scholarly journals i<sub>N</sub>RACM: Incorporating <sup>15</sup>N into the Regional Atmospheric Chemistry Mechanism (RACM) for assessing the role photochemistry plays in controlling the isotopic composition of NO<sub>x</sub>, NO<sub>y</sub>, and atmospheric nitrate.

2020 ◽  
Author(s):  
Greg Michalski ◽  
Huan Fang ◽  
Wendell W. Walters ◽  
David Mase
Keyword(s):  
Atmospheric Chemistry ◽  
Isotope Fractionation ◽  
Isotope Effects ◽  
Trace Gases ◽  
Organic Aerosols ◽  
Organic Nitrates ◽  
Tropospheric Photochemistry ◽  
Atmospheric Nitrate ◽  
N Compounds ◽  
Fractionation Factors

Abstract. Nitrogen oxides, classified as NOx (nitric oxide (NO) + nitrogen dioxide (NO2)) and NOy (NOx + NO3, N2O5 HNO3, + HNO4 + HONO + Peroxyacetyl nitrate (PAN) + organic nitrates + any oxidized N compound), are important trace gases in the troposphere, which play an important role in the formation of ozone, particulate matter (PM), and secondary organic aerosols (SOA). Among many uncertainties in movement of atmospheric N compounds, nowadays understanding of NOy cycling is limited by NOx emission budget, unresolved issues within the heterogeneous uptake coefficients of N2O5, the formation of organic nitrates in urban forests, etc. A photochemical mechanism used to simulate tropospheric photochemistry was altered to include 15N compounds and reactions as a means to simulate δ15N values in NOy compounds. The 16 N compounds and 96 reactions involving N used in Regional Atmospheric Chemistry Mechanism (RACM) were replicated using 15N in a new mechanism called iNRACM. The 192 N reactions in iNRACM were tested to see if isotope effects were relevant with respect to significantly changing the δ15N values (±1 ‰) of NOx, HONO, and/or HNO3. The isotope fractionation factors (α) for relevant reactions were assigned based on recent experimental or calculated values. Each relevant reaction in the iNRACM mechanism was tested individually and in concert in order to assess the controlling reactions. The final mechanism was tested by running simulations under different conditions that are typical of pristine, rural, urban, and highly polluted environments. The results of these simulations predicted several interesting δ15N variations.

scholarly journals i<sub>N</sub>RACM: incorporating <sup>15</sup>N into the Regional Atmospheric Chemistry Mechanism (RACM) for assessing the role photochemistry plays in controlling the isotopic composition of NO<sub><i>x</i></sub>, NO<sub><i>y</i></sub>, and atmospheric nitrate

2021 ◽  
Vol 14 (8) ◽  
pp. 5001-5022
Author(s):  
Huan Fang ◽  
Wendell W. Walters ◽  
David Mase ◽  
Greg Michalski
Keyword(s):  
Particulate Matter ◽  
Atmospheric Chemistry ◽  
Isotope Fractionation ◽  
Isotope Effects ◽  
Nox Emission ◽  
Organic Nitrates ◽  
Atmospheric Nitrate ◽  
N Isotopes ◽  
N Compounds ◽  
Fractionation Factors

Abstract. Nitrogen oxides, classified as NOx (nitric oxide (NO) + nitrogen dioxide (NO2)) and NOy (NOx+ NO3, N2O5 HNO3, + HNO4+ HONO + Peroxyacetyl nitrate (PAN) + organic nitrates + any oxidized N compound), are important trace gases in the troposphere, which play an important role in the formation of ozone, particulate matter (PM), and secondary organic aerosols (SOA). There remain many uncertainties in the origin and fate of atmospheric N compounds including the understanding of NOy cycling, NOx emission budgets, unresolved issues within the heterogeneous uptake coefficients of N2O5, and the formation of organic nitrates in urban forests, to name a few. A potential tool to resolve some of these uncertainties are using natural abundance N isotopes in NOy compounds. Here we have developed a photochemical mechanism used to simulate tropospheric photochemistry to include 15N compounds and reactions as a means to simulate δ15N values in NOy compounds. The 16 N compounds and 96 reactions involving N used in the Regional Atmospheric Chemistry Mechanism (RACM) were replicated using 15N in a new mechanism called iNRACM. The 192 N reactions in iNRACM were tested to see if isotope effects were relevant with respect to significantly changing the δ15N values (±1 ‰) of NOx, HONO, and/or HNO3. The isotope fractionation factors (α) for relevant reactions were assigned based on recent experimental or calculated values. Each relevant reaction in the iNRACM mechanism was tested individually and in concert in order to assess the controlling reactions. The controlling reactions and their diurnal importance are discussed. A comparison between iNRACM predictions and observed δ15N NO3- in particulate matter from Tucson, Arizona, suggests the model, and isotope fractionation factors incorporated into it, are accurately capturing the isotope effects occurring during the photochemistry of NOy. The implication is that measurements of δ15N in NOy compounds may be a new way of tracing in situ N chemistry and a means of assessing NOx emission budgets.

