scholarly journals Quantifying the nitrogen isotope effects during photochemical equilibrium between NO and NO<sub>2</sub>: implications for <i>δ</i><sup>15</sup>N in tropospheric reactive nitrogen

2020 ◽  
Vol 20 (16) ◽  
pp. 9805-9819 ◽  
Author(s):  
Jianghanyang Li ◽  
Xuan Zhang ◽  
John Orlando ◽  
Geoffrey Tyndall ◽  
Greg Michalski
Keyword(s):  
Atmospheric Chemistry ◽  
Isotopic Exchange ◽  
Isotope Effects ◽  
Isotopic Fractionation ◽  
Field Measurements ◽  
Nitrogen Isotope ◽  
Dominant Factor ◽  
Reactive Nitrogen ◽  
Fractionation Factor ◽  
The Impact

Abstract. Nitrogen isotope fractionations between nitrogen oxides (NO and NO2) play a significant role in determining the nitrogen isotopic compositions (δ15N) of atmospheric reactive nitrogen. Both the equilibrium isotopic exchange between NO and NO2 molecules and the isotope effects occurring during the NOx photochemical cycle are important, but both are not well constrained. The nighttime and daytime isotopic fractionations between NO and NO2 in an atmospheric simulation chamber at atmospherically relevant NOx levels were measured. Then, the impact of NOx level and NO2 photolysis rate on the combined isotopic fractionation (equilibrium isotopic exchange and photochemical cycle) between NO and NO2 was calculated. It was found that the isotope effects occurring during the NOx photochemical cycle can be described using a single fractionation factor, designated the Leighton cycle isotope effect (LCIE). The results showed that at room temperature, the fractionation factor of nitrogen isotopic exchange is 1.0289±0.0019, and the fractionation factor of LCIE (when O3 solely controls the oxidation from NO to NO2) is 0.990±0.005. The measured LCIE factor showed good agreement with previous field measurements, suggesting that it could be applied in an ambient environment, although future work is needed to assess the isotopic fractionation factors of NO+RO2/HO2→NO2. The results were used to model the NO–NO2 isotopic fractionations under several NOx conditions. The model suggested that isotopic exchange was the dominant factor when NOx>20 nmol mol−1, while LCIE was more important at low NOx concentrations (<1 nmol mol−1) and high rates of NO2 photolysis. These findings provided a useful tool to quantify the isotopic fractionations between tropospheric NO and NO2, which can be applied in future field observations and atmospheric chemistry models.

scholarly journals Iron-Manganese System for Preparation of Radiocarbon Ams Targets: Characterization of Procedural Chemical-Isotopic Blanks and Fractionation

Radiocarbon ◽  
1997 ◽  
Vol 39 (3) ◽  
pp. 269-283 ◽  
Author(s):  
R. Michael Verkouteren ◽  
Donna B. Klinedinst ◽  
Lloyd A. Currie
Keyword(s):  
Sample Size ◽  
Isotope Effects ◽  
Isotopic Fractionation ◽  
Chemical Yield ◽  
Standard Reference Materials ◽  
Methane Formation ◽  
Fractionation Factor ◽  
Beam Geometry ◽  
C Fibers ◽  
Counting Statistics

We report a practical system to mass-produce accelerator mass spectrometry (AMS) targets with 10–100 μg carbon samples. Carbon dioxide is reduced quantitatively to graphite on iron fibers via manganese metal, and the Fe-C fibers are melted into a bead suitable for AMS. Pretreatment, reduction and melting processes occur in sealed quartz tubes, allowing parallel processing for otherwise time-intensive procedures.Chemical and isotopic (13C, 14C) blanks, target yields and isotopic fractionation were investigated with respect to levels of sample size, amounts of Fe and Mn, pretreatment and reduction time, and hydrogen pressure. With 7-day pretreatments, carbon blanks exhibited a lognormal mass distribution of 1.44 μg (central mean) with a dispersion of 0.50 μg (standard deviation). Reductions of 10 μg carbon onto targets were complete in 3–6 h with all targets, after correction for the blank, reflecting the 13C signature of the starting material. The 100 μg carbon samples required at least 15 h for reduction; shorter durations resulted in isotopic fractionation as a function of chemical yield. The trend in the 13C data suggested the presence of kinetic isotope effects during the reduction. The observed CO2-graphite 13C fractionation factor was 3–4% smaller than the equilibrium value in the simple Rayleigh model. The presence of hydrogen promoted methane formation in yields up to 25%.Fe-C beaded targets were made from NIST Standard Reference Materials and compared with graphitic standards. Although the 12C ion currents from the beads were one to two orders of magnitude lower than currents from the graphite, measurements of the beaded standards were reproducible and internally consistent. Measurement reproducibility was limited mainly by Poisson counting statistics and blank variability, translating to 14C uncertainties of 5–1% for 10–100 μg carbon samples, respectively. A bias of 5–7% (relative) was observed between the beaded and graphitic targets, possibly due to variations in sputtering fractionation dependent on sample size, chemical form and beam geometry.

