scholarly journals Dissociation, recombination and attachment processes in the upper atmosphere—I

1937 ◽  
Vol 163 (915) ◽  
pp. 542-553 ◽  
Keyword(s):  
Reaction Rates ◽  
Upper Atmosphere ◽  
Radio Waves ◽  
Ionization Density ◽  
Pressure Discharge ◽  
Mechanical Methods ◽  
Order Of Magnitude ◽  
Quantitative Manner ◽  
The Individual ◽  
Atmospheric Ionization

The study of the properties of the earth’s upper atmosphere has now progressed so far as to provide what should be a sufficient basis for the development of a detailed theory. Since the state of the upper atmosphere approximates closely to that of the gas in a low-pressure discharge tube (except for the absence of solid boundaries), it is clear that such a theory must deal with the individual collision processes which can occur in such a system. Until the last few years no satisfactory theory of these phenomena was available, but it is now possible to apply quantum mechanical methods with reasonable expectation of results accurate at least as regards order of magnitude. We therefore propose to make use of these methods to obtain a deeper understanding of the physics of the ionosphere. In this paper we confine ourselves particularly to the qualitative study of certain problems associated with the two upper ionized layers (the E and F regions), making use of information already available concerning the probabilities of the various collision reactions which are important. The detailed evaluation of these reaction rates is being carried out, and in later papers it is hoped to deal with the various problems in a more nearly quantitative manner. The two main strata of atmospheric ionization are the E region extending roughly from 120 to 160 km. and the F region from 180 to 300 km., at night. During the day each splits into two distinct strata forming the E 1 and E 2 and the F 1 and F 2 regions. The ionization density in each region, as determined from experiments with radio waves, exhibits characteristic annual and diurnal variations besides irregular variations of considerable magnitude. The first problem which arises is the reason for the existence of the stratification. This being understood it is then necessary to account for the observed variations of density, the daytime splitting of the layers, and so on.

scholarly journals Acetylcholinesterase active centre and gorge conformations analysed by combinatorial mutations and enantiomeric phosphonates

2003 ◽  
Vol 373 (1) ◽  
pp. 33-40 ◽  
Author(s):  
Zrinka KOVARIK ◽  
Zoran RADIĆ ◽  
Harvey A. BERMAN ◽  
Vera SIMEON-RUDOLF ◽  
Elsa REINER ◽  
...  
Keyword(s):  
Reaction Rates ◽  
Side Chain ◽  
Active Centre ◽  
Catalytic Potential ◽  
Steady State Kinetic ◽  
Order Of Magnitude ◽  
Small Change ◽  
The Individual ◽  
State Kinetic ◽  
Absolute Stereochemistry

A series of eight double and triple mutants of mouse acetylcholinesterase (AChE; EC 3.1.1.7), with substitutions corresponding to residues found largely within the butyrylcholinesterase (BChE; EC 3.1.1.8) active-centre gorge, was analysed to compare steady-state kinetic constants for substrate turnover and inhibition parameters for enantiomeric methylphosphonate esters. The mutations combined substitutions in the acyl pocket (Phe295→Leu and Phe297→Ile) with the choline-binding site (Tyr337→Ala and Phe338→Ala) and with a side chain (Glu202→ Gln) N-terminal to the active-site serine, Ser203. The mutations affected catalysis by increasing Km and decreasing kcat, but these constants were typically affected by an order of magnitude or less, a relatively small change compared with the catalytic potential of AChE. To analyse the constraints on stereoselective phosphonylation, the mutant enzymes were reacted with a congeneric series of SP- and RP-methylphosphonates of known absolute stereochemistry. Where possible, the overall reaction rates were deconstructed into the primary constants for formation of the reversible complex and intrinsic phosphonylation. The multiple mutations greatly reduced the reaction rates of the more reactive SP-methylphosphonates, whereas the rates of reaction with the RP-methylphosphonates were markedly enhanced. With the phosphonates of larger steric bulk, the enhancement of rates for the RP enantiomers, coupled with the reduction of the SP enantiomers, was sufficient to invert markedly the enantiomeric preference. The sequence of mutations to enlarge the size of the AChE active-centre gorge, resembling in part the more spacious gorge of BChE, did not show an ordered conversion into BChE reactivity as anticipated for a rigid template. Rather, the individual aromatic residues may mutually interact to confer a distinctive stereospecificity pattern towards organophosphates.

