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991.
The Impact of Precipitation Scavenging on the Transport of Trace Gases: A 3-Dimensional Model Sensitivity Study 总被引:1,自引:0,他引:1
With the global Chemistry-Transport model MATCHsensitivity simulations were performed to determinethe degree to which especially upward transport ofgases from the earth's surface is limited byconvective and large-scale precipitation scavenging.When only dissolution of species in the liquid phaseis taken into account, mixing ratio reductions in themiddle and upper troposphere by 10% arecalculated for gases with a Henry's Law constant H of103 mol/l/atm. The removal increases to 50% forH = 104 mol/l/atm, and to 90% for H =105 mol/l/atm. We also consider scavenging by theice phase, which is generally much less efficient thanby the aqueous phase. In fact, rejection of gases fromfreezing water droplets may be a source of trace gasat higher altitudes.H2O2 and the strong acids (H2SO4,HNO3, HCl, HBr, HI) have such large solubilitiesthat they become largely removed by precipitation.When significant concentrations of these gases andsulfate aerosol exist above the liquid water domain ofthe atmosphere, they have likely been produced thereor at higher altitudes, although some could have comefrom trace gas rejection from ice particles or fromevaporating hydrometeors. Several other gases areaffected by precipitation, but not strongly enough toprevent fractional transfer to the middle and uppertroposphere: e.g., HNO4, HNO2 at pH 5,CH2O, the organic acids at pH 6,CH3SOCH3, HOCl, HOBr, and HOI. NH3 islargely removed by liquid phase scavenging at pH 7 and SO2 atpH 7. At pH less thanabout 6, upward transport of SO2 should largelydepend on the efficiency of oxidation processes in thewater droplets by O3 and H2O2.Most gases have solubilities which are too low forsignificant precipitation scavenging and aqueous phaseoxidation to occur. This holds, e.g., for O3, CO,the hydrocarbons, NO, NO2, HCN, CH3CN,CH3SCH3, CH3O2H, CH3CHOandhigher aldehydes, CH3OH and higher alcohols,peroxyacetylnitrate (PAN), CH3COCH3 andother ketones (note that some of these are not listedin Table I because their solubilities are below 10mol/l/atm). Especially for the short-lived gases,transfer from the boundary layer to the middle andupper troposphere is actually promoted by the enhancedupward transport that occurs in clouds. 相似文献
992.
Localized spectral analysis on the sphere 总被引:5,自引:0,他引:5
993.
994.
The NASA/Goddard Institute for Space Studies (GISS) climatemodel is forced with globally observed sea-surfacetemperatures (SST) in five simulations, 1969–1991,with individual runs beginning from altered initialatmospheric conditions. The interannual variability ofmodeled anomalies of the Southern Oscillation Index,mid-tropospheric temperatures, 850 mb zonal winds andOutgoing Longwave Radiation over the tropical PacificOcean, which has the largest SST anomaly forcing, arestrongly correlated with observed trends which reflectENSO cycles. The model's rainfall variability overthree agriculturally intensive regions, two tropicaland one mid-latitude, is investigated in order toevaluate the potential usefulness of GCM predictionsfor agricultural planning. The correct sign ofZimbabwe seasonal precipitation anomalies was hindcastwithin a useful range of consensus only for selectseasons corresponding to extreme ENSO events for whichanomalous circulation patterns were ratherrealistically simulated. The correlation betweenhindcasts of Nordeste monthly precipitation andobservations increases with time smoothing, reaching0.64 for 5-month running means. Consensus betweenindividual runs is directly proportional to theabsolute value of Niño3 SST so that during ElNiño and La Niña years most simulations agreeon the sign of predicted Nordeste rainfall anomalies.We show that during selected seasons the uppertropospheric divergent circulation and near surfacemeridional displacements of the ITCZ are realisticallyrepresented by the ensemble mean of the simulations.This realistic simulation of both the synopticmechanisms and the resulting precipitation changesincreases confidence in the GCM's potential forseasonal climate prediction. 相似文献
995.
996.
