Uptake Of NO3 Research Articles

Uptake of NO2 to Deliquesced Dihydroxybenzoate Aerosol Particles

The uptake of nitrogen dioxide (NO2), a major trace gas in the atmosphere, to deliquesced particles containing the sodium salts of hydroquinone (1,4-dihydroxybenzene) or gentisic (2,5-dihydroxybenzoic) acid was investigated at 40% relative humidity and 23 degrees C in an aerosol flow tube. The experiments were performed using the short-lived radioactive tracer 13N and a denuder technique. The observed uptake coefficient for NO2 was up to approximately 6 x 10(-3) for the hydroquinone disodium salt aerosol, which exceeds previously reported data in the range 10(-4) to 10(-3). The measured time dependence of NO2 uptake was fitted using a kinetic model taking into account reactant consumption in the particle phase, and keeping the bulk accommodation coefficient, alpha(b), and the rate constants for the reaction of dissolved NO2 with the deprotonated forms of the mentioned phenolic compounds as variables. We obtained alpha(b) = 0.024(-0.003)(+0.018) as a best estimate. For gentisic acid, the second-order rate constant was k2 = (2.9 +/- 0.1) x 10(8) L mol(-1) s(-1) and is reported for the first time. The data are consistent with bulk reaction limited uptake, without indications for a surface component in the kinetics.

Salt marsh ecosystem biogeochemical responses to nutrient enrichment: a paired 15N tracer study

We compared processing and fate of dissolved NO3- in two New England salt marsh ecosystems, one receiving natural flood tide concentrations of approximately 1-4 micromol NO3-/ L and the other receiving experimentally fertilized flood tides containing approximately 70-100 micromol NO3-/ L. We conducted simultaneous 15NO3- (isotope) tracer additions from 23 to 28 July 2005 in the reference (8.4 ha) and fertilized (12.4 ha) systems to compare N dynamics and fate. Two full tidal cycles were intensively studied during the paired tracer additions. Resulting mass balances showed that essentially 100% (0.48-0.61 mol NO3-N.ha(-1).h(-1)) of incoming NO3- was assimilated, dissimilated, sorbed, or sedimented (processed) within a few hours in the reference system when NO3- concentrations were 1.3-1.8 micromol/L. In contrast, only 50-60% of incoming NO3- was processed in the fertilized system when NO3- concentrations were 84-96 micromol/L; the remainder was exported in ebb tidewater. Gross NO3- processing was approximately 40 times higher in the fertilized system at 19.34-24.67 mol NO3-N.ha(-1).h(-1). Dissimilatory nitrate reduction to ammonium was evident in both systems during the first 48 h of the tracer additions but <1% of incoming 15NO3- was exported as 15NH4+. Nitrification rates calculated by 15NO3- dilution were 6.05 and 4.46 mol.ha(-1).h(-1) in the fertilized system but could not be accurately calculated in the reference system due to rapid (<4 h) NO3- turnover. Over the five-day paired tracer addition, sediments sequestered a small fraction of incoming NO3-, although the efficiency of sequestration was 3.8% in the reference system and 0.7% in the fertilized system. Gross sediment N sequestration rates were similar at 13.5 and 12.6 mol.ha(-1).d(-1), respectively. Macrophyte NO3- uptake efficiency, based on tracer incorporation in aboveground tissues, was considerably higher in the reference system (16.8%) than the fertilized system (2.6%), although bulk uptake of NO3- by plants was lower in the reference system (1.75 mol NO3-.ha(-1).d(-1)) than the fertilized system (approximately 10 mol NO3-.ha(-1).d(-1)). Nitrogen processing efficiency decreased with NO3- load in all pools, suggesting that the nutrient processing capacity of the marsh ecosystem was exceeded in the fertilized marsh.