scholarly journals Theoretical Estimates of Equilibrium Carbon and Hydrogen Isotope Effects in Microbial Methane Production and Anaerobic Oxidation of Methane

2020 ◽  
Author(s):  
Jonathan Gropp ◽  
Mark Iron ◽  
Itay Halevy
Keyword(s):  
Isotopic Composition ◽  
Methane Production ◽  
Isotope Fractionation ◽  
Isotope Effects ◽  
Anaerobic Oxidation Of Methane ◽  
Anaerobic Oxidation ◽  
Oxidation Of Methane ◽  
Production And Consumption ◽  
Fractionation Factors ◽  
Equilibrium Fractionation

Microbial production and consumption of methane are widespread in natural and artificial environments, with important economic and climatic implications. Attempts to use the isotopic composition of methane to constrain its sources are complicated by incomplete understanding of the mechanisms of variation in methane's isotopic composition. Knowledge of the equilibrium isotope fractionations among the large organic intracellular intermediates in the microbial pathways of methane production and consumption must form the basis of any exploration of the mechanisms of isotopic variation, but estimates of these equilibrium isotope fractionations are currently unavailable. To address this gap, we calculated the equilibrium isotopic fractionation of carbon (<sup>13</sup>C/<sup>12</sup>C) and hydrogen (D/H) isotopes among compounds in anaerobic methane metabolisms, as well as the abundance of multiple isotope substitutions ("clumping," e.g., <sup>13</sup>C--D) in these compounds. The Density Functional Theory calculations employed the M06-L/def2-TZVP level of theory and the SMD implicit solvation model, which we have recently optimized for large organic molecules and tested against measured equilibrium isotope fractionations. The computed <sup>13</sup>beta and <sup>2</sup>beta values decrease with decreasing average oxidation state of the carbon atom in the molecules, resulting in a preference for enrichment of the molecules with more oxidized carbon in <sup>13</sup>C and D. Using the computed $\beta$ values, we calculated the equilibrium isotope fractionation factors in the prominent methanogenesis pathways (hydrogenotrophic, methylotrophic and acetoclastic) and in the pathway for anaerobic oxidation of methane (AOM) over a temperature range of 0-700 degrees Celsius. Our calculated equilibrium fractionation factors compare favorably with experimental constrains, where available, and we used them to investigate the relation between the apparent isotope fractionation during methanogenesis and AOM and the thermodynamic drive for these reactions. We show that a detailed map of the equilibrium fractionation factors along these metabolic pathways allows an evaluation of the contribution of equilibrium and kinetic isotope effects to apparent isotope fractionations observed in laboratory, natural and artificial settings. The comprehensive set of equilibrium isotope fractionation factors calculated in this study provides a firm basis for future explorations of isotope effects in methane metabolism.

scholarly journals Theoretical Estimates of Equilibrium Carbon and Hydrogen Isotope Effects in Microbial Methane Production and Anaerobic Oxidation of Methane

2020 ◽  
Author(s):  
Jonathan Gropp ◽  
Mark Iron ◽  
Itay Halevy
Keyword(s):  
Isotopic Composition ◽  
Methane Production ◽  
Isotope Fractionation ◽  
Isotope Effects ◽  
Anaerobic Oxidation Of Methane ◽  
Anaerobic Oxidation ◽  
Oxidation Of Methane ◽  
Production And Consumption ◽  
Fractionation Factors ◽  
Equilibrium Fractionation

