Spis treści
Gotowa bibliografia na temat „Earth’s Atmosphere - Radical‐ Molecule Reactions”
Utwórz poprawne odniesienie w stylach APA, MLA, Chicago, Harvard i wielu innych
Zobacz listy aktualnych artykułów, książek, rozpraw, streszczeń i innych źródeł naukowych na temat „Earth’s Atmosphere - Radical‐ Molecule Reactions”.
Przycisk „Dodaj do bibliografii” jest dostępny obok każdej pracy w bibliografii. Użyj go – a my automatycznie utworzymy odniesienie bibliograficzne do wybranej pracy w stylu cytowania, którego potrzebujesz: APA, MLA, Harvard, Chicago, Vancouver itp.
Możesz również pobrać pełny tekst publikacji naukowej w formacie „.pdf” i przeczytać adnotację do pracy online, jeśli odpowiednie parametry są dostępne w metadanych.
Artykuły w czasopismach na temat "Earth’s Atmosphere - Radical‐ Molecule Reactions"
1
Raofie, Farhad, Graydon Snider i Parisa A. Ariya. "Reaction of gaseous mercury with molecular iodine, atomic iodine, and iodine oxide radicals — Kinetics, product studies, and atmospheric implications". Canadian Journal of Chemistry 86, nr 8 (1.08.2008): 811–20. http://dx.doi.org/10.1139/v08-088.
Pełny tekst źródłaStreszczenie:
Mercury is present in the Earth’s atmosphere mainly in elemental form. The chemical transformation of mercury in the atmosphere may influence its bioaccumulation in the human food chain as well as its global cycling. We carried out the first kinetic and product studies of the reactions of gaseous mercury with molecular iodine, atomic iodine, and iodine oxide radicals at tropospheric pressure (~740 Torr) and 296 ± 2 K in air and in N2 (1 Torr = 133.322 4 Pa; 0 °C = 273.15 K). Atomic iodine was formed using UV photolysis of CH2I2. IO radicals were formed by the UV photolysis of CH2I2 in the presence of ozone The reaction kinetics were studied using absolute rate techniques with gas chromatographic and mass spectroscopic detection (GC–MS). The measured rate coefficient for the reaction of Hg0 with I2 was ≤ (1.27 ± 0.58) × 10–19 cm3 molecule–1 s–1. The reaction products were analyzed in the gas phase from the suspended aerosols and from deposits on the walls of the reaction chambers using six complementary methods involving chemical ionization and electron impact mass spectrometry, GC–MS, a MALDI-TOF mass spectrometer, a cold vapor atomic fluorescence spectrometer (CVAFS), and a high-resolution transmission electron microscope (HRTEM) coupled to an energy dispersive spectrometer (EDS). The major reaction products identified were HgI2, HgO, and HgIO or HgOI. The implications of the results are discussed with regards to both the chemistry of atmospheric mercury and its potential implications in the biogeochemical cycling of mercury.Key words: mercury, molecular iodine, atomic iodine, iodine oxide radicals kinetics, product study, atmospheric chemistry.
2
Campbell, Laurence, Dale L. Muccignat i Michael J. Brunger. "Inclusion of Electron Interactions by Rate Equations in Chemical Models". Atoms 10, nr 2 (10.06.2022): 62. http://dx.doi.org/10.3390/atoms10020062.
Pełny tekst źródłaStreszczenie:
The concept of treating subranges of the electron energy spectrum as species in chemical models is investigated. This is intended to facilitate simple modification of chemical models by incorporating the electron interactions as additional rate equations. It is anticipated that this embedding of fine details of the energy dependence of the electron interactions into rate equations will yield an improvement in computational efficiency compared to other methods. It will be applicable in situations where the electron density is low enough that the electron interactions with chemical species are significant compared to electron–electron interactions. A target application is the simulation of electron processes in the D-region of the Earth’s atmosphere, but it is anticipated that the method would be useful in other areas, including enhancement of Monte Carlo simulation of electron–liquid interactions and simulations of chemical reactions and radical generation induced by electrons and positrons in biomolecular systems. The aim here is to investigate the accuracy and practicality of the method. In particular, energy must be conserved, while the number of subranges should be small to reduce computation time and their distribution should be logarithmic in order to represent processes over a wide range of electron energies. The method is applied here to the interaction by inelastic and superelastic collisions of electrons with a gas of molecules with only one excited vibrational level. While this is unphysical, it allows the method to be validated by checking for accuracy, energy conservation, maintenance of equilibrium and evolution of a Maxwellian electron spectrum.
3
Grankin, Dmitry, Irina Mironova, Galina Bazilevskaya, Eugene Rozanov i Tatiana Egorova. "Atmospheric Response to EEP during Geomagnetic Disturbances". Atmosphere 14, nr 2 (30.01.2023): 273. http://dx.doi.org/10.3390/atmos14020273.