Radicals in the marine boundary layer during NEAQS 2004: a model study of day-time and night-time sources and sinks

Abstract. This paper describes a modelling study of several HOx and NOx species (OH, HO2, organic peroxy radicals, NO3 and N2O5) in the marine boundary layer. A model based upon the Master Chemical Mechanism (MCM) was constrained to observations of chemical and physical parameters made onboard the NOAA ship R/V Brown as part of the New England Air Quality Study (NEAQS) in the summer of 2004. The model was used to calculate [OH] and to determine the composition of the peroxy radical pool. Modelled [NO3] and [N2O5] were compared to in-situ measurements by Cavity Ring-Down Spectroscopy. The comparison showed that the model generally overestimated the measurements by 30–50%, on average. The model results were analyzed with respect to several chemical and physical parameters, including uptake of NO3 and N2O5 on fog droplets and on aerosol, dry deposition of NO3 and N2O5, gas-phase hydrolysis of N2O5 and reactions of NO3 with NMHCs and peroxy radicals. The results suggest that fog, when present, is an important sink for N2O5 via rapid heterogeneous uptake. The comparison between the model and the measurements were consistent with values of the heterogeneous uptake coefficient of N2O5 (γN2O5)&gt;1×10−2, independent of aerosol composition in this marine environment. The analysis of the different loss processes of the nitrate radical showed the important role of the organic peroxy radicals, which accounted for a significant fraction (median: 15%) of NO3 gas-phase removal, particularly in the presence of high concentrations of dimethyl sulphide (DMS).

Increased Oxidative-nitrative Stress In The Hypoxic Human Brain

Oxidative stress may be responsible for the reduction in systemic nitric oxide (NO) bioavailability and impaired neurovascular reactivity recently observed in hypoxia (DM Bailey et al., J. Physiol. in-the-press). However, the brain's contribution to these systemic changes in redox homeostasis remains unknown. PURPOSE: The present study determined the trans-cerebral exchange kinetics of oxidative-nitrative stress biomarkers in response to hypoxia. Specifically, we hypothesized that hypoxia would increase the net cerebral output of free radicals that would inactivate NO as indicated by a decrease in the net uptake or consumption of nitrite (NO2) and increased output of 3-nitrotyrosine (3-NT). METHODS: Ten healthy males aged 27 (mean) ± 4 (SD) years were examined in normoxia (N: 20.9%O2) and following 9h passive exposure to hypoxia (H: 12.9%O2). Global cerebral blood flow (CBF) was measured using the Kety-Schmidt technique with paired samples obtained from the radial artery and jugular venous bulb. Global cerebral plasma flow (CPF) was determined as CBF × (1-hematocrit). The serum concentration of spin-trapped a-phenyl-tert-butylnitrone (PBN)-adducts was assessed via X-band electron paramagnetic resonance spectroscopy. Plasma NO2 was determined by ozone-based chemiluminescence using modified tri-iodide reagent and 3-NT via ELISA. Trans-cerebral net exchange was calculated as the arterio-jugular venous concentration difference x CPF. Data were analyzed with a two-way repeated measures ANOVA and post-hoc Bonferroni-corrected paired samples t-tests. RESULTS: Despite a marked reduction in PaO2 (N: 107 ± 6 to H: 46 ± 3 mmHg, P < 0.05), the cerebral metabolic rate for O2 was preserved (N: 2.4 ± 0.5 vs.H: 2.5 ± 0.3 μmol/g/min) due to a trend towards increased CBF (N: 85 ± 15 to H: 94 ± 17 mL/100g/min). Hypoxia increased the net cerebral output of PBN-adducts from -73 ± 192 to -360 ± 253 AU/min/g (P < 0.05). This was associated with an attenuation in the net uptake of NO2 (N: 126.4 ± 93.9 to H: 16.0 ± 46.7 nmol/min/g, P < 0.05) and increased output of 3-NT (N: -3.1 ± 9.0 to H: -10.7 ± 18.2 nmol/min/g, P < 0.05). CONCLUSION: These findings provide direct evidence for increased oxidative-nitrative stress across the hypoxic human brain and suggest that neurovascular NO bioavailability may be subject to redox-regulation.