scholarly journals Isotopic signatures of production and uptake of H<sub>2</sub> by soil

2015 ◽  
Vol 15 (22) ◽  
pp. 13003-13021 ◽  
Author(s):  
Q. Chen ◽  
M. E. Popa ◽  
A. M. Batenburg ◽  
T. Röckmann
Keyword(s):  
Isotope Effects ◽  
Deposition Velocity ◽  
Isotopic Fractionation ◽  
Isotopic Signature ◽  
Deuterium Content ◽  
Forest Site ◽  
Fractionation Factor ◽  
Isotopic Signatures ◽  
The Difference ◽  
Isotope Equilibrium

Abstract. Molecular hydrogen (H2) is the second most abundant reduced trace gas (after methane) in the atmosphere, but its biogeochemical cycle is not well understood. Our study focuses on the soil production and uptake of H2 and the associated isotope effects. Air samples from a grass field and a forest site in the Netherlands were collected using soil chambers. The results show that uptake and emission of H2 occurred simultaneously at all sampling sites, with strongest emission at the grassland sites where clover (N2 fixing legume) was present. The H2 mole fraction and deuterium content were measured in the laboratory to determine the isotopic fractionation factor during H2 soil uptake (αsoil) and the isotopic signature of H2 that is simultaneously emitted from the soil (δDsoil). By considering all net-uptake experiments, an overall fractionation factor for deposition of αsoil = kHD / kHH = 0.945 ± 0.004 (95 % CI) was obtained. The difference in mean αsoil between the forest soil 0.937 ± 0.008 and the grassland 0.951 ± 0.026 is not statistically significant. For two experiments, the removal of soil cover increased the deposition velocity (vd) and αsoil simultaneously, but a general positive correlation between vd and αsoil was not found in this study. When the data are evaluated with a model of simultaneous production and uptake, the isotopic composition of H2 that is emitted at the grassland site is calculated as δDsoil = (−530 ± 40) ‰. This is less deuterium depleted than what is expected from isotope equilibrium between H2O and H2.

scholarly journals Isotopic signatures of production and uptake of H<sub>2</sub> by soil

2015 ◽  
Vol 15 (17) ◽  
pp. 23457-23506 ◽  
Author(s):  
Q. Chen ◽  
M. E. Popa ◽  
A. M. Batenburg ◽  
T. Röckmann
Keyword(s):  
Isotope Effects ◽  
Deposition Velocity ◽  
Isotopic Fractionation ◽  
Isotopic Signature ◽  
Deuterium Content ◽  
Forest Site ◽  
Fractionation Factor ◽  
Isotopic Signatures ◽  
The Difference ◽  
Isotope Equilibrium

Abstract. Molecular hydrogen (H2) is the second most abundant reduced trace gas (after methane) in the atmosphere, but its biogeochemical cycle is not well understood. Our study focuses on the soil production and uptake of H2 and the associated isotope effects. Air samples from a grass field and a forest site in the Netherlands were collected using soil chambers. The results show that uptake and emission of H2 occurred simultaneously at all sampling sites, with strongest emission at the grassland sites where clover (N2 fixing legume) was present. The H2 mole fraction and deuterium content were measured in the laboratory to determine the isotopic fractionation factor during H2 soil uptake (αsoil) and the isotopic signature of H2 that is simultaneously emitted from the soil (δDsoil). By considering all net-uptake experiments, an overall fractionation factor for deposition of αsoil = kHD/kHH = 0.945 ± 0.004 (95 % CI) was obtained. The difference in mean αsoil between the forest soil 0.937 ± 0.008 and the grassland 0.951 ± 0.025 is not statistically significant. For two experiments, the removal of soil cover increased the deposition velocity (vd) and αsoil simultaneously, but a general positive correlation between vd and αsoil was not found in this study. When the data are evaluated with a model of simultaneous production and uptake, the isotopic composition of H2 that is emitted at the grassland site is calculated as δDsoil = (−530 ± 40) ‰. This is less deuterium-depleted than what is expected from isotope equilibrium between H2O and H2.