Pełny tekst źródłaStreszczenie:
Energetic electron precipitation (EEP) is associated with solar activity and space weather and plays an important role in the Earth’s polar atmosphere. Energetic electrons from the radiation belt precipitate into the atmosphere during geomagnetic disturbances and cause additional ionization rates in the polar middle atmosphere. These induced atmospheric ionization rates lead to the formation of radicals in ion-molecular reactions at the heights of the mesosphere and upper stratosphere with the formation of reactive compounds of odd nitrogen NOy and odd hydrogen HOx groups. These compounds are involved in catalytic reactions that destroy the ozone. In this paper, we present the calculation of atmospheric ionization rates during geomagnetic disturbances using reconstructed spectra of electron precipitation from balloon observations; estimation of ozone destruction during precipitation events using one-dimensional photochemical radiation-convective models, taking into account both parameterization and ion chemistry; as well as provide an estimation of electron density during these periods.
4
Keßel, Stephan, David Cabrera-Perez, Abraham Horowitz, Patrick R. Veres, Rolf Sander, Domenico Taraborrelli, Maria Tucceri i in. "Atmospheric chemistry, sources and sinks of carbon suboxide, C<sub>3</sub>O<sub>2</sub>". Atmospheric Chemistry and Physics 17, nr 14 (20.07.2017): 8789–804. http://dx.doi.org/10.5194/acp-17-8789-2017.
Pełny tekst źródłaStreszczenie:
Abstract. Carbon suboxide, O = C = C = C = O, has been detected in ambient air samples and has the potential to be a noxious pollutant and oxidant precursor; however, its lifetime and fate in the atmosphere are largely unknown. In this work, we collect an extensive set of studies on the atmospheric chemistry of C3O2. Rate coefficients for the reactions of C3O2 with OH radicals and ozone were determined as kOH = (2.6 ± 0.5) × 10−12 cm3 molecule−1 s−1 at 295 K (independent of pressure between ∼ 25 and 1000 mbar) and kO3 < 1.5 × 10−21 cm3 molecule−1 s−1 at 295 K. A theoretical study on the mechanisms of these reactions indicates that the sole products are CO and CO2, as observed experimentally. The UV absorption spectrum and the interaction of C3O2 with water (Henry's law solubility and hydrolysis rate constant) were also investigated, enabling its photodissociation lifetime and hydrolysis rates, respectively, to be assessed. The role of C3O2 in the atmosphere was examined using in situ measurements, an analysis of the atmospheric sources and sinks and simulation with the EMAC atmospheric chemistry–general circulation model. The results indicate sub-pptv levels at the Earth's surface, up to about 10 pptv in regions with relatively strong sources, e.g. influenced by biomass burning, and a mean lifetime of ∼ 3.2 days. These predictions carry considerable uncertainty, as more measurement data are needed to determine ambient concentrations and constrain the source strengths.
5
Sipilä, M., T. Jokinen, T. Berndt, S. Richters, R. Makkonen, N. M. Donahue, R. L. Mauldin III i in. "Reactivity of stabilized Criegee intermediates (sCIs) from isoprene and monoterpene ozonolysis toward SO<sub>2</sub> and organic acids". Atmospheric Chemistry and Physics 14, nr 22 (19.11.2014): 12143–53. http://dx.doi.org/10.5194/acp-14-12143-2014.
Pełny tekst źródłaStreszczenie:
Abstract. Oxidation processes in Earth's atmosphere are tightly connected to many environmental and human health issues and are essential drivers for biogeochemistry. Until the recent discovery of the atmospheric relevance of the reaction of stabilized Criegee intermediates (sCIs) with SO2, atmospheric oxidation processes were thought to be dominated by a few main oxidants: ozone, hydroxyl radicals (OH), nitrate radicals and, e.g. over oceans, halogen atoms such as chlorine. Here, we report results from laboratory experiments at 293 K and atmospheric pressure focusing on sCI formation from the ozonolysis of isoprene and the most abundant monoterpenes (α-pinene and limonene), and subsequent reactions of the resulting sCIs with SO2 producing sulfuric acid (H2SO4). The measured total sCI yields were (0.15 ± 0.07), (0.27 ± 0.12) and (0.58 ± 0.26) for α-pinene, limonene and isoprene, respectively. The ratio between the rate coefficient for the sCI loss (including thermal decomposition and the reaction with water vapour) and the rate coefficient for the reaction of sCI with SO2, k(loss) /k(sCI + SO2), was determined at relative humidities of 10 and 50%. Observed values represent the average reactivity of all sCIs produced from the individual alkene used in the ozonolysis. For the monoterpene-derived sCIs, the relative rate coefficients k(loss) / k(sCI + SO2) were in the range (2.0–2.4) × 1012 molecules cm−3 and nearly independent of the relative humidity. This fact points to a minor importance of the sCI + H2O reaction in the case of the sCI arising from α-pinene and limonene. For the isoprene sCIs, however, the ratio k(loss) / k(sCI + SO2) was strongly dependent on the relative humidity. To explore whether sCIs could have a more general role in atmospheric oxidation, we investigated as an example the reactivity of acetone oxide (sCI from the ozonolysis of 2,3-dimethyl-2-butene) toward small organic acids, i.e. formic and acetic acid. Acetone oxide was found to react faster with the organic acids than with SO2; k(sCI + acid) / k(sCI + SO2) = (2.8 ± 0.3) for formic acid, and k(sCI + acid) / k(sCI + SO2) = (3.4 ± 0.2) for acetic acid. This finding indicates that sCIs can play a role in the formation and loss of other atmospheric constituents besides SO2.
6
Sipilä, M., T. Jokinen, T. Berndt, S. Richters, R. Makkonen, N. M. Donahue, R. L. Mauldin III i in. "Reactivity of stabilized Criegee intermediates (sCI) from isoprene and monoterpene ozonolysis toward SO<sub>2</sub> and organic acids". Atmospheric Chemistry and Physics Discussions 14, nr 2 (29.01.2014): 3071–98. http://dx.doi.org/10.5194/acpd-14-3071-2014.
Pełny tekst źródłaStreszczenie:
Abstract. Oxidation processes in Earth's atmosphere are tightly connected to many environmental and human health issues and are essential drivers for biogeochemistry. Until the recent discovery of the atmospheric relevance of stabilized Criegee intermediates (sCI), atmospheric oxidation processes were thought to be dominated by few main oxidants: ozone, hydroxyl radicals (OH), nitrate radicals and, e.g. over oceans, halogen atoms such as chlorine. Here, we report results from laboratory experiments at 293 K and atmospheric pressure focusing on sCI formation from the ozonolysis of isoprene and the most abundant monoterpenes (α-pinene and limonene), and subsequent reactions of the resulting sCIs with SO2 producing sulphuric acid (H2SO4). The measured sCI yields were (0.15 ± 0.07), (0.27 ± 0.12) and (0.58 ± 0.26) for the ozonolysis of α-pinene, limonene and isoprene, respectively. The ratio between the rate coefficient for the sCI loss (including thermal decomposition and the reaction with water vapour) and the rate coefficient for the reaction of sCI with SO2, k(loss) / k(sCI + SO2), was determined at relative humidities of 10% and 50%. Observed values represent the average reactivity of all sCIs produced from the individual alkene used in the ozonolysis. For the monoterpene derived sCIs, the relative rate coefficients k(loss) / k(sCI + SO2) were in the range (2.0–2.4) × 1012 molecule cm−3 and nearly independent on the relative humidity. This fact points to a minor importance of the sCI + H2O reaction in the case of the sCI arising from α-pinene and limonene. For the isoprene sCIs, however, the ratio k(loss) / k(sCI + SO2) was strongly dependent on the relative humidity. To explore whether sCIs could have a more general role in atmospheric oxidation, we investigated as an example the reactivity of acetone oxide (sCI from the ozonolysis of 2,3-dimethyl-2-butene) toward small organic acids, i.e. formic and acetic acid. Acetone oxide was found to react faster with the organic acids than with SO2; k(sCI + acid) / k(sCI + SO2) = (2.8 ± 0.3) for formic acid and k(sCI + acid) / k(sCI + SO2) = (3.4 ± 0.2) for acetic acid. This finding suggests that sCIs can play a role in the formation and loss of several atmospheric constituents besides SO2.
7
Kang, Zhiqin, Zhijing Wang, Yang Lu, Ran Cao, Dongwei Huang i Qiaorong Meng. "Investigation on the Effect of Atmosphere on the Pyrolysis Behavior and Oil Quality of Jimusar Oil Shale". Geofluids 2022 (2.03.2022): 1–9. http://dx.doi.org/10.1155/2022/1408690.
Pełny tekst źródłaStreszczenie:
High-temperature H2O and CO2 can improve the pyrolysis behavior of oil shale. Therefore, in this paper, Jimusar oil shale was selected as the research object and the effect of the reaction atmosphere (H2O, CO2, and N2) on its pyrolysis behavior, pyrolysate distribution, and pyrolysis oil quality was fully compared and studied. The results showed that compared with the N2 atmosphere, the presence of H2O and CO2 both increased the weight loss and weight loss rate during pyrolysis of oil shale and the existence of H2O advanced the initial precipitation temperature of volatiles by 17°C. The comprehensive release characteristic indices of volatiles during pyrolysis of oil shale in the CO2 and H2O atmospheres increased by 49.34% and 114.35%, respectively, which significantly improved its pyrolysis reactivity. Both H2O and CO2 atmospheres improved the pyrolysis oil yield of oil shale, and the pyrolysis oil yield in the H2O atmosphere performed better than that in the CO2 atmosphere. Especially, the H2O atmosphere could increase the pyrolysis oil yield by 41.42%. The existence of CO2 prevented methyl radicals from accepting hydrogen radicals during pyrolysis and reduced the alkane yield, while CO2 participated in the addition reaction of alkane, which increased the alkene yield. High-temperature H2O provided more hydrogen source, which increased the alkane yield and inhibited the alkene formation. Both H2O and CO2 atmospheres promoted the cracking of polycyclic aromatics and increased the yield of small-molecular aromatics in the pyrolysis oil. During the pyrolysis process of oil shale, CO2 and H2O underwent reforming reaction with the heavy oil, which increased the light component fraction, thereby increasing the H/C ratio of pyrolysis oil. Thus, the existence of H2O and CO2 atmospheres improved the quality of pyrolysis oil and the effect of H2O was better than CO2. The H2O and CO2 atmosphere promoted the formation of a well-developed pore structure, which was conducive to mass and heat transfer during pyrolysis of oil shale.
8
Romanias, Manolis N., i Thanh Lam Nguyen. "Evaluating the Atmospheric Loss of H2 by NO3 Radicals: A Theoretical Study". Atmosphere 13, nr 8 (18.08.2022): 1313. http://dx.doi.org/10.3390/atmos13081313.
Pełny tekst źródłaStreszczenie:
Molecular hydrogen (H2) is now considered among the most prominent substitute for fossil fuels. The environmental impacts of a hydrogen economy have received more attention in the last years, but still, the knowledge is relatively poor. In this work, the reaction of H2 with NO3 radical (the dominant night-time detergent of the atmosphere) is studied for the first time using high-level composite G3B3 and modification of high accuracy extrapolated ab initio thermochemistry (mHEAT) methods in combination with statistical kinetics analysis using non-separable semi-classical transition state theory (SCTST). The reaction mechanism is characterized, and it is found to proceed as a direct H-abstraction process to yield HNO3 plus H atom. The reaction enthalpy is calculated to be 12.8 kJ mol−1, in excellent agreement with a benchmark active thermochemical tables (ATcT) value of 12.2 ± 0.3 kJ mol−1. The energy barrier of the title reaction was calculated to be 74.6 and 76.7 kJ mol−1 with G3B3 and mHEAT methods, respectively. The kinetics calculations with the non-separable SCTST theory give a modified-Arrhenius expression of k(T) = 10−15 × T0.7 × exp(−6120/T) (cm3 s−1) for T = 200–400 K and provide an upper limit value of 10−22 cm3 s−1 at 298 K for the reaction rate coefficient. Therefore, as compared to the main consumption pathway of H2 by OH radicals, the title reaction plays an unimportant role in H2 loss in the Earth’s atmosphere and is a negligible source of HNO3.
9
Klein, Frieder, Jesse D. Tarnas i Wolfgang Bach. "Abiotic Sources of Molecular Hydrogen on Earth". Elements 16, nr 1 (1.02.2020): 19–24. http://dx.doi.org/10.2138/gselements.16.1.19.
Pełny tekst źródłaStreszczenie:
The capacity for molecular hydrogen (H2) to hydrogenate oxygen and carbon is critical to the origin of life and represents the basis for all known life-forms. Major sources of H2 that strictly involve nonbiological processes and inorganic reactants include (1) the reduction of water during the oxidation of iron in minerals, (2) water splitting due to radioactive decay, (3) degassing of magma at low pressures, and (4) the reaction of water with surface radicals during mechanical breaking of silicate rocks. None of these processes seem to significantly affect the current global atmospheric budget of H2, yet there is substantial H2 cycling in a wide range of Earth’s subsurface environments, with multifaceted implications for microbial ecosystems.
10
Egorov, O. V., i Yu N. Kalugina. "Analysis of radial cross sections of the potential energy of the interacting O3-O2 complex". Izvestiya vysshikh uchebnykh zavedenii. Fizika, nr 3 (2022): 10–16. http://dx.doi.org/10.17223/00213411/65/3/10.
Pełny tekst źródłaStreszczenie:
The article presents preliminary results for the potential energy calculations of the interacting O3-O2 complex. The study of this complex is important for modeling the reaction of the ozone formation in the stratospheric layer of the Earth's atmosphere. The calculations were carried out from the first principles ( ab initio ) of the quantum theory using the explicitly correlated spin-unrestricted coupled clusters method [UCCSD(T)-F12a] in a combination with the correlation-consistent basis set [aug-cc-pVTZ] to describe the molecular orbitals. The obtained radial dependences at the selected angular orientations are discussed in comparison with the O3-N2 complex.
Książki na temat "Earth’s Atmosphere - Radical‐ Molecule Reactions"
1
Calvert, Jack G., John J. Orlando, William R. Stockwell i Timothy J. Wallington. The Mechanisms of Reactions Influencing Atmospheric Ozone. Oxford University Press, 2015. http://dx.doi.org/10.1093/oso/9780190233020.001.0001.
Pełny tekst źródłaStreszczenie:
Ozone, an important trace component, is critical to life on Earth and to atmospheric chemistry. The presence of ozone profoundly impacts the physical structure of the atmosphere and meteorology. Ozone is also an important photolytic source for HO radicals, the driving force for most of the chemistry that occurs in the lower atmosphere, is essential to shielding biota, and is the only molecule in the atmosphere that provides protection from UV radiation in the 250-300 nm region. However, recent concerns regarding environmental issues have inspired a need for a greater understanding of ozone, and the effects that it has on the Earth's atmosphere. The Mechanisms of Reactions Influencing Atmospheric Ozone provides an overview of the chemical processes associated with the formation and loss of ozone in the atmosphere, meeting the need for a greater body of knowledge regarding atmospheric chemistry. Renowned atmospheric researcher Jack Calvert and his coauthors discuss the various chemical and physical properties of the earth's atmosphere, the ways in which ozone is formed and destroyed, and the mechanisms of various ozone chemical reactions in the different spheres of the atmosphere. The volume is rich with valuable knowledge and useful descriptions, and will appeal to environmental scientists and engineers alike. A thorough analysis of the processes related to tropospheric ozone, The Mechanisms of Reactions Influencing Atmospheric Ozone is an essential resource for those hoping to combat the continuing and future environmental problems, particularly issues that require a deeper understanding of atmospheric chemistry.
Części książek na temat "Earth’s Atmosphere - Radical‐ Molecule Reactions"
1
Calvert, Jack G., John J. Orlando, William R. Stockwell i Timothy J. Wallington. "Ozone in the Atmosphere". W The Mechanisms of Reactions Influencing Atmospheric Ozone. Oxford University Press, 2015. http://dx.doi.org/10.1093/oso/9780190233020.003.0004.
Pełny tekst źródłaStreszczenie:
The importance of ozone to life on Earth and to atmospheric chemistry cannot be overstated. Nucleic acids and other macromolecules essential to life absorb strongly in the ultraviolet (UV) and are damaged by UV radiation with wavelengths of less than approximately 300 nm. For proper functioning, such biological macromolecules need to be shielded from the full intensity of solar radiation. Molecular oxygen (O2) absorbs strongly and blocks solar radiation with wavelengths below 230–240 nm from reaching the Earth’s surface. However, oxygen is transparent at wavelengths above approximately 245 nm. Fortunately, absorption of UV radiation of wavelengths of less than 242 nm by molecular oxygen (O2) yields oxygen atoms that add to O2 to form ozone which has a very strong absorption band at 200–300 nm. Even though it is present in only trace amounts in the atmosphere, absorption by ozone effectively blocks harsh solar UV radiation from reaching the Earth’s surface. There is no other molecule in the atmosphere that provides protection from solar UV radiation in the 250–300 nm region. The development of the ozone layer is intimately connected to the development of life on Earth. Oxygen levels in the prebiotic atmosphere were less than 5 ×10−9 of the current level. Photosynthesis after the appearance of life on the planet more than 3.5 billion years ago led to increased oxygen levels in the atmosphere. By approximately 600 million years ago, the O2 concentration had exceeded 10% of the current level, and the corresponding layer of ozone was sufficient to offer an effective UV shield for the migration of life onto land (Wayne, 1991). Life on Earth as we know it would not have developed without the protection offered by the ozone layer, and, equally, the ozone layer would not have developed without life on Earth. In addition to its obviously important physical role in shielding biota from the damaging effects of harsh UV radiation, ozone plays an essential chemical role as a photolytic source for HO radicals.
2
Calvert, Jack G., John J. Orlando, William R. Stockwell i Timothy J. Wallington. "Mechanisms of Ozone Reactions in the Troposphere". W The Mechanisms of Reactions Influencing Atmospheric Ozone. Oxford University Press, 2015. http://dx.doi.org/10.1093/oso/9780190233020.003.0005.
Pełny tekst źródłaStreszczenie:
In Chapter I, we identified the origin of stratospheric ozone and its role in limiting the short wavelengths of sunlight reaching the Earth. We also saw the importance of trace impurities of NOx and hydrocarbons in the development of tropospheric ozone. In this chapter, we review and evaluate the chemical reactions of ozone that create the important hydroxyl (HO) radical. It is the photodecomposition of tropospheric ozone that is the major source of the important HO radical, and it is the HO radical that initiates the destruction of most of the reactive trace gases that are emitted into the atmosphere. Ozone also serves as a major reactant for removal of the alkenes and other reactive unsaturated compounds, and, in this chapter, we review and evaluate the rate coefficients and mechanisms of these reactions and the expected products that result from them. The reactions that generate oxygen atoms in their first excited electronic state, O(1D) atoms, and ultimately HO radicals within the atmosphere are initiated through ozone photodecomposition: . . . O3 (X1A1) + hν → O(1D) + O2(a1Δg) (I) . . . . . . → O(1D) + O2(X3Σ–g) (II) . . . A fraction of the O(1D) atoms formed in the reactions (I) and (II) react with water molecules to generate HO radicals in reaction (1) and a larger fraction are deactivated by collisions with N2 and O2 molecules to form ground state O(3P) atoms in reaction (2): . . . O(1D) + H2O → HO + HO (1) . . . . . . O(1D) + M (N2, O2) → O(3P) + M (N2, O2) (2) . . . The competition between H2O and other air molecules (N2, O2) for reaction with O(1D) atoms results in HO generation being dependent on relative humidity. Rate coefficients for reaction of O(1D) with H2O, N2, and O2 at 298 K (in units of 10−10 cm3 molecule−1 s−1) recommended by the International Union of Pure and Applied Chemistry (IUPAC) panel are 2.14, 0.31, and 0.40, respectively (Atkinson et al., 2004). To better understand the factors that control HO formation, we will review ozone photochemistry, its cross sections, quantum yields of its major photodecomposition modes, and its photolysis frequencies under varied atmospheric conditions.
3
Atkins, Peter. "Irritating Atmospheres: Atmospheric Photochemistry". W Reactions. Oxford University Press, 2011. http://dx.doi.org/10.1093/oso/9780199695126.003.0030.