Microbial production and consumption of methane are widespread in natural and artificial environments, with important economic and climatic implications. Attempts to use the isotopic composition of methane to constrain its sources are complicated by incomplete understanding of the mechanisms of variation in methane's isotopic composition. Knowledge of the equilibrium isotope fractionations among the large organic intracellular intermediates in the microbial pathways of methane production and consumption must form the basis of any exploration of the mechanisms of isotopic variation, but estimates of these equilibrium isotope fractionations are currently unavailable. To address this gap, we calculated the equilibrium isotopic fractionation of carbon (<sup>13</sup>C/<sup>12</sup>C) and hydrogen (D/H) isotopes among compounds in anaerobic methane metabolisms, as well as the abundance of multiple isotope substitutions ("clumping," e.g., <sup>13</sup>C--D) in these compounds. The Density Functional Theory calculations employed the M06-L/def2-TZVP level of theory and the SMD implicit solvation model, which we have recently optimized for large organic molecules and tested against measured equilibrium isotope fractionations. The computed <sup>13</sup>beta and <sup>2</sup>beta values decrease with decreasing average oxidation state of the carbon atom in the molecules, resulting in a preference for enrichment of the molecules with more oxidized carbon in <sup>13</sup>C and D. Using the computed $\beta$ values, we calculated the equilibrium isotope fractionation factors in the prominent methanogenesis pathways (hydrogenotrophic, methylotrophic and acetoclastic) and in the pathway for anaerobic oxidation of methane (AOM) over a temperature range of 0-700 degrees Celsius. Our calculated equilibrium fractionation factors compare favorably with experimental constrains, where available, and we used them to investigate the relation between the apparent isotope fractionation during methanogenesis and AOM and the thermodynamic drive for these reactions. We show that a detailed map of the equilibrium fractionation factors along these metabolic pathways allows an evaluation of the contribution of equilibrium and kinetic isotope effects to apparent isotope fractionations observed in laboratory, natural and artificial settings. The comprehensive set of equilibrium isotope fractionation factors calculated in this study provides a firm basis for future explorations of isotope effects in methane metabolism.

Hydrogen isotope fractionation between methanol and diphenylphosphine or dimethylphosphine in the gas phase and in aprotic solvents

1996 ◽  
Vol 74 (8) ◽  
pp. 1465-1469
Author(s):  
Andrzej Wawer ◽  
Jerzy Szydtlowski
Keyword(s):  
Interaction Energy ◽  
Gas Phase ◽  
Hydrogen Isotope ◽  
Isotope Fractionation ◽  
Isotope Effects ◽  
Solvent Polarity ◽  
Exchange Reactions ◽  
Fractionation Factor ◽  
The Given ◽  
Fractionation Factors

D/H fractionation factors between MeOH and Ph2PH in dilute solutions of tetrachloroethylene, benzene, tetrahydrofuran, pyridine, and acetonitrile and T/H fractionation factors between MeOH and Me2PH vapors were measured. The experimental results agree very well with values calculated from the statistical theory of isotope effects formulated by Bigeleisen and Mayer. There are correlations between observed fractionation factors and solvent polarity, and the interaction energy of methanol with the given solvent. Another correlation has been found between enthalpy of the exchange reactions and the interaction energy between methanol and the given solvent. Key words: isotope effects, fractionation factor, diphenylphosphine, methanol.

scholarly journals Theoretical Estimates of Equilibrium Carbon and Hydrogen Isotope Effects in Microbial Methane Production and Anaerobic Oxidation of Methane

2020 ◽  
Author(s):  
Jonathan Gropp ◽  
Mark Iron ◽  
Itay Halevy
Keyword(s):  
Isotopic Composition ◽  
Methane Production ◽  
Isotope Fractionation ◽  
Isotope Effects ◽  
Anaerobic Oxidation Of Methane ◽  
Anaerobic Oxidation ◽  
Oxidation Of Methane ◽  
Production And Consumption ◽  
Fractionation Factors ◽  
Equilibrium Fractionation