Local and regional variations in Central American arc lavas controlled by variations in subducted sediment input 总被引:9,自引:0,他引:9
Lina C. Patino Michael J. Carr Mark D. Feigenson 《Contributions to Mineralogy and Petrology》2000,138(3):265-283
The sedimentary section (at DSDP Site 495) on the subducting Cocos Plate has large stratigraphic changes in incompatible
elements and element ratios, the result of early carbonate deposition followed by late hemipelagic deposition. Lavas from
Central America define both local and regional geochemical trends that reflect the strong influence of the two Cocos Plate
sediment units. Element ratios with large stratigraphic variations on the Cocos Plate (e.g. Ba/Th, U/La) define local variations
within individual volcanic centers in Central America, indicating that marine stratigraphy controls some geochemical characteristics
of the lavas. These local trends can be explained by changing the proportions of hemipelagic sediment input into the magma
generation process. These local trends are observed in all the segments of the arc, regardless of the intensity of the slab
signature. Regional variations are most clearly seen in element ratios that are nearly constant through the Cocos Plate sediment
stratigraphy (e.g. Ba/La, U/Th), suggesting that regional variations reflect differences in the intensity of the flux from
the subducting slab. The slab signal is strongest in Nicaragua and along the volcanic front. The signal decreases to the northwest
and southeast of Nicaragua and toward the back arc. The large slab signature in the lavas from western Nicaragua occurs in
the area with the thinnest continental crust and steepest dip of the slab. The mass flux of incompatible elements into the
system is easily estimated, except for elements, like Pb, that have high and variable abundances in the basaltic oceanic crust
section. The mass flux of elements out of the system depends on eruption rates, which are variable along the arc and only
approximately known. Comparison of input and output fluxes for five different segments of the arc reveals that some elements
(K, B, Cs, and Rb) are very efficiently delivered to the volcanoes from the subducted slab. Other elements (Sr, Ba, and U)
are returned to the surface with moderate efficiency, whereas some elements (REEs) may come mostly from the mantle wedge with
minor slab contribution. The relative order of recycling efficiencies of incompatible elements implies that a hydrous fluid
dominates the transfer of material from the slab to mantle.
Received: 7 September 1998 / Accepted: 30 September 1999 相似文献
997.
T. Mark Harrison Marty Grove Oscar M. Lovera E. J. Catlos Jessica DAndrea 《Journal of Asian Earth Sciences》1999,17(5-6)
The key to comprehending the tectonic evolution of the Himalaya is to understand the relationships between large-scale faulting, anatexis, and inverted metamorphism. The great number and variety of mechanisms that have been proposed to explain some or all of these features reflects the fact that fundamental constraints on such models have been slow in coming. Recent developments, most notably in geophysical imaging and geochronology, have been key to coalescing the results of varied Himalayan investigations into constraints with which to test proposed evolutionary models. These models fall into four general types: (1) the inverted metamorphic sequences within the footwall of the Himalayan thrust and adjacent hanging wall anatexis are spatially and temporally related by thrusting; (2) thrusting results from anatexis; (3) anatexis results from normal faulting; and (4) apparent inverted metamorphism in the footwall of the Himalayan thrust is produced by underplating of right-way-up metamorphic sequences. We review a number of models and find that many are inconsistent with available constraints, most notably the recognition that the exposed crustal melts and inverted metamorphic sequences not temporally related. The generalization that appears to best explain the observed distribution of crustal melts and inverted metamorphic sequences is that, due to specific petrological and tectonic controls, episodic magmatism and out-of-sequence thrusting developed during continuous convergence juxtaposing allochthonous igneous and metamorphic rocks. This coincidental juxtaposition has proven to be something of a red herring, unduly influencing attention toward finding a causal relationship between anatexis and inverted metamorphism. 相似文献
998.
Cochran Anita Veverka Joseph Bell James Belton Michael Benkhoff Johannes Benkhoff Andrew Clark Benton Feldman Paul Kissel Jochen Mahaffy Paul Malin Michael Murchie Scott Neimann Hasso Owen Tobias Robinson Mark Schwehm Gerhard Squyres Steve Thomas Peter Whipple Fred Yeomans Donald 《Earth, Moon, and Planets》2000,89(1-4):289-300
In 1997, the COmet Nucleