共查询到20条相似文献，搜索用时 609 毫秒
1.
Miller RL Suits AG Houston PL Toumi R Mack JA Wodtke AM 《Science (New York, N.Y.)》1994,265(5180):1831-1838
Highly vibrationally excited O(2)(X(3)sigmag(-), v >/= 26) has been observed from the photodissociation of ozone (O(3)), and the quantum yield for this reaction has been determined for excitation at 226 nanometers. This observation may help to address the "ozone deficit" problem, or why the previously predicted stratospheric O(3) concentration is less than that observed. Recent kinetic studies have suggested that O(2)(X(3)sigmag(-), v >/= 26) can react rapidly with O(2) to form O(3) + O and have led to speculation that, if produced in the photodissociation of O(3), this species might be involved in resolving the discrepancy. The sequence O(3) + hv --> O(2)(X(3)sigmag(-), v >/= 26) + O; O(2)(X(3)sigmag(-), v >/= 26) + O(2) --> O(3) + O (where hv is a photon) would be an autocatalytic mechanism for production of odd oxygen. A two-dimensional atmospheric model has been used to evaluate the importance of this new mechanism. The new mechanism can completely account for the tropical O(3) deficit at an altitude of 43 kilometers, but it does not completely account for the deficit at higher altitudes. The mechanism also provides for isotopic fractionation and may contribute to an explanation for the anomalously high concentration of heavy O(3) in the stratosphere. 相似文献
2.
We propose an isotopic fractionation mechanism, based on photolytic destruction, to explain the 15N/14N and 18O/16O fractionation of stratospheric nitrous oxide (N2O) and reconcile laboratory experiments with atmospheric observations. The theory predicts that (i) the isotopomers 15N14N16O and 14N15N16O have very different isotopic fractionations in the stratosphere, and (ii) laboratory photolysis experiments conducted at 205 nanometers should better simulate the observed isotopic fractionation of stratospheric N2O. Modeling results indicate that there is no compelling reason to invoke a significant chemical source of N2O in the middle atmosphere and that individual N2O isotopomers might be useful tracers of stratospheric air parcel motion. 相似文献
3.
Because of its high resistivity and subsequent low electroactivity, sulfur is not normally considered a room-temperature battery cathode. An elemental sulfur cathode has been made with a measured capacity of over 900 ampere.hours per kilogram, more than 90 percent of the theoretical storage capacity of solid sulfur at room temperature, accessed by means of a lightweight, highly conductive, aqueous polysulfide interface through the electrocatalyzed reaction S + H(2)O + 2e(-) --> HS(-) + OH(-). This solid sulfur cathode was first used in a battery with an aluminum anode for an overall discharge reaction 2Al + 3S + 3OH(-) + 3H(2)O --> 2Al(OH)(3) + 3HS(-), giving a cell potential of 1.3 volts. The theoretical specific energy of the aluminum-sulfur battery (based on potassium salts) is 910 watt.hours per kilogram with an experimental specific energy of up to 220 watt.hours per kilogram. 相似文献
4.
Nitrous oxide in the earth's atmosphere contributes to catalytic stratospheric ozone destruction and is also a greenhouse gas component. A precise budgetary accounting of N(2)O sources has remained elusive, and there is an apparent lack of source identification. One source of N(2)O is as a by-product in the manufacture of nylon, specifically in the preparation of adipic acid. Characterization of the reaction N(2)O stoichiometry and its isotopic composition with a simulated industrial adipic acid synthesis indicates that because of high rates of global adipic acid production, this N(2)O may account for approximately 10 percent of the increase observed for atmospheric N(2)O. 相似文献
5.
Existence of an 16O-rich gaseous reservoir in the solar nebula 总被引：1，自引：0，他引：1
Krot AN McKeegan KD Leshin LA MacPherson GJ Scott ER 《Science (New York, N.Y.)》2002,295(5557):1051-1054
Carbonaceous chondrite condensate olivine grains from two distinct petrographic settings, calcium-aluminum-rich inclusion (CAI) accretionary rims and amoeboid olivine aggregates (AOAs), are oxygen-16 (16O) enriched at the level previously observed inside CAIs. This requires that the gas in the nebular region where these grains condensed was 16O-rich. This contrasts with an 16O-poor gas present during the formation of chondrules, suggesting that CAIs and AOAs formed in a spatially restricted region of the solar nebula containing 16O-rich gas. The 16O-rich gas composition may have resulted either from mass-independent isotopic chemistry or from evaporation of regions with enhanced dust/gas ratios, possibly in an X-wind environment near the young Sun. 相似文献
6.