Microbial production and consumption of methane are widespread in natural and artificial environments, with important economic and climatic implications. Attempts to use the isotopic composition of methane to constrain its sources are complicated by incomplete understanding of the mechanisms of variation in methane's isotopic composition. Knowledge of the equilibrium isotope fractionations among the large organic intracellular intermediates in the microbial pathways of methane production and consumption must form the basis of any exploration of the mechanisms of isotopic variation, but estimates of these equilibrium isotope fractionations are currently unavailable. To address this gap, we calculated the equilibrium isotopic fractionation of carbon (<sup>13</sup>C/<sup>12</sup>C) and hydrogen (D/H) isotopes among compounds in anaerobic methane metabolisms, as well as the abundance of multiple isotope substitutions ("clumping," e.g., <sup>13</sup>C--D) in these compounds. The Density Functional Theory calculations employed the M06-L/def2-TZVP level of theory and the SMD implicit solvation model, which we have recently optimized for large organic molecules and tested against measured equilibrium isotope fractionations. The computed <sup>13</sup>beta and <sup>2</sup>beta values decrease with decreasing average oxidation state of the carbon atom in the molecules, resulting in a preference for enrichment of the molecules with more oxidized carbon in <sup>13</sup>C and D. Using the computed $\beta$ values, we calculated the equilibrium isotope fractionation factors in the prominent methanogenesis pathways (hydrogenotrophic, methylotrophic and acetoclastic) and in the pathway for anaerobic oxidation of methane (AOM) over a temperature range of 0-700 degrees Celsius. Our calculated equilibrium fractionation factors compare favorably with experimental constrains, where available, and we used them to investigate the relation between the apparent isotope fractionation during methanogenesis and AOM and the thermodynamic drive for these reactions. We show that a detailed map of the equilibrium fractionation factors along these metabolic pathways allows an evaluation of the contribution of equilibrium and kinetic isotope effects to apparent isotope fractionations observed in laboratory, natural and artificial settings. The comprehensive set of equilibrium isotope fractionation factors calculated in this study provides a firm basis for future explorations of isotope effects in methane metabolism.

scholarly journals Theoretical Estimates of Equilibrium Carbon and Hydrogen Isotope Effects in Microbial Methane Production and Anaerobic Oxidation of Methane

2020 ◽  
Author(s):  
Jonathan Gropp ◽  
Mark Iron ◽  
Itay Halevy
Keyword(s):  
Isotopic Composition ◽  
Methane Production ◽  
Isotope Fractionation ◽  
Isotope Effects ◽  
Anaerobic Oxidation Of Methane ◽  
Anaerobic Oxidation ◽  
Oxidation Of Methane ◽  
Production And Consumption ◽  
Fractionation Factors ◽  
Equilibrium Fractionation

Microbial production and consumption of methane are widespread in natural and artificial environments, with important economic and climatic implications. Attempts to use the isotopic composition of methane to constrain its sources are complicated by incomplete understanding of the mechanisms of variation in methane's isotopic composition. Knowledge of the equilibrium isotope fractionations among the large organic intracellular intermediates in the microbial pathways of methane production and consumption must form the basis of any exploration of the mechanisms of isotopic variation, but estimates of these equilibrium isotope fractionations are currently unavailable. To address this gap, we calculated the equilibrium isotopic fractionation of carbon (<sup>13</sup>C/<sup>12</sup>C) and hydrogen (D/H) isotopes among compounds in anaerobic methane metabolisms, as well as the abundance of multiple isotope substitutions ("clumping," e.g., <sup>13</sup>C--D) in these compounds. The Density Functional Theory calculations employed the M06-L/def2-TZVP level of theory and the SMD implicit solvation model, which we have recently optimized for large organic molecules and tested against measured equilibrium isotope fractionations. The computed <sup>13</sup>beta and <sup>2</sup>beta values decrease with decreasing average oxidation state of the carbon atom in the molecules, resulting in a preference for enrichment of the molecules with more oxidized carbon in <sup>13</sup>C and D. Using the computed $\beta$ values, we calculated the equilibrium isotope fractionation factors in the prominent methanogenesis pathways (hydrogenotrophic, methylotrophic and acetoclastic) and in the pathway for anaerobic oxidation of methane (AOM) over a temperature range of 0-700 degrees Celsius. Our calculated equilibrium fractionation factors compare favorably with experimental constrains, where available, and we used them to investigate the relation between the apparent isotope fractionation during methanogenesis and AOM and the thermodynamic drive for these reactions. We show that a detailed map of the equilibrium fractionation factors along these metabolic pathways allows an evaluation of the contribution of equilibrium and kinetic isotope effects to apparent isotope fractionations observed in laboratory, natural and artificial settings. The comprehensive set of equilibrium isotope fractionation factors calculated in this study provides a firm basis for future explorations of isotope effects in methane metabolism.

scholarly journals Unravelling the process of petroleum hydrocarbon biodegradation in different filter materials of constructed wetlands by stable isotope fractionation and labelling studies