Kinetics of esterification of monoisopropylphenols with phosphoryl trichloride

1989 ◽  
Vol 54 (5) ◽  
pp. 1311-1317
Author(s):  
Miroslav Magura ◽  
Ján Vojtko ◽  
Ján Ilavský
Keyword(s):  
Liquid Phase ◽  
Rate Constants ◽  
Reaction Rates ◽  
Activation Energies ◽  
The Individual ◽  
Kinetics Of

The kinetics of liquid-phase isothermal esterification of POCl3 with 2-isopropylphenol and 4-isopropylphenol have been studied within the temperature intervals of 110 to 130 and 90 to 110 °C, respectively. The rate constants and activation energies of the individual steps of this three-step reaction have been calculated from the values measured. The reaction rates of the two isomers markedly differ: at 110 °C 4-isopropylphenol reacts faster by the factors of about 7 and 20 for k1 and k3, respectively. This finding can be utilized in preparation of mixed triaryl phosphates, since the alkylation mixture after reaction of phenol with propene contains an excess of 2-isopropylphenol over 4-isopropylphenol.

scholarly journals Statistical Uncertainty of DNS in Geometries without Homogeneous Directions

2021 ◽  
Vol 11 (4) ◽  
pp. 1399
Author(s):  
Jure Oder ◽  
Cédric Flageul ◽  
Iztok Tiselj
Keyword(s):  
Channel Flow ◽  
A Priori ◽  
Time Integration ◽  
Navier Stokes ◽  
Statistical Uncertainty ◽  
Time Step ◽  
Order Of Magnitude ◽  
Averaging Time ◽  
The Individual ◽  
Time Required

In this paper, we present uncertainties of statistical quantities of direct numerical simulations (DNS) with small numerical errors. The uncertainties are analysed for channel flow and a flow separation case in a confined backward facing step (BFS) geometry. The infinite channel flow case has two homogeneous directions and this is usually exploited to speed-up the convergence of the results. As we show, such a procedure reduces statistical uncertainties of the results by up to an order of magnitude. This effect is strongest in the near wall regions. In the case of flow over a confined BFS, there are no such directions and thus very long integration times are required. The individual statistical quantities converge with the square root of time integration so, in order to improve the uncertainty by a factor of two, the simulation has to be prolonged by a factor of four. We provide an estimator that can be used to evaluate a priori the DNS relative statistical uncertainties from results obtained with a Reynolds Averaged Navier Stokes simulation. In the DNS, the estimator can be used to predict the averaging time and with it the simulation time required to achieve a certain relative statistical uncertainty of results. For accurate evaluation of averages and their uncertainties, it is not required to use every time step of the DNS. We observe that statistical uncertainty of the results is uninfluenced by reducing the number of samples to the point where the period between two consecutive samples measured in Courant–Friedrichss–Levy (CFL) condition units is below one. Nevertheless, crossing this limit, the estimates of uncertainties start to exhibit significant growth.

scholarly journals The return of radio waves from the middle atmosphere—I

1937 ◽  
Vol 161 (905) ◽  
pp. 181-196 ◽  
Keyword(s):  
Physical Characteristic ◽  
Middle Atmosphere ◽  
Upper Atmosphere ◽  
Radio Waves ◽  
Sharp Distinction ◽  
Fundamental Significance ◽  
High Ionization ◽  
Collisional Damping

One of us introduced the name “Ionosphere” to designate that region of the upper atmosphere of which the most prominent physical characteristic was the occurrence of sustained high ionization densities, and which was, in consequence, of fundamental significance in the propagation of radio waves. The name, after finding its way into many languages, has been formally adopted by the Union Radio Scientifique Internationale for international use, and is now commonly applied to the region of the atmosphere above the first 90 km. It is an object of this present paper to show that this sharp distinction, although very broadly justified, is less happy than might have been hoped. It has been customary to regard the return of radio waves of measurable intensity from regions sensibly below 90-100 km. as very improbable in any save exceptional conditions, the collisional damping at lower levels being believed to ensure severe attenuation of such waves as might otherwise be returned from any temporarily densely ionized regions at moderate levels, while at still lower levels the rate of recombination seemed likely to prevent the maintenance of substantial ionization densities.