Suzuki S Green PG Bumgarner RE Dasgupta S Goddard WA Blake GA 《Science (New York, N.Y.)》1992,257(5072):942-945
Fully rotationally resolved spectra of three isotopic species of 1:1 clusters of benzene with water (H(2)O, D(2)O, and HDO) were fit to yield moments of inertia that demonstrate unambiguously that water is positioned above the benzene plane in nearly free internal rotation with both hydrogen atoms pointing toward the pi cloud. Ab initio calculations (MP2 level of electron correlation and 6-31 G(**) basis set with basis set superposition error corrections) predict a binding energy D(e) greater, similar 1.78 kilocalories per mole. In both the experimental and theoretical structures, water is situated nearly 1 angstrom within the van der Waals contacts of the monomers, a clear manifestation of hydrogen bond formation in this simple model of aqueous-pi electron interactions. 相似文献
7.
Friedman I Lipman PW Obradovich JD Gleason JD Christiansen RL 《Science (New York, N.Y.)》1974,184(4141):1069-1072
Oxygen isotope analyses of sanidine phenocrysts from rhyolitic sequences in Nevada, Colorado, and the Yellowstone Plateau volcanic field show that delta(18)O decreased in these magmas as a function of time. This decrease in delta(18)O may have been caused by isotopic exchange between the magma and groundwater low in (18)O. For the Yellowstone Plateau rhyolites, 7000 cubic kilometers of magma could decrease in delta(18)O by 2 per mil in 600,000 years by reacting with water equivalent to 3 millimeters of precipitation per year, which is only 0.3 percent of the present annual precipitation in this region. The possibility of reaction between large magmatic bodies and meteoric water at liquidus temperatures has major implications in the possible differentiation history of the magma and in the generation of ore deposits. 相似文献
8.
Xi-ShunZhao 《绵阳农专学报》2004,19(3):329-340
In this paper, the class of regular disjunction-free default theories is introduced and investigated. A transformation from regular default theories to normal default theories is established. The initial theory and the transformed theory have the same extensions when restricted to old variables. Hence, regular default theories enjoy some similar properties (e.g., existence of extensions, semi-monotonicity) as normal default theories. Then, a new algorithm for credulous reasoning of regular theories is developed. This algorithm runs in a time not more than O(1.45^n), where n is the number of defaults. In case of regular prerequisite-free or semi-2CNF default theories, the credulous reasoning can be solved in polynomial time. However, credulous reasoning for semi-Horn default theories is shown to be NP-complete although it is tractable for Horn default theories. Moreover, skeptical reasoning for regular unary default theories is co-NP-complete. 相似文献
9.
目的探讨原花青素对β-淀粉样肽25－35（Aβ25-35）诱导PCI2细胞凋亡及氧化应激反应的影响。方法3×10^8／LPCI2细胞培养24h,分别加入培养液（对照组）、10μmol／LAβ25-35（诱导组）、30mg/L原花青素＋10μmol／LAβ25－35（保护组）。24h后收集细胞,以Hoechst33258荧光染色观察细胞凋亡,比色分析测定MDA、H2O2含量和CAT活性。结果诱导组细胞可见凋亡特异性核固缩、核碎裂;MDA和H2O2含量与对照组比较明显增加（P〈0．01）,CAT活性明显降低（P〈0．05）。保护组与诱导组比较,凋亡特异性核固缩、核碎裂减少;MDA和H2O2含量明显降低（P〈0．01）,而CAT活性显著增强（P〈0．01）。结论原花青素可抑制Aβ25-35诱导的PC12细胞凋亡,可能与其抑制Aβ25-35引起的氧化应激反应有关。 相似文献
10.