DOC and N2O dynamics in upland and peatland forest soils after clear-cutting and soil preparation

Forest clear-cutting followed by soil preparation means disturbance for soil microorganisms and disruption of N and C cycles. We measured fluxes of N2O and dissolved organic carbon (DOC) in upland soil (podzol) and adjacent peat within a clear-cut forest catchment. Both soil types behaved in a similar way, showing net uptake of N2O in the first year after the clear-cutting, and turning to net release in the second. The N2O flux dynamics were similar to those of N content in logging residues, as reported from a nearby site. As organic matter is used in the food web of the decomposers, we attempted to explain the dynamics of N2O uptake and release by measuring the concurrent dynamics of the low molecular weight (LMW) fraction and the aromaticity of DOC in a soil solution. The labile and most readily available LMW fractions of DOC were nearly absent in the year following the clear-cutting, but rose after two years. The more refractory high molecular weight (HMW) fraction of DOC decreased two years after the clear-cutting. The first year’s net uptake of N2O could be accounted for by the growth of decomposer biomass in the logging residues and detritus from the degenerating ground vegetation, resulting in immobilization of nitrogen. Simultaneously, the labile, LMW fraction of DOC became almost completely exhausted. The low availability of the LMW fraction could retard the growth and cause the accumulated decomposer biomass to collapse. During the following winter and summer the fraction of LMW clearly increased, followed by increased N2O emissions. The presence of LMW DOC fractions, not the concentration of DOC, seems to be an important controller for N2O liberation after a major disturbance such as clear-cutting and site preparation. The complex connection between DOC characteristics, nitrification or denitrification merits further studies.

Transcerebral Exchange Kinetics of Nitrite and Calcitonin Gene-Related Peptide in Acute Mountain Sickness

High-altitude headache is the primary symptom associated with acute mountain sickness, which may be caused by nitric oxide-mediated activation of the trigeminovascular system. Therefore, the present study examined the effects of inspiratory hypoxia on the transcerebral exchange kinetics of the vasoactive molecules, nitrite (NO(2)(*)), and calcitonin gene-related peptide (CGRP). Ten males were examined in normoxia and after 9-hour exposure to hypoxia (12.9% O(2)). Global cerebral blood flow was measured by the Kety-Schmidt technique with paired samples obtained from the radial artery and jugular venous bulb. Plasma CGRP and NO(2)(*) were analyzed via radioimmunoassay and ozone-based chemiluminescence. Net cerebral exchange was calculated by the Fick principle and acute mountain sickness/headache scores assessed via clinically validated questionnaires. Hypoxia increased cerebral blood flow with a corresponding increase in acute mountain sickness and headache scores (P<0.05 vs normoxia). Hypoxia blunted the cerebral uptake of NO(2)(*), whereas CGRP exchange remained unaltered. No relationships were observed between the change (hypoxia-normoxia) in cerebral NO(2)(*) or CGRP exchange and acute mountain sickness/headache scores (P>0.05). These findings argue against sustained trigeminovascular system activation as a significant event in acute mountain sickness.

Drought turns a Central European Norway spruce forest soil from an N2O source to a transient N2O sink