scholarly journals An integrated microfluidic platform for quantifying drug permeation across biomimetic vesicle membranes

2019 ◽  
Author(s):  
Michael Schaich ◽  
Jehangir Cama ◽  
Kareem Al Nahas ◽  
Diana Sobota ◽  
Kevin Jahnke ◽  
...  
Keyword(s):  
Drug Transport ◽  
Membrane Composition ◽  
Drug Permeability ◽  
Bacterial Membranes ◽  
Drug Molecules ◽  
Total Analysis System ◽  
Transport Assay ◽  
Order Of Magnitude ◽  
Analysis System ◽  
The Individual

The low membrane permeability of candidate drug molecules is a major challenge in drug development and insufficient permeability is one reason for the failure of antibiotic treatment against bacteria. Quantifying drug transport across specific pathways in living systems is challenging since one typically lacks knowledge of the exact lipidome and proteome of the individual cells under investigation. Here, we quantify drug permeability across biomimetic liposome membranes, with comprehensive control over membrane composition. We integrate the microfluidic octanol-assisted liposome assembly platform with an optofluidic transport assay to create a complete microfluidic total analysis system for quantifying drug permeability. Our system enables us to form liposomes with charged lipids mimicking the negative charge of bacterial membranes at physiological salt and pH levels, which proved difficult with previous liposome formation techniques. Furthermore, the microfluidic technique yields an order of magnitude more liposomes per experiment than previous assays. We demonstrate the feasibility of the assay by determining the permeability coefficient of norfloxacin across biomimetic liposomes.

On the Control of Chaos in Extended Structures

1997 ◽  
Vol 119 (4) ◽  
pp. 551-556 ◽  
Author(s):  
C. Barratt
Keyword(s):  
Nearest Neighbor ◽  
Duffing Oscillator ◽  
Vertical Plane ◽  
Small Time ◽  
Time Dependent ◽  
External Forcing ◽  
Control Of Chaos ◽  
Chaotic Vibrations ◽  
Order Of Magnitude ◽  
The Individual

A mechanism is proposed for synchronizing the chaotic vibrations of an externally forced array of oscillators with nearest-neighbor viscoelastic coupling. The proposed mechanism involves the application of small time-dependent perturbations to the individual oscillators. The perturbations required to preserve the coherence are of the order of magnitude of any noise present. The mechanism works with any form of external forcing. A modification of the mechanism is used to control the forced chaotic vibrations of a single Duffing oscillator allowed to vibrate out of the vertical plane.

Kinetic study of key species and reactions of atmospheric pressure pulsed corona discharge in humid air

2021 ◽  
Author(s):  
Yongkang Peng ◽  
Xiaoyue Chen ◽  
Yeqiang Deng ◽  
Lan Lei ◽  
Zhan Haoyu ◽  
...  
Keyword(s):  
Chemical Reactions ◽  
Chemical Reaction ◽  
Atmospheric Pressure ◽  
Discharge Plasma ◽  
Reaction Rates ◽  
Global Model ◽  
Discharge Process ◽  
Humid Air ◽  
Pressure Discharge ◽  
Atmospheric Pressure Discharge

Abstract The traditional corona discharge fluid model considers only electrons, positive and negative ions, and the discharge parameters are determined using the simplified weighting method involving the partial pressure ratio. Atmospheric pressure discharge plasma in humid air involves three main neutral gas molecule types: N2, O2, and H2O(g). However, in these conditions, the discharge process involves many types of particles and chemical reactions, and the charge and substance transfer processes are complex. At present, the databases of plasma chemical reaction equations are still expanding based on scholarly research. In this study, we examined the key particles and chemical reactions that substantially influence plasma characteristics. In summarizing the chemical reaction model for the discharge process of N2–O2–H2O(g) mixed gases, 65 particle types and 673 chemical reactions were investigated. On this basis, a global model of atmospheric pressure humid air discharge plasma was developed, with a focus on the variation of charged particles densities and chemical reaction rates with time under the excitation of a 0–200 Td pulsed electric field. Particles with a density greater than 1% of the electron density were classified as key particles. For such particles, the top ranking generation or consumption reactions (i.e., where the sum of their rates was greater than 95% of the total rate of the generation or consumption reactions) were classified as key chemical reactions On the basis of the key particles and reactions identified, a simplified global model was derived. A comparison of the global model with the simplified global model in terms of the model parameters, particle densities, reaction rates (with time), and calculation efficiencies demonstrated that both models can adequately identify the key particles and chemical reactions reflecting the chemical process of atmospheric pressure discharge plasma in humid air. Thus, by analyzing the key particles and chemical reaction pathways, the charge and substance transfer mechanism of atmospheric pressure pulse discharge plasma in humid air was revealed, and the mechanism underlying water vapor molecules’ influence on atmospheric pressure air discharge was elucidated.