Pełny tekst źródłaStreszczenie:
The problem of photochemically generated smog begins inside internal combustion engines, where at the high temperatures within the combustion cylinders and the hot exhaust manifold nitrogen molecules and oxygen molecule combine to form nitric oxide, NO. Almost as soon as it is formed, and when the exhaust gases mingle with the atmosphere, some NO is oxidized to the pungent and chemically pugnacious brown gas nitrogen dioxide, NO2, 1. We need to watch what happens when one of these NO2 molecules is exposed to the energetic ultraviolet photons in sunlight. We see a photon strike the molecule and cause a convulsive tremor of its electron cloud. In the brief instant that the electron cloud has swarmed away from one of the bonding regions, an O atom makes its escape, leaving behind an NO molecule. We now continue to watch the liberated O atom. We see it collide with an oxygen molecule, O2, and stick to it to form ozone, O3, 2. This ozone is formed near ground level and is an irritant; ozone at stratospheric levels is a benign ultraviolet shield. Now keep your eye on the ozone molecule. In one instance we see it collide with an NO molecule, which plucks off one of ozone’s O atoms, forming NO2 and letting O3 revert to O2. Another fate awaiting NO2 is for it to react with oxygen and any unburned hydrocarbon fuel and its fragments that have escaped into the atmosphere. We can watch that happening too where the air includes surviving fragments of hydrocarbon fuel molecules. A lot of little steps are involved, and they occur at a wide range of rates. Let’s suppose that some unburned fuel escapes as ethane molecules, CH3CH3, 3. Although ethane is not present in gasoline, a CH3CH2· radical (Reaction 12) would have been formed in its combustion and then combined with an H atom in the tumult of reactions going on there. You already know that vicious little O atoms are lurking in the sunlit NO2-ridden air. We catch sight of one of their venomous acts: in a collision with an H2O molecule they extract an H atom, so forming two ·OH radicals.
4
"The Chemical Logic for Major Reaction Types". W Natural Product Biosynthesis, 22–46. The Royal Society of Chemistry, 2022. http://dx.doi.org/10.1039/bk9781839165641-00022.
Pełny tekst źródłaStreszczenie:
This chapter defines a core set of central metabolites that are thermodynamically activated but sufficiently stable kinetically to serve as diffusible molecules that power coupled reaction equilibria to drive biosynthesis in both primary and secondary pathways. Three such molecules are adenosine triphosphate (ATP), acetyl-coenzyme A (CoA), and the reduced nicotinamide coenzymes NADH and NADPH, which serve as cellular currencies for phosphoryl-, acetyl-, and electron transfers, respectively. ATP's thermodynamic activation arises from its kinetically stable side chain phosphoric anhydride linkages; acetyl-CoA from its acyl thioester grouping, and NAD(P)H from the dihydropyridinium ion linkage. S-Adenosylmethionine, with its activated sulfonium cation group, can transfer methyl, aminobutyryl, and adenosyl groups to cosubstrates as electrophilic or as radical fragments. Carbamoyl phosphate is a biologic carbamoylating reagent due to its mixed acyl phosphoric anhydride core. UDP-glucose and congeneric NDP-hexoses are fragmentable enzymatically into C1-glucosyl electrophiles for capture by cosubstrate nucleophiles. The delta 2- and 3-double bonds in isopentenyl-PP isomers serve as electrophilic and nucleophilic partners, respectively, for C–C bond-forming alkylations at the start of all isoprenoid biosynthetic pathways. Adenosine-5′-phosphosulfate is activated for sulfuryl group transfer via its mixed sulfuric-phosphoric acid side chain linkage. Molecular oxygen (O2) is kinetically stable enough to comprise 21% of Earth's atmosphere, but is thermodynamically activated to be the terminal electron acceptor in aerobic metabolism. Its controlled reductive cleavage is the driving force for introduction of diverse oxygen functional groups in a plethora of natural product maturations.
5
Calvert, Jack G., John J. Orlando, William R. Stockwell i Timothy J. Wallington. "The Hydroxyl Radical and Its Role in Ozone Formation". W The Mechanisms of Reactions Influencing Atmospheric Ozone. Oxford University Press, 2015. http://dx.doi.org/10.1093/oso/9780190233020.003.0007.