scholarly journals Examining the impact of heterogeneous nitryl chloride production on air quality across the United States

2012 ◽  
Vol 12 (2) ◽  
pp. 6145-6183 ◽  
Author(s):  
G. Sarwar ◽  
H. Simon ◽  
P. Bhave ◽  
G. Yarwood
Keyword(s):  
United States ◽  
Air Quality ◽  
Atmospheric Chemistry ◽  
Forest Fires ◽  
The United States ◽  
Anthropogenic Sources ◽  
Reactive Nitrogen ◽  
Nitryl Chloride ◽  
The Impact ◽  
Hydrolysis Of

Abstract. The heterogeneous hydrolysis of dinitrogen pentoxide (N2O5) has typically been modeled as only producing nitric acid. However, recent field studies have confirmed that the presence of particulate chloride alters the reaction product to produce nitryl chloride (ClNO2) which undergoes photolysis to generate chlorine atoms and nitrogen dioxide (NO2). Both chlorine and NO2 affect atmospheric chemistry and air quality. We present an updated gas-phase chlorine mechanism that can be combined with the Carbon Bond 05 mechanism and incorporate the combined mechanism into the Community Multiscale Air Quality modeling system. We then update the current model treatment of heterogeneous hydrolysis of N2O5 to include ClNO2 as a product. The model, in combination with a comprehensive inventory of chlorine compounds, reactive nitrogen, particulate matter, and organic compounds, is used to evaluate the impact of the heterogeneous ClNO2 production on air quality across the United States for the months of February and September in 2006. The heterogeneous production increases ClNO2 in coastal as well as many in-land areas in the United States. Particulate chloride derived from sea-salts, anthropogenic sources, and forest fires activates the heterogeneous production of ClNO2. With current estimates of tropospheric emissions burden, it modestly enhances monthly mean 8-h ozone (up to 1–2 ppbv or 3–4%) but causes large increases (up to 13 ppbv) in isolated episodes. It also substantially reduce the mean total nitrate by up to 0.8–2.0 μg m−3 or 11–21%. Modeled ClNO2 accounts for up to 3–4% of the monthly mean total reactive nitrogen. Sensitivity results of the model suggest that ClNO2 formation is limited more by the presence of particulate chloride than by the abundance of N2O5.

scholarly journals A simple modeling approach to study the regional impact of a Mediterranean forest isoprene emission on anthropogenic plumes

2004 ◽  
Vol 4 (6) ◽  
pp. 7691-7724 ◽  
Author(s):  
J. Cortinovis ◽  
F. Solmon ◽  
D. Serça ◽  
C. Sarrat ◽  
R. Rosset
Keyword(s):  
Atmospheric Chemistry ◽  
Field Measurements ◽  
Mediterranean Ecosystems ◽  
Tropospheric Chemistry ◽  
Ozone Production ◽  
Transfer Model ◽  
Data Sets ◽  
Isoprene Emission ◽  
On Line ◽  
The Impact