2021 ◽  
Author(s):  
Andrea Watzinger ◽  
Melanie Hager ◽  
Thomas Reichenauer ◽  
Gerhard Soja ◽  
Paul Kinner
Keyword(s):  
Stable Isotope ◽  
Constructed Wetlands ◽  
Hydrogen Isotope ◽  
Petroleum Hydrocarbon ◽  
Isotope Fractionation ◽  
Isotope Effects ◽  
Filter Material ◽  
Hydrocarbon Biodegradation ◽  
Stable Isotope Fractionation ◽  
Filter Materials

AbstractMaintaining and supporting complete biodegradation during remediation of petroleum hydrocarbon contaminated groundwater in constructed wetlands is vital for the final destruction and removal of contaminants. We aimed to compare and gain insight into biodegradation and explore possible limitations in different filter materials (sand, sand amended with biochar, expanded clay). These filters were collected from constructed wetlands after two years of operation and batch experiments were conducted using two stable isotope techniques; (i) carbon isotope labelling of hexadecane and (ii) hydrogen isotope fractionation of decane. Both hydrocarbon compounds hexadecane and decane were biodegraded. The mineralization rate of hexadecane was higher in the sandy filter material (3.6 µg CO2 g−1 day−1) than in the expanded clay (1.0 µg CO2 g−1 day−1). The microbial community of the constructed wetland microcosms was dominated by Gram negative bacteria and fungi and was specific for the different filter materials while hexadecane was primarily anabolized by bacteria. Adsorption / desorption of petroleum hydrocarbons in expanded clay was observed, which might not hinder but delay biodegradation. Very few cases of hydrogen isotope fractionation were recorded in expanded clay and sand & biochar filters during decane biodegradation. In sand filters, decane was biodegraded more slowly and hydrogen isotope fractionation was visible. Still, the range of observed apparent kinetic hydrogen isotope effects (AKIEH = 1.072–1.500) and apparent decane biodegradation rates (k = − 0.017 to − 0.067 day−1) of the sand filter were low. To conclude, low biodegradation rates, small hydrogen isotope fractionation, zero order mineralization kinetics and lack of microbial biomass growth indicated that mass transfer controlled biodegradation.

scholarly journals Measurement on Diffusion Coefficients and Isotope Fractionation Factors by a Through-Diffusion Experiment

Minerals ◽  
2021 ◽  
Vol 11 (2) ◽  
pp. 208
Author(s):  
Takuma Hasegawa ◽  
Kotaro Nakata ◽  
Rhys Gwynne
Keyword(s):  
Groundwater Flow ◽  
Diffusion Coefficients ◽  
Isotope Fractionation ◽  
Free Water ◽  
Effective Porosity ◽  
Diffusion Experiment ◽  
Fractionation Factor ◽  
Through Diffusion ◽  
Geological Formations ◽  
Fractionation Factors

For radioactive waste disposal, it is important that local groundwater flow is slow as groundwater flow is the main transport medium for radioactive nuclides in geological formations. When the groundwater flow is very slow, diffusion is the dominant transport mechanism (diffusion-dominant domain). Key pieces of evidence indicating a diffusion-dominant domain are the separation of components and the fractionation of isotopes by diffusion. To prove this, it is necessary to investigate the different diffusion coefficients for each component and the related stable isotope fractionation factors. Thus, in this study, through-diffusion and effective-porosity experiments were conducted on selected artificial materials and natural rocks. We also undertook measurements relating to the isotope fractionation factors of Cl and Br isotopes for natural samples. For natural rock samples, the diffusion coefficients of water isotopes (HDO and H218O) were three to four times higher than those of monovalent anions (Cl−, Br- and NO3−), and the isotope fractionation factor of 37Cl (1.0017–1.0021) was slightly higher than that of free water. It was experimentally confirmed that the isotope fractionation factor of 81Br was approximately 1.0007–1.0010, which is equivalent to that of free water. The enrichment factor of 81Br was almost half that of 37Cl. The effective porosity ratios of HDO and Cl were slightly different, but the difference was not significant compared to the ratio of their diffusion coefficients. As a result, component separation was dominated by diffusion. For artificial samples, the diffusion coefficients and effective porosities of HDO and Cl were almost the same; it was thus difficult to assess the component separation by diffusion. However, isotope fractionation of Cl and Br was confirmed using a through-diffusion experiment. The results show that HDO and Cl separation and isotope fractionation of Cl and Br can be expected in diffusion-dominant domains in geological formations.

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