AbstractBased on current climate scenarios, a higher frequency of summer drought periods followed by heavy rainfall events is predicted for Central Europe. It is expected that drying/rewetting events induce an increased matter cycling in soils and may contribute considerably to increased emissions of the greenhouse gas N2O on annual scales. To investigate the influence of drying/rewetting events on N2O emissions in a mature Norway spruce forest in the Fichtelgebirge area (NE Bavaria, Germany), a summer drought period of 46 days was induced by roof installations on triplicate plots, followed by a rewetting event of 66 mm experimental rainfall in 2 days. Three nonmanipulated plots served as controls. The experimentally induced soil drought was accompanied by a natural drought. During the drought period, the soil of both the throughfall exclusion and control plots served as an N2O sink. This was accompanied by subambient N2O concentrations in upper soil horizons. The sink strength of the throughfall exclusion plots was doubled compared with the control plots. We conclude that the soil water status together with the soil nitrate availability was an important driving factor for the N2O sink strength. Rewetting quickly turned the soil into a source for atmospheric N2O again, but it took almost 4 months to turn the cumulative soil N2O fluxes from negative (sink) to positive (source) values. N2O concentration and isotope analyses along soil profiles revealed that N2O produced in the subsoil was subsequently consumed during upward diffusion along the soil profile throughout the entire experiment. Our results show that long drought periods can lead to drastic decreases of N2O fluxes from soils to the atmosphere or may even turn forest soils temporarily to N2O sinks. Accumulation of more field‐scale data on soil N2O uptake as well as a better understanding of underlying mechanisms would essentially advance our knowledge of the global N2O budget.

Removal of Nitrite Ions from Aqueous Solutions Using Conducting Electroactive Polymers

This paper deals with a new application of polypyrrole (PPy) and polyaniline (PAni) electroactive polymers synthesized chemically, coated on sawdust and then used for removal of NO2 - ions from aqueous solutions. Adsorption studies were performed using both batch and column systems at room temperature. The effects of some important parame- ters such as initial concentration, adsorbent dosage, and contact time on the uptake of NO2 - ions were also investigated. PPy and PAni conducting electroactive polymers were found to be effective adsorbents for the uptake of nitrite ions from aqueous solutions. Adsorption studies have shown that pH of the nitrite solution has an important influence on the nitrite ion uptake. In order to find out the possibility of the frequent use of the adsorbent, desorption study was also carried out in this investigation.

Kinetic Modeling of NOx Storage/Reduction on Pt/BaO/Al2O3 Monolith Catalysts

A balance of the complexity of the reactor model and the chemical reaction mechanism has to be made in order to predict the dynamic nature of NOx storage/reduction processes in real time. In this work, a one-dimensional, two-phase model is used to simulate the transient behavior of a monolithic Pt/BaO/Al2O3 catalyst for NOx storage/reduction. The following aspects of the process are discussed: (i) kinetics of NO and NO2 adsorption on BaO sites, (ii) effects of CO2 and H2O on NO/NO2 adsorption, and (iii) reduction of surface nitrates using H2. NOx adsorption with excess oxygen involves two kinetic routes, namely, NO2 disproportionation and direct NO adsorption, both of which form nitrates on the catalyst at 300 °C. A model with two time scales was found to be necessary to describe NO2 adsorption on the 20 wt % BaO catalyst. The model and parameters required to fit the NOx breakthrough curves suggest that CO2 and H2O in the feed reduce the number of sites for NO adsorption by changing the surface morphology of the Ba phase. The rate constants for both fast and slow NO2 uptake are decreased in the presence of CO2 and H2O, but the total capacity remains the same. Under reaction conditions, H2 reduction of surface NOx is limited by the supply of the reductant; that is, the rate of surface NOx removal is limited by the flux of inlet H2. NH3 serves as the reducing intermediate/H carrier during the H2 reduction process. The confined reduction front moving along the channel localizes the heat generation, thus leading to a surface temperature in the reduction front about 35 °C higher than the inlet gas temperature for our reaction conditions.

Whole‐Stream Response to Nitrate Loading in Three Streams Draining Agricultural Landscapes