Abstract. Research over the past year has outlined the importance of biogenic isoprene emission in tropospheric chemistry, and notably in the context of regional ozone photo-oxidant pollution. The first part of this article deals with the development of a simple isoprene emission scheme based upon the classical Guenther's algorithm coupled with a soil-vegetation-atmosphere transfer model. The resulting emission scheme is tested in a "stand-alone" version at the canopy scale. Experimental data sets coming from Boreal, Tropical, Temperate and Mediterranean ecosystems are used to estimate the robustness of the scheme over contrasted climatic and ecological conditions. Considering the simple hypothesis used, simulated isoprene fluxes are generally consistent with field measurements and the emission scheme is thus deemed suitable for regional application. Limitations of the model are outlined as well as further improvements. In the second part of the article, the emission scheme is used on line in the broader context of a meso-scale atmospheric chemistry scheme. Dynamically idealized simulations are carried out to study the chemical interactions of pollutant plumes with realistic isoprene emissions coming from a Mediterranean oak forest. Two chemical scenarios are considered with anthropogenic emissions, respectively representative of the Marseille (urban) and Martigues (industrial) French Mediterranean areas. For the Marseille scenario, the impact of biogenic emission on ozone production is larger when the forest is situated in a sub-urban configuration (i.e. downwind distance TOWN-FOREST <30 km) and decrease quite rapidly as the distance increases. For the Martigues scenario, the biogenic impact on the plume is detectable even at a longer TOWN-FOREST distance of 100 km. For both cases, the importance of the VOC/NOx ratio, which characterizes the aging of advected pollutant plumes over the day, is outlined. Finally, possible applications of this work for real-case studies are discussed.

scholarly journals Viewpoint: Isotopic fractionation by plant nitrate reductase, twenty years later

2006 ◽  
Vol 33 (6) ◽  
pp. 531 ◽  
Author(s):  
Guillaume Tcherkez ◽  
Graham D. Farquhar
Keyword(s):  
Nitrate Reductase ◽  
Isotope Effect ◽  
Nitrate Reduction ◽  
Isotope Effects ◽  
Isotopic Fractionation ◽  
Nitrogen Isotope ◽  
Reaction Rates ◽  
Rate Limiting Step ◽  
18O Isotope ◽  
N Isotopes

Plant nitrate reductase, the enzyme that reduces nitrate (NO3–) to nitrite (NO2–), is known to fractionate N isotopes, depleting nitrite in 15N compared with substrate nitrate. Nearly 20 years ago, the nitrogen isotope effect associated with this reaction was found to be around 1.015. However, the relationships between the isotope effect and the mechanism of the reaction have not yet been examined in the light of recent advances regarding the catalytic cycle and enzyme structure. We thus give here the mathematical bases of the 14N / 15N and also the 16O / 18O isotope effects as a function of reaction rates. Enzymatic nitrate reduction involves steps other than NO3– reduction itself, in which the oxidation number of N changes from +V (nitrate) to +III (nitrite). Using some approximations, we give numerical estimates of the intrinsic N and O isotope effects and this leads us to challenge the assumptions of nitrate reduction itself as being a rate-limiting step within the nitrate reductase reaction, and of the formation of a bridging oxygen as a reaction intermediate.

Solvent and substrate isotope effects on the enolization and carbon-acid ionization of isobutyrophenone

1996 ◽  
Vol 74 (12) ◽  
pp. 2481-2486 ◽  
Author(s):  
J.R. Keeffe ◽  
A.J. Kresge
Keyword(s):  
Isotope Effect ◽  
Isotope Effects ◽  
Isotopic Fractionation ◽  
Medium Effect ◽  
Solvent Isotope Effect ◽  
Fractionation Factor ◽  
Solvent Isotope ◽  
Carbon Acid ◽  
Fractionation Factors ◽  
Acid Ionization

Bromine scavenging was used to measure rates of acid-catalyzed enolization of isobutyrophenone in H2O and in D2O solution and of isobutyrophenone-α-d in D2O solution. The results provide the solvent isotope effect kH +/kD + = 0.56 and the substrate isotope effect kH/kD = 6.2 on the enolization reaction, both of which are consistent with the generally accepted mechanism for this process. The present results in combination with literature information also provide the solvent isotope effect on the enolization equilibrium, KE(H2O)/KE(D2O) = 0.92, and the solvent isotope effect on the ionization of isobutyrophenone as a carbon acid, kaK(H2O)/kaK(D2O) = 5.4, as well as the product of isotopic fractionation factor and medium effect, [Formula: see text], for isobutyrophenone enol and the medium effect, Φ = 0.47, for its enolate ion. The isotope effect on KE is the first ever determined for the keto–enol equilibrium of a simple aldehyde or ketone; its near-unit value is consistent with expectation on the basis of fractionation factors for the species involved. Key words: isobutyrophenone, keto–enol equilibrium, carbon-acid ionization, solvent isotope effects, isotopic fractionation factors.