Wiechert U Halliday AN Lee DC Snyder GA Taylor LA Rumble D 《Science (New York, N.Y.)》2001,294(5541):345-348
We have determined the abundances of 16O, 17O, and 18O in 31 lunar samples from Apollo missions 11, 12, 15, 16, and 17 using a high-precision laser fluorination technique. All oxygen isotope compositions plot within +/-0.016 per mil (2 standard deviations) on a single mass-dependent fractionation line that is identical to the terrestrial fractionation line within uncertainties. This observation is consistent with the Giant Impact model, provided that the proto-Earth and the smaller impactor planet (named Theia) formed from an identical mix of components. The similarity between the proto-Earth and Theia is consistent with formation at about the same heliocentric distance. The three oxygen isotopes (delta17O) provide no evidence that isotopic heterogeneity on the Moon was created by lunar impacts. 相似文献
11.
本文通过石墨共沉淀这一化学改性法，证实廉价易得的石墨材料对Mno_2电极的可充性有一定的改善。经过化学反应使Mn（OH）_2与石墨共沉淀，通氧气使Mn（OH）_2氧化成Mn_2O_3，干燥脱水后的Mn_2O_3再经酸化歧化成为所需MnO_2样品，并探讨了石墨选型对MnO_2电极改性的影响． 相似文献
12.
Atmospheric gases trapped in polar ice at the firn to ice transition layer are enriched in heavy isotopes (nitrogen-15 and oxygen-18) and in heavy gases (O(2)/N(2) and Ar/N(2) ratios) relative to the free atmosphere. The maximum enrichments observed follow patterns predicted for gravitational equilibrium at the base of the firn layer, as calculated from the depth to the transition layer and the temperature in the firn. Gas ratios exhibit both positive and negative enrichments relative to air: the negative enrichments of heavy gases are consistent with observed artifacts of vacuum stripping of gases from fractured ice and with the relative values of molecular diameters that govern capillary transport. These two models for isotopic and elemental fractionation provide a basis for understanding the initial enrichments of carbon-13 and oxygen-18 in trapped CO(2), CH(4), and O(2) in ice cores, which must be known in order to decipher ancient atmospheric isotopic ratios. 相似文献
13.
The oxygen-18 (18O) content of atmospheric carbon dioxide (CO2) is an important indicator of CO2 uptake on land. It has generally been assumed that during photosynthesis, oxygen in CO2 reaches isotopic equilibrium with oxygen in 18O-enriched water in leaves. We show, however, large differences in the activity of carbonic anhydrase (which catalyzes CO2 hydration and 18O exchange in leaves) among major plant groups that cause variations in the extent of 18O equilibrium (theta(eq)). A clear distinction in theta(eq) between C3 trees and shrubs, and C4 grasses makes atmospheric C18OO a potentially sensitive indicator to changes in C3 and C4 productivity. We estimate a global mean theta(eq) value of approximately 0.8, which reasonably reconciles inconsistencies between 18O budgets of atmospheric O2 (Dole effect) and CO2. 相似文献
14.
McKeegan KD Kallio AP Heber VS Jarzebinski G Mao PH Coath CD Kunihiro T Wiens RC Nordholt JE Moses RW Reisenfeld DB Jurewicz AJ Burnett DS 《Science (New York, N.Y.)》2011,332(6037):1528-1532
All planetary materials sampled thus far vary in their relative abundance of the major isotope of oxygen, (16)O, such that it has not been possible to define a primordial solar system composition. We measured the oxygen isotopic composition of solar wind captured and returned to Earth by NASA's Genesis mission. Our results demonstrate that the Sun is highly enriched in (16)O relative to the Earth, Moon, Mars, and bulk meteorites. Because the solar photosphere preserves the average isotopic composition of the solar system for elements heavier than lithium, we conclude that essentially all rocky materials in the inner solar system were enriched in (17)O and (18)O, relative to (16)O, by ~7%, probably via non-mass-dependent chemistry before accretion of the first planetesimals. 相似文献
15.
A full quantum dynamical study of the reactions of a hydrogen atom with water, on an accurate ab initio potential energy surface, is reported. The theoretical results are compared with available experimental data for the exchange and abstraction reactions in H + D2O and H + H2O. Clear agreement between theory and experiment is revealed for available thermal rate coefficients and the effects of vibrational excitation of the reactants. The excellent agreement between experiment and theory on integral cross sections for the exchange reaction is unprecedented beyond atom-diatom reactions. However, the experimental cross sections for abstraction are larger than the theoretical values by more than a factor of 10. Further experiments are required to resolve this. 相似文献
16.