Physical, chemical, hydrologic, and biologic factors affecting nitrate (NO3(-)) removal were evaluated in three agricultural streams draining orchard/dairy and row crop settings. Using 3-d "snapshots" during biotically active periods, we estimated reach-level NO3(-) sources, NO3(-) mass balance, in-stream processing (nitrification, denitrification, and NO3(-) uptake), and NO3(-) retention potential associated with surface water transport and ground water discharge. Ground water contributed 5 to 11% to stream discharge along the study reaches and 8 to 42% of gross NO3(-) input. Streambed processes potentially reduced 45 to 75% of ground water NO3(-) before discharge to surface water. In all streams, transient storage was of little importance for surface water NO3(-) retention. Estimated nitrification (1.6-4.4 mg N m(-2) h(-1)) and unamended denitrification rates (2.0-16.3 mg N m(-2) h(-1)) in sediment slurries were high relative to pristine streams. Denitrification of NO3(-) was largely independent of nitrification because both stream and ground water were sources of NO3(-). Unamended denitrification rates extrapolated to the reach-scale accounted for <5% of NO3(-) exported from the reaches minimally reducing downstream loads. Nitrate retention as a percentage of gross NO3(-) inputs was >30% in an organic-poor, autotrophic stream with the lowest denitrification potentials and highest benthic chlorophyll a, photosynthesis/respiration ratio, pH, dissolved oxygen, and diurnal NO3(-) variation. Biotic processing potentially removed 75% of ground water NO3(-) at this site, suggesting an important role for photosynthetic assimilation of ground water NO3(-) relative to subsurface denitrification as water passed directly through benthic diatom beds.

THE RELATIVE IMPORTANCE OF WATER MOTION ON NITROGEN UPTAKE BY THE SUBTIDAL MACROALGA ADAMSIELLA CHAUVINII (RHODOPHYTA) IN WINTER AND SUMMER(1).

The influence of seawater velocity (1.5-12 cm · s(-1) ) on inorganic nitrogen (N) uptake by the soft-sediment perennial macroalga Adamsiella chauvinii (Harv.) L. E. Phillips et W. A. Nelson (Rhodophyta) was determined seasonally by measuring uptake rate in a laboratory flume. Regardless of N tissue content, water velocity had no influence on NO3 (-) uptake in either winter or summer, indicating that NO3 (-) -uptake rate was biologically limited. However, when thalli were N limited, increasing water velocity increased NH4 (+) uptake, suggesting that mass-transfer limitation of NH4 (+) is likely during summer for natural populations. Uptake kinetics (Vmax , Ks ) were similar among three populations of A.chauvinii at sites with different mean flow speeds; however, uptake rates of NO3 (-) and NH4 (+) were lower in summer (when N status was generally low) than in winter. Our results highlight how N uptake can be affected by seasonal changes in the physiology of a macroalga and that further investigation of N uptake of different macroalgae (red, brown, and green) during different seasons is important in determining the relative influence of water velocity on nutrient uptake.

Reactive Uptake of NO3, N2O5, NO2, HNO3, and O3on Three Types of Polycyclic Aromatic Hydrocarbon Surfaces

We investigated the reactive uptake of NO3, N2O5, NO2, HNO3, and O3 on three types of solid polycyclic aromatic hydrocarbons (PAHs) using a coated wall flow tube reactor coupled to a chemical ionization mass spectrometer. The PAH surfaces studied were the 4-ring systems pyrene, benz[a]anthracene, and fluoranthene. Reaction of NO3 radicals with all three PAHs was observed to be very fast with the reactive uptake coefficient, gamma, ranging from 0.059 (+0.11/-0.049) for benz[a]anthracene at 273 K to 0.79 (+0.21/-0.67) for pyrene at room temperature. In contrast to the NO3 reactions, reactions of the different PAHs with the other gas-phase species (N2O5, NO2, HNO3, and O3) were at or below the detection limit (gamma <or= 6.6 x 10(-5)) in all cases, illustrating that these reactions are at best slow. For NO3 we also investigated the time dependence of the reactive uptake to determine if the surface-bound PAH molecules were active participants in the reaction (i.e., reactants). Reaction of NO3 on all three PAH surfaces slowed down at 263 K after long NO3 exposure times, suggesting that the PAH molecules were reactants. Additionally, NO2 and HNO3 were identified as major gas-phase products. Our results show that under certain atmospheric conditions, NO3 radicals can be a more important sink for PAHs than NO2, HNO3, N2O5, or O3 and impact tropospheric lifetimes of surface-bound PAHs.