New Evidence for Symmetry Dependent Isotope Effects: O+CO Reaction

1989 ◽  
Vol 44 (5) ◽  
pp. 435-444 ◽  
Author(s):  
S. K. Bhattacharya ◽  
M. H. Thiemens
Keyword(s):  
Isotopic Exchange ◽  
Isotope Effects ◽  
Isotopic Fractionation ◽  
Large Mass ◽  
Reaction Rates ◽  
Kinetic Evaluation ◽  
Mass Independent Fractionation ◽  
New Evidence ◽  
Activated Complex ◽  
Fractionation Mechanism

The isotopic fractionation associated with the O + CO reaction has been studied using oxygen atoms produced by room temperature O2 photolysis at two different wavelengths, 185 and 130 nm. A large mass-independent isotopic fractionation is observed in the product CO2, extending the range of this type of reaction beyond O + O2 and SF5 + SF5. Kinetic evaluation of the data restricts the source of the mass-independent fractionation mechanism to the O + CO recombination step rather than O2 photolysis, secondary ozone formation, or O2 photodissociation. At least one, and most likely two other fractionation processes appear to occur in the experiments, and interpretation of the isotopic results is tentative at present. Based on the relevant reaction rates and the value for the reduced partition function for isotopic exchange between O and CO, it is suggested that this process may occur prior to the δ17O≅δ18O recombination process. Secondary CO2 photolysis may superimpose an additional fractionation. The experimental data are also examined in the context of a model based upon energy randomization rates versus the lifetime of the activated complex.

scholarly journals Photolytic control of the nitrate stable isotope signal in snow and atmosphere of East Antarctica and implications for reactive nitrogen cycling

2009 ◽  
Vol 9 (3) ◽  
pp. 12559-12596 ◽  
Author(s):  
M. M. Frey ◽  
J. Savarino ◽  
S. Morin ◽  
J. Erbland ◽  
J. M. F. Martins
Keyword(s):  
Isotopic Exchange ◽  
Isotopic Fractionation ◽  
Ice Cores ◽  
Skin Layer ◽  
Reactive Nitrogen ◽  
Lab Experiments ◽  
15N Enrichment ◽  
Oxygen Stable Isotopes ◽  
Significant Enrichment ◽  
Isotopic Signal

Abstract. The nitrogen (δ15N) and triple oxygen (δ17/18O) isotopic composition of nitrate (NO3−) was measured year-round in the atmosphere and snow pits at Dome C (DC, 75.1° S, 123.3° E), and in surface snow on a transect between DC and the coast. Snow pit profiles of δ15N (δ18O) in NO3− show significant enrichment (depletion) of >200 (<40) ‰ compared to the isotopic signal in atmospheric NO3−, whereas post-depositional fractionation in Δ17O(NO3−) is small, allowing reconstruction of past shifts in tropospheric oxidation pathways from ice cores. Assuming a Rayleigh-type process we find in the DC04 (DC07) pit fractionation factors ε of −50±10 (−71±12) ‰, 6±3 (9±2) ‰ and 1±0.2 (2±0.6) ‰, for δ15N, δ18O and Δ17O, respectively. A photolysis model reproduces ε for δ15N within the range of uncertainty at DC and for lab experiments reported by Blunier et al. (2005), suggesting that the current literature value for photolytic isotopic fractionation in snow is significantly underestimated. Depletion of oxygen stable isotopes is attributed to photolysis followed by isotopic exchange with water and hydroxyl radicals. Conversely, 15N enrichment of the NO3− fraction in the snow implies 15N depletion of emissions. Indeed, δ15N in atmospheric NO3− shows a strong decrease from background levels (4.4±6.8‰) to −35.1‰ in spring followed by recovery during summer, consistent with significant snow pack emissions of reactive nitrogen. Field and lab evidence therefore suggest that photolysis dominates fractionation and associated NO3− loss from snow in the low-accumulation regions of the East Antarctic Ice Sheet (EAIS). The Δ17O signature confirms previous coastal measurements that the peak of atmospheric NO3− in spring is of stratospheric origin. After sunrise photolysis drives then redistribution of NO3− from the snowpack photic zone to the atmosphere and a snow surface skin layer, thereby concentrating NO3− at the surface. Little NO3− is exported off the EAIS plateau, still snow emissions from as far as 600 km inland can contribute to the coastal NO3− budget.

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