Pełny tekst źródłaStreszczenie:
Although the HO radical is present in the sunlight-irradiated troposphere at very low concentrations, only about 106 molecules cm−3, it is the most important trace component in our atmosphere. It is a highly reactive transient species and is responsible for initiating the oxidation of the majority of organic compounds in the troposphere. It initiates the chain reactions that produce ozone. All the saturated, H-atom containing molecules react with HO through abstraction of an H atom. In the case of the simplest alkane, methane, reaction (1) leads to the formation of a water molecule and an alkyl (CH3) radical: . . . HO + CH4 → H2O + CH3 (1) . . . The CH3 radical released into the oxygen-rich atmosphere quickly adds O2 to give the methyl peroxy radical in reaction (2), which in NO-containing atmospheres can react to form NO2, and an alkoxy radical, CH3O, in reaction (3). In turn, this radical reacts with O2 to give an HO2 radical and a molecule of formaldehyde in (4). An HO radical can be regenerated as the HO2 molecule oxidizes NO to NO2 in (5), and the chain of events, reactions (1) through (5), leads to ozone generation through the photolysis of the NO2 molecule in reactions (6) and (7): . . . CH3 + O2 → CH3O2 (2) . . . . . . CH3O2 + NO → CH3O + NO2 (3) . . . . . . CH3O + O2 → HO2 + CH2O (4) . . . HO2 + NO → HO + NO2 (5) . . . . . . NO2 + hν → O + NO (6) . . . . . . O + O2 (+ M) → O3 (+ M) (7) . . . Methane is the least reactive of the alkanes with HO. Urban atmospheres contain a complex mixture of the more reactive larger alkanes (RH). The number of different possible geometric isomers and stereoisomers of the alkanes that can be formed by association of C and H atoms is astounding (Calvert et al., 2008). For example, there are more than a thousand structurally different molecules of molecular formula C12H26, more than a million C20H22, more than a billion of formula C25H52, and more than a trillion possible different isomers of molecular formula C31H64.
6
DAMIÁN-SALINAS, Daniela, Dulce Arely RIVERA-RODRÍGUEZ, Liliana LIZÁRRAGA-MENDIOLA i Gabriela A. VÁZQUEZ-RODRÍGUEZ. "Andanzas de un elemento mágico: el ciclo biogeoquímico del manganeso". W CIERMMI Women in Science Biology, Chemistry and Life Sciences Handbook T-XIV, 39–58. ECORFAN-Mexico, S.C., 2021. http://dx.doi.org/10.35429/h.2021.14.39.58.
Pełny tekst źródłaStreszczenie:
This review is devoted to the biogeochemical cycle of manganese and the chemical characteristics of this element that make such a cycle possible, particularly its redox transformations. Through a journey of the five environmental spheres, namely the Earth's crust (and specifically the soil), the different parts of the hydrosphere, the biosphere, the anthroposphere, and the atmosphere, the main manganese species in each of these compartments are analyzed, among which manganese oxides (MnOx) stand out. The formation of submarine deposits of MnOx at the crust/hydrosphere interface is also presented since they represent the largest reservoir of this element in the Earth's crust. In reviewing the manganese redox reactions in the hydrosphere, its speciation in different types of natural water is presented, as well as the circumstances that turn this element into a matter of concern. The section dedicated to the biosphere shows how the terrestrial history of manganese is intimately intertwined with the emergence of photosynthesis and the oxygenation of the atmosphere. It also examines how manganese chemistry was crucial in fortuitously providing a defense against the free radicals that have, since its emergence, accompanied molecular oxygen and aerobic metabolism. Besides, some microbial redox transformations, the role of manganese as a nutrient, and relevant aspects of its toxicology are examined. Socio-industrial uses of manganese, which span several thousands of years, are summarized in the anthroposphere section. The article concludes with an overview of the non-redox mechanisms that mobilize this "magic" element between soil and water.
7
Calvert, Jack G., John J. Orlando, William R. Stockwell i Timothy J. Wallington. "Mechanisms of Reactions of HO2 and RO2 Radicals". W The Mechanisms of Reactions Influencing Atmospheric Ozone. Oxford University Press, 2015. http://dx.doi.org/10.1093/oso/9780190233020.003.0008.
Pełny tekst źródłaStreszczenie:
The peroxy radicals are an important link in the reaction chain that develops ozone in the atmosphere through their reactions with NO. This chapter explores the kinetics and mechanisms of these RO2 reactions. In Chapters III and IV, the kinetics and mechanisms of the reactions of organic compounds with the major atmospheric oxidants [HO, NO3, and O3] were discussed. The organic radicals formed in these reactions add O2 to form organic peroxy radicals (RO2). The rate coefficient for these reaction is typically of the order of (10−12–10−11) cm3 molecule−1 s−1 under tropospheric conditions. One atmosphere (1 atm) of air contains 5 × 1018 molecule cm−3 of O2, and the lifetime of organic radicals with respect to addition of O2 to give peroxy radicals is 10–100 nanoseconds. Addition of O2 is essentially the sole atmospheric fate of the organic radicals formed during the oxidation of organic compounds. As examples, consider the HO-initiated oxidation of ethane and acetone (M is a third body, such as N2, which collisionally deactivates the nascent peroxy radical): . . . HO + CH3CH3 → CH3CH2 + H2O . . . . . . CH3CH2 + O2 + M → CH3CH2O2 + M . . . . . . HO + CH3C(O)CH3 → CH3C(O)CH2 + H2O . . . . . . CH3C(O)CH2 + O2 + M → CH3C(O)CH2O2 + M . . . Because of the rapidity and exclusivity of the O2 addition to alkyl radicals, the organic peroxy radicals (CH3CH2O2 and CH3C(O)CH2O2) can be thought of as the primary products of the initial oxidation step. HO2 radicals are formed in reactions of O2 with alkoxy radicals (e.g., CH3O) and by the association reaction of H atoms with O2: . . . CH3O + O2 → CH2O + HO2 . . . . . . H + O2 + M → HO2 + M . . . Peroxy radicals (HO2 and RO2) have a rich atmospheric chemistry and undergo reactions with NO, NO2, HO2, and other peroxy radicals (R′O2). Unimolecular isomerization is also an important fate for larger organic peroxy radicals where the peroxy radical can abstract a hydrogen atom from another part of the organic moiety (the peroxy radical bites its own tail). Reactions of peroxy radicals with NO3 radicals at night, and ClO and BrO radicals in maritime environments, can also be of importance on local scales.
