for CYP1A-inducing effects by measuring 7-ethoxyresorufin-O-deethylase activity, a multistep fractionation procedure, and various methods Of Chemical characterization. The identified nonpriority pollutants were found to be significantly more potent than the reference compound, benzo[a]pyrene, and among the most potent polycyclic inducers known. On the basis of the history of industrial activity at the contaminated site, the heterocyclic compounds identified in this study are thought to stem from 2-naphthol production. 2-Naphthol is one of the most high tonnage products of the aniline dye industry in general, thereby indicating the potential environmental relevance of the identified heterocyclic aromatic compounds. To date, however, no or very little knowledge exists about their occurrence, fate, and biological effects.

2003

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Aerosol hygroscopicity and CCN activation kinetics in a boreal forest environment during the 2007 EUCAARI campaign

Athanasios Nenes

Measurements of size-resolved cloud condensation nuclei (CCN) concentrations, subsaturated hygroscopic growth, size distribution, and chemical composition were collected from March through May, 2007, in the remote Boreal forests of Hyytiälä, Finland, as part of the European Integrated project on Aerosol Cloud Climate and Air Quality Interactions (EUCAARI) campaign. Hygroscopicity parameter, κ, distributions were derived independently from Continuous Flow-Streamwise Thermal Gradient CCN Chamber (CFSTGC) and Hygroscopicity Tandem Differential Mobility Analyzer (HTDMA) measurements. CFSTGC-derived κ values for 40, 60, and 80 nm particles range mostly between 0.10 and 0.40 with an average characteristic of highly oxidized organics of 0.20 ± 0.10, indicating that organics play a dominant role for this environment. HTDMA-derived κ were generally 30% lower. Diurnal trends of κ show a minimum at sunrise and a maximum in the late afternoon; this trend covaries with inorganic mass fraction and the m/z 44 organic mass fraction given by a quadrupole aerosol mass spectrometer, further illustrating the importance of organics in aerosol hygroscopicity. The chemical dispersion inferred from the observed κ distributions indicates that while 60 and 80 nm dispersion increases around midday, 40 nm dispersion remains constant. Additionally, 80 nm particles show a markedly higher level of chemical dispersion than both 40 and 60 nm particles. An analysis of droplet activation kinetics for the sizes considered indicates that most of the CCN activate as rapidly as (NH 4) 2SO 4 calibration aerosol. © 2011 Author(s).

2011

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