Photoenhanced uptake of NO2 on mineral dust: Laboratory experiments and model simulations

Mineral dust contains material such as TiO2 that is well known to have photocatalytic activity. In this laboratory study, mixed TiO2‐SiO2, Saharan dust and Arizona Test Dust were exposed to NO2 in a coated wall flow tube reactor. While uptake in the dark was negligible, photoenhanced uptake of NO2 was observed on all samples. For the mixed TiO2‐SiO2, the uptake coefficients increased with increasing TiO2 mass fraction, with BET uptake coefficients ranging from 0.12 to 1.9 × 10−6. HONO was observed from all samples, with varying yields, e.g., 80% for Saharan dust. Three‐dimensional modeling indicates that photochemistry of dust may reduce the NO2 level up to 37% and ozone up to 5% during a dust event in the free troposphere.

Plant nitrogen dynamics in fertilized and natural New England salt marshes: a paired 15N tracer study

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 354:35-46 (2008) - DOI: https://doi.org/10.3354/meps07170 Plant nitrogen dynamics in fertilized and natural New England salt marshes: a paired 15N tracer study D. C. Drake1,3,*, B. J. Peterson1, L. A. Deegan1, L. A. Harris1, E. E. Miller2, R. S. Warren2 1The Ecosystems Center, Marine Biological Laboratory, 7 MBL Street, Woods Hole, Massachusetts 02543, USA 2Connecticut College, Box 5362, New London, Connecticut 06320, USA 3Present address: National Institute for Water and Atmospheric Research, 10 Kyle Street, Riccarton, Christchurch, New Zealand *Email: d.drake@niwa.co.nz ABSTRACT: We examined the effects of increased nutrient availability on nitrogen (N) dynamics in dominant New England salt marsh plants (tall and stunted Spartina alterniflora and S. patens) using paired large-scale nutrient and 15NO3– tracer additions. This study is one component of a long-term, large-scale, salt marsh nutrient and trophic manipulation study (the Trophic Cascades and Interacting Control Processes in a Detritus-based Aquatic Ecosystem [TIDE] Project). We compared physiological variables of plants in fertilized (~17× ambient N and P in incoming tidal water) and reference marsh systems to quantify NO3– uptake and uptake efficiency, allocation of N to tissues, end-of-season N resorption, leaf litter quality and other potential responses to increased nutrient availability. Reference system plants sequestered ~24.5 g NO3-N ha–1 d–1 in aboveground pools during mid-summer, while fertilized plants sequestered ~140 g NO3-N ha–1 d–1. However, NO3– uptake efficiency (% of total incoming NO3-N sequestered aboveground) was higher in the reference system (16.8%) than in the fertilized system (2.6%), suggesting that our fertilization rate (~70 µM NO3– in incoming water) approaches or exceeds the uptake saturation point for this vegetation community. Leaf litter quality was clearly affected by N availability; N resorption efficiency was lower in all plants of the fertilized system; senesced leaves from the fertilized creek contained ~43% (tall S. alterniflora), 23% (stunted S. alterniflora) and 15% (S. patens) more N per unit biomass than reference creek leaves. KEY WORDS: Spartina alterniflora · Spartina patens · Plant ecophysiology · Eutrophication · Nitrogen isotopes · Nitrogen cycling · Marsh ecosystem Full text in pdf format PreviousNextCite this article as: Drake DC, Peterson BJ, Deegan LA, Harris LA, Miller EE, Warren RS (2008) Plant nitrogen dynamics in fertilized and natural New England salt marshes: a paired 15N tracer study. Mar Ecol Prog Ser 354:35-46. https://doi.org/10.3354/meps07170 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 354. Online publication date: February 07, 2008 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2008 Inter-Research.