8
Calvert, Jack G., John J. Orlando, William R. Stockwell i Timothy J. Wallington. "Photodecomposition of Light-Absorbing Oxygenates and Its Influence on Ozone Levels in the Atmosphere". W The Mechanisms of Reactions Influencing Atmospheric Ozone. Oxford University Press, 2015. http://dx.doi.org/10.1093/oso/9780190233020.003.0011.
Pełny tekst źródłaStreszczenie:
Photochemistry provides the important driving force that initiates chemistry in the atmosphere. We saw in Chapter II how light absorbed by ozone generates the important HO radical, and, in Chapter III, we reviewed how light absorption by NO2 leads to ozone formation. In this chapter, we discuss the photochemistry of the light-absorbing oxygenates: their photochemical lifetimes and the nature of the modes of photodecomposition they undergo. Of course, light of sufficient energy per quantum must be absorbed by a molecule if its photodecomposition is to occur. The hydrocarbons do not absorb tropospheric sunlight, as seen in Figure VIII-A-1. The light gray and dark gray lines, respectively, show the distribution of actinic flux present in the troposphere and upper stratosphere for overhead Sun. It can be seen that the larger alkanes, alkenes, and aromatic hydrocarbons absorb at somewhat longer wavelengths than the first member of the family, but none can be electronically excited by tropospheric radiation. Among the hydrocarbons, only the polycyclic aromatics absorb appreciable tropospheric sunlight, and their π → π* excitation does not result in decomposition but likely generates O2(1Δg) molecules by energy transfer; these molecules are usually quenched by collision to ground state O2(3Σg−) molecules (see Calvert et al., 2000). As atmospheric oxidation of the hydrocarbons occurs, initiated largely by HO radicals, a multitude of oxygenated organic species are generated. The absorption region for the oxygenates is generally shifted to longer wavelengths, although the alcohols, ethers, acids, and esters still show no overlap of the regions of tropospheric actinic flux. For the families of compounds shown, the only significant absorbers of tropospheric sunlight are the aldehydes (e.g., CH2O) and the ketones (e.g., CH3C(O)CH3). Formic acid and methyl formate, as well as the larger members of the acid and ester families, absorb sunlight available only at the higher altitudes of the stratosphere, where they are expected to photodecompose. However, these species are not expected to be present in the stratosphere because they are removed in the troposphere largely via HO reactions. In this chapter, we focus on the rates and pathways for photodecomposition of the aldehydes and ketones with less detailed considerations of the other less prevalent light-absorbing trace compounds.
9
Berner, Robert A. "Atmospheric O2 over Phanerozoic Time". W The Phanerozoic Carbon Cycle. Oxford University Press, 2004. http://dx.doi.org/10.1093/oso/9780195173338.003.0008.
Pełny tekst źródłaStreszczenie:
The chemical reactions that affect atmospheric O2 on a multimillion-year time scale involve the most abundant elements in the earth’s crust that undergo oxidation and reduction. This includes carbon, sulfur, and iron. (Other redox elements, such as manganese, are not abundant enough to have an appreciable effect on O2.) Iron is the most abundant of the three, but it plays only a minor role in O2 control (Holland, 1978). This is because during oxidation the change between Fe+2 and Fe+3 involves the uptake of only one-quarter of an O2 molecule, whereas the oxidation of sulfide to sulfate involves two O2 molecules, and the oxidation of reduced carbon, including organic matter and methane, involves between one and two O2 molecules. The same stoichiometry applies to reduction of the three elements. Because iron is not sufficiently abundant enough to counterbalance its low relative O2 consumption/release, the iron cycle is omitted in most discussions of controls on atmospheric oxygen. In contrast, the sulfur cycle, although subsidiary to the carbon cycle as to its effect on atmospheric O2, is nevertheless non-negligible and must be included in any discussion of the evolution of atmospheric O2. In this chapter the methods and results of modeling the long-term carbon and sulfur cycles are presented in terms of calculations of past levels of atmospheric oxygen. The modeling results are then compared with independent, indirect evidence of changes in O2 based on paleobiological observations and experimental studies that simulate the response of forest fires to changes in the levels of O2. Because the sulfur cycle is not discussed anywhere else in this book, it is briefly presented first. The long-term sulfur cycle is depicted as a panorama in figure 6.1. Sulfate is added to the oceans, via rivers, originating from the oxidative weathering of pyrite (FeS2) and the dissolution of calcium sulfate minerals (gypsum and anhydrite) on the continents. Volcanic, metamorphic/hydrothermal, and diagenetic reactions add reduced sulfur to the oceans and atmosphere where it is oxidized to sulfate. Sulfur is removed from the oceans mainly via formation of sedimentary pyrite and calcium sulfate.