Tectonic and geomorphic controls on the lacustrine deposits of the Neogene Vinchina basin, northwestern Argentina

2020 ◽  
Vol 90 (2) ◽  
pp. 250-267 ◽  
Author(s):  
Sergio A. Marenssi ◽  
Carlos O. Limarino ◽  
Laura J. Schencman ◽  
Patricia L. Ciccioli
Keyword(s):  
Water Depth ◽  
Drainage Basin ◽  
Low Frequency ◽  
Sedimentary Facies ◽  
Coarse Grained ◽  
Lacustrine Deposits ◽  
Facies Associations ◽  
Order Of Magnitude ◽  
Fluvial Sedimentation ◽  
The Individual

ABSTRACT Two episodes of lacustrine sedimentation, separated by an erosional surface and fluvial sedimentation, took place in the southern part of the broken foreland Vinchina basin (NW Argentina) between 11 and 5 Ma. The lacustrine deposits, 768 and 740 meters thick, are recorded in the upper part of the Vinchina Formation (“Vinchina lake”) and the lower part of the Toro Formation (“Toro Negro lake”) respectively. According to sedimentological features, four sedimentary facies associations (FAs) are recognized in the lacustrine deposits: 1) thinly laminated mudstones facies association (FA 1), 2) coarsening- and thickening-upward muddy to sandy cycles (FA 2), 3) medium- to coarse-grained sandstones (FA 3), and 4) mudstones, sandstones, and oolitic limestones (FA 4). Altogether, these facies correspond to ephemeral, shallow, lacustrine systems including saline mudflats. The total thickness of each lacustrine interval, the thickness of the individual cycles and their lithology, and the overall aggradational facies arrangement suggest that both lakes developed during underfilled stages of the basin. The coarsening-upward cycles can be regarded as lacustrine parasequences representing cyclic episodes of expansion and contraction of the lake, but unlike marine parasequences these cycles do not correlate to water depth. The development of lacustrine conditions and continuous base-level rise, together with the coeval southward-directed paleoflow indicators, suggest axial drainages and that the basin was externally closed (endorheic) at that time. The large thicknesses of each lacustrine interval also points to high accommodation in the southern part of the Vinchina basin during these times. Lake filling cycles are one order of magnitude thicker than lake depth, so we postulate that subsidence (tectonic) and rise of the spill point (geomorphology) increased accommodation but not water depth. Thus, unlike marine parasequences, the analyzed coarsening-upward cycles do not correlate to water depth, but rather they are controlled by more complex basinal accommodation processes. We hypothesize that the coeval uplift of the Umango and Espinal basement block to the south, coupled with the initial doming of the Sierra de Los Colorados to the east, may have generated the damming of the southward-directed drainage and a zone of maximum accommodation, then controlling the location of the two lakes and the preservation of their thick sedimentary records. Therefore, localized accommodation was enhanced by a combination of tectonic subsidence and topographic growth. The two lacustrine intervals and the intervening fluvial deposits record changing contributions from axial to transverse drainages and different cycles of closed and open conditions in the basin. A low-frequency, closed to open and back to closed (axial to transverse and return to axial drainage) basin evolution, is envisaged by the development of the two lakes (closed stages) and the erosional surface followed by the interval of fluvial sedimentation that separates them (open stage). In addition, several high-frequency lake fluctuations (expansion–contraction) are represented by the coarsening-upward cycles within each lacustrine interval. The thick lacustrine intervals and their intermediate incision surfaces record cyclic filling and re-excavation stages and localized episodes of increased subsidence in the Vinchina basin, which seem to be a common feature of tectonically active broken foreland basins.

scholarly journals Predictions of diffusion rates of organic molecules in secondary organic aerosols using the Stokes-Einstein and fractional Stokes-Einstein relations

2019 ◽  
Author(s):  
Erin Evoy ◽  
Adrian M. Maclean ◽  
Grazia Rovelli ◽  
Ying Li ◽  
Alexandra P. Tsimpidi ◽  
...  
Keyword(s):  
Citric Acid ◽  
Organic Molecules ◽  
Transport Model ◽  
Reaction Rates ◽  
Einstein Relation ◽  
Acid Mixture ◽  
Chemical Transport Model ◽  
Order Of Magnitude ◽  
Diffusion Rates ◽  
Almost All

Abstract. Information on the rate of diffusion of organic molecules within secondary organic aerosol (SOA) is needed to accurately predict the effects of SOA on climate and air quality. Often, researchers have predicted diffusion rates of organic molecules within SOA using measurements of viscosity and the Stokes-Einstein relation (D ∝ 1/η where D is the diffusion coefficient and η is viscosity). However, the accuracy of this relation for predicting diffusion in SOA remains uncertain. We measured diffusion coefficients over eight orders in magnitude in proxies of SOA including citric acid, sorbitol, and a sucrose-citric acid mixture. These results were combined with literature data to evaluate the Stokes-Einstein relation for predicting diffusion of organic molecules in SOA. Although almost all the data agrees with the Stokes-Einstein relation within a factor of ten, a fractional Stokes-Einstein relation (D ∝ C/ηt) with t = 0.93 and C = 1.66 is a better model for predicting diffusion of organic molecules in the SOA proxies studied. In addition, based on the output from a chemical transport model, the Stokes-Einstein relation can over predict mixing times of organic molecules within SOA by as much as one order of magnitude at an altitude ~ 3 km, compared to the fractional Stokes-Einstein relation with t = 0.93 and C = 1.66. These differences can be important for predicting growth, evaporation, and reaction rates of SOA in the middle and upper part of the troposphere. These results also have implications for other areas where diffusion of organic molecules within organic-water matrices is important.

scholarly journals Towards the distributed burning regime in turbulent premixed flames

2019 ◽  
Vol 871 ◽  
pp. 1-21 ◽  
Author(s):  
A. J. Aspden ◽  
M. S. Day ◽  
J. B. Bell
Keyword(s):  
Laminar Flame ◽  
Three Dimensional ◽  
Lewis Number ◽  
Reaction Rates ◽  
Turbulent Flames ◽  
Fuel Temperature ◽  
Transport Models ◽  
Eddy Viscosity Model ◽  
Order Of Magnitude ◽  
Number Species

Three-dimensional numerical simulations of canonical statistically steady, statistically planar turbulent flames have been used in an attempt to produce distributed burning in lean methane and hydrogen flames. Dilatation across the flame means that extremely large Karlovitz numbers are required; even at the extreme levels of turbulence studied (up to a Karlovitz number of 8767) distributed burning was only achieved in the hydrogen case. In this case, turbulence was found to broaden the reaction zone visually by around an order of magnitude, and thermodiffusive effects (typically present for lean hydrogen flames) were not observed. In the preheat zone, the species compositions differ considerably from those of one-dimensional flames based a number of different transport models (mixture averaged, unity Lewis number and a turbulent eddy viscosity model). The behaviour is a characteristic of turbulence dominating non-unity Lewis number species transport, and the distinct limit is again attributed to dilatation and its effect on the turbulence. Peak local reaction rates are found to be lower in the distributed case than in the lower Karlovitz cases but higher than in the laminar flame, which is attributed to effects that arise from the modified fuel-temperature distribution that results from turbulent mixing dominating low Lewis number thermodiffusive effects. Finally, approaches to achieve distributed burning at realisable conditions are discussed; factors that increase the likelihood of realising distributed burning are higher pressure, lower equivalence ratio, higher Lewis number and lower reactant temperature.

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