Discontinuities in stream nutrient uptake below lakes in mountain drainage networks

In many watersheds, lakes and streams are hydrologically linked in spatial patterns that influence material transport and retention. We hypothesized that lakes affect stream nutrient cycling via modifications to stream hydrogeomorphology, source‐waters, and biological communities. We tested this hypothesis in a lake district of the Sawtooth Mountains, Idaho. Uptake of NO3− and PO4−3 was compared among 25 reaches representing the following landscape positions: lake inlets and outlets, reaches &gt;1‐km downstream from lakes, and reference reaches with no nearby lakes. We quantified landscape‐scale hydrographic and reach‐scale hydrogeomorphic, source‐water, and biological variables to characterize these landscape positions and analyze relationships to nutrient uptake. Nitrate uptake was undetectable at most lake outlets, whereas PO4−3 uptake was higher at outlets as compared to reference and lake inlet reaches. Patterns in nutrient demand farther downstream were similar to lake outlets with a gradual shift toward reference‐reach functionality. Nitrate uptake was most correlated to sediment mobility and channel morphology, whereas PO4−3 uptake was most correlated to source‐water characteristics. The best integrated predictor of these patterns in nutrient demand was % contributing area (the proportion of watershed area not routing through a lake). We estimate that NO3− and PO4−3 demand returned to 50% of pre‐lake conditions within 1‐4‐km downstream of a small headwater lake and resetting of nutrient demand was slower downstream of a larger lake set lower in a watershed. Full resetting of these nutrient cycling processes was not reached within 20‐km downstream, indicating that lakes can alter stream ecosystem functioning at large spatial scales throughout mountain watersheds.

The Role of Metal Contact in the Sensitivity of Single-Walled Carbon Nanotubes to NO2

The interaction of NO2 with single-walled C nanotubes and highly oriented pyrolitic graphite was studied at different temperatures by using high-energy resolution core level photoemission spectroscopy. During NO2 uptake below 200 K, the C1s peak exhibits a similar behavior for the two samples and correspondent NOx (x = 1−3) adspecies form on the surface of nanotubes and graphite. These results indicate the occurrence of equivalent chemical reactions and of comparable charge transfer from the flat and curved C lattice to the oxidizing adsorbates. Starting from 200 K, no NO2 adsorption onand therefore no interaction withC nanotubes takes place. This excludes the possibility that any tube−adsorbate charge transfer influencing the nanotube conductivity occurs above this temperature. However, at 200 K NO2 strongly interacts with Rh nanoclusters dispersed among the C nanotube bundles. The increased work function of the oxidized Rh changes the alignment of the nanotube bands and simulates an efficient hole dopin...

Kinetics of NO3‐ net fluxes in Pinus pinaster, Rhizopogon roseolus and their ectomycorrhizal association, as affected by the presence of NO3‐ and NH4+

The impact of mineral N supply, N-free or NO3(-) with or without NH4+, on the subsequent uptake of NO3(-) by maritime pine seedlings associated with the ectomycorrhizal fungus Rhizopogon roseolus was studied using ion-selective microelectrodes. NO3(-) net fluxes into N-starved non-mycorrhizal short roots (NMSRs) were low and measurable only over the NO3(-) concentration range of 0-70 microM. The simple kinetics observed in those roots may reflect the dominant operation of a high-affinity NO3(-) transport system (HATS) which is constitutive. NO3(-) pretreatment increased the NO3(-) net fluxes and led to a complex kinetics that may reflect the operation of other HATS. A simple kinetics was observed in plants pre-incubated at high NH4+ concentration. In contrast, NO3(-) uptake kinetics presented only one saturation phase in the fungus, whether associated with the plant or not. NO3(-) uptake was greater after a pretreatment in N-free or NO3 (-) solution, but NH4+ pretreatment led to a threefold reduction in NO3 (-) uptake. These results suggest that the regulation of NO3(-) transport systems varies between the host and the fungal partner. This variation is likely to contribute to the positive effect of mycorrhizal association on N uptake in plants when the N supply is low and fluctuating.