A new type of host-dependent mutant, azure mutant, of bacteriophage f2 has been isolated. Growth of these mutants was restricted specifically by amber suppressor genes in the host bacteria. Restriction of the formation of infective centers by different bacterial suppressor genes was 98 percent, 90 percent, and 70 percent with Su-3, Su-1, and Su-2 genes, respectively. Restriction, like suppression, was the dominant phenotype. The block in growth of the mutants occurred in an early stage of the infection cycle. Once infection was established, however, an infected cell produced approximately the same number of progeny phage as a cell without the suppressor genes did. It is proposed that the azure codon is the same as the amber codon (uracil, adenine, guanine) and that restriction results from improper termination of protein chains of the phage RNA polymerase. Similar mutants may exist in other systems. 相似文献
17.
The Eocene greenhouse climate state has been linked to a more vigorous hydrologic cycle at mid- and high latitudes; similar information on precipitation levels at low latitudes is, however, limited. Oxygen isotopic fluxes track moisture fluxes and, thus, the δ(18)O values of ocean surface waters can provide insight into hydrologic cycle changes. The offset between tropical δ(18)O values from sampled Eocene sirenian tooth enamel and modern surface waters is greater than the expected 1.0 per mil increase due to increased continental ice volume. This increased offset could result from suppression of surface-water δ(18)O values by a tropical, annual moisture balance substantially wetter than that of today. Results from an atmospheric general circulation model support this interpretation and suggest that Eocene low latitudes were extremely wet. 相似文献
18.
Fluid processes in subduction zones 总被引：10，自引：0，他引：10
Peacock SA 《Science (New York, N.Y.)》1990,248(4953):329-337
Fluids play a critical role in subduction zones and arc magmatism. At shallow levels in subduction zones (<40 kilometers depth), expulsion of large volumes of pore waters and CH(4)-H(2)O fluids produced by diagenetic and low-grade metamorphic reactions affect the thermal and rheological evolution of the accretionary prism and provide nutrients for deep-sea biological communities. At greater depths, H(2)O and CO(2) released by metamorphic reactions in the subducting oceanic crust may alter the bulk composition in the overlying mantle wedge and trigger partial melting reactions. The location and conse-quences of fluid production in subduction zones can be constrained by consideration of phase diagrams for relevant bulk compositions in conjunction with fluid and rock pressure-temperature-time paths predicted by numerical heat-transfer models. Partial melting of subducting, amphibole-bearing oceanic crust is predicted only within several tens of million years of the initiation of subduction in young oceanic lithosphere. In cooler subduction zones, partial melting appears to occur primarily in the overlying mantle wedge as a result of fluid infiltration. 相似文献
19.
Arpigny C Jehin E Manfroid J Hutsemékers D Schulz R Stüwe JA Zucconi JM Ilyin I 《Science (New York, N.Y.)》2003,301(5639):1522-1524
High-resolution spectra of the CN B2 summation operator +-X2 summation operator + (0,0) band at 390 nanometers yield isotopic ratios for comets C/1995 O1 (Hale-Bopp) and C/2000 WM1 (LINEAR) as follows: 165 +/- 40 and 115 +/- 20 for 12C/13C, 140 +/- 35 and 140 +/- 30 for 14N/15N. Our N isotopic measurements are lower than the terrestrial 14N/15N = 272 and the ratio for Hale-Bopp from measurements of HCN, the presumed parent species of CN. This isotopic anomaly suggests the existence of other parent(s) of CN, with an even lower N isotopic ratio. Organic compounds like those found in interplanetary dust particles are good candidates. 相似文献
20.
Hydroxyl radicals are often called the "detergent" of the atmosphere because they control the atmosphere's capacity to cleanse itself of pollutants. Here, we show that the reaction of electronically excited nitrogen dioxide with water can be an important source of tropospheric hydroxyl radicals. Using measured rate data, along with available solar flux and atmospheric mixing ratios, we demonstrate that the tropospheric hydroxyl contribution from this source can be a substantial fraction (50%) of that from the traditional O(1D) + H2O reaction in the boundary-layer region for high solar zenith angles. Inclusion of this chemistry is expected to affect modeling of urban air quality, where the interactions of sunlight with emitted NOx species, volatile organic compounds, and hydroxyl radicals are central in determining the rate of ozone formation. 相似文献