The dependence of NO2 and CO2 uptake and release on BaO content for NOx storage catalysts made using different precursor salts

Here we compare the uptake of CO2 on BaO–Al2O3 with that of NO2 for nine samples made by one, two or three incipient wetness impregnations using acetate and nitrates as the precursor salts. The amounts of CO2 and NO2 adsorbed to the point of breakthrough at 300°C are similar, proportional to the BaO content and largely independent of the preparation method. The breakthrough of CO2 is sharp. Its desorption occurs over a wide range of temperatures. Displacement by NO2 will remove the one-quarter retained at 550°C. Unlike the adsorption of CO2, the uptake of NO2 continues for prolonged period after initial breakthrough. It occurs with disproportionation of NO2 to gaseous NO and stored nitrate. Measurement of the overall O:N ratio in the gaseous decomposition products shows that the material stored to the point of breakthrough retains the overall stoichiometry NO2. It decomposes in two stages, initially by disproportionation to NO and nitrate followed by decomposition of the latter to NO and O2 in the ratio 4:3 starting at 450°C. The O:N ratio of material stored after prolonged exposure corresponds to nitrate. In accordance with the literature, these species decompose to NO2 and a small amount of O2 with maximum evolution rates below 500°C and to NO and O2 in the 4:3 ratio above that. The amount of NO2 produced is largely independent of the BaO content while that of NO is proportional to it.

Uptake of NO3 on soot and pyrene surfaces

The reaction of NO3 with methane soot, hexane soot, and solid pyrene was investigated using a flow tube reactor. The uptake of NO3 on fresh soot was fast (uptake coefficient &gt;0.1). Based on this result and an assumed density of reactive sites on soot, the time to process or oxidize 90% of a soot surface in the atmosphere would take only approximately five minutes. This suggests that NO3 chemistry can rapidly oxidize soot surfaces under atmospheric conditions. After exposing soot films to NO3 for approximately 180 minutes in the laboratory, the uptake reaches a steady‐state value. The steady state uptake coefficients (assuming a geometric surface area) were 0.0054 ± 0.0027 and 0.0025 ± 0.0018 for methane and hexane soot, respectively. These numbers are used to show that heterogeneous reactions between NO3 and soot are not likely a significant sink of gas‐phase NO3 under most atmospheric conditions. The uptake of NO3 on fresh pyrene surfaces was also fast (uptake coefficient &gt;0.1), and much faster than previously suggested. We argue that under certain atmospheric conditions reactions between NO3 and surface‐bound polycyclic aromatic hydrocarbons (PAHs) may be an important loss process of PAHs in the atmosphere.

Size‐fractionated nitrogen uptake measurements in the equatorial Pacific and confirmation of the low Si–high‐nitrate low‐chlorophyll condition

The equatorial Pacific Ocean is the largest natural source of CO2 to the atmosphere, and it significantly impacts the global carbon cycle. Much of the large flux of upwelled CO2 to the atmosphere is due to incomplete use of the available nitrate (NO3) and low net productivity. This high‐nutrient low‐chlorophyll (HNLC) condition of the equatorial upwelling zone (EUZ) has been interpreted from modeling efforts to be due to low levels of silicate (Si(OH)4) that limit the new production of diatoms. These ideas were incorporated into an ecosystem model, CoSINE. This model predicted production by the larger phytoplankton and the picoplankton and effects on air‐sea CO2 fluxes in the Pacific Ocean. However, there were no size‐fractionated rates available for verification. Here we report the first size‐fractionated new and regenerated production rates (obtained with 15N−NO3 and 15N−NH4 incubations) for the EUZ with the objective of validating the conceptual basis and functioning of the CoSINE model. Specifically, the larger phytoplankton (with cell diameters &gt; 5 μm) had greater rates of new production and higher f‐ratios (i.e., the proportion of NO3 to the sum of NO3 and NH4 uptake) than the picoplankton that had high rates of NH4 uptake and low f‐ratios. The way that the larger primary producers are regulated in the EUZ is discussed using a continuous chemostat approach. This combines control of Si(OH)4 production by supply rate (bottom‐up) and control of growth rate (or dilution) by grazing (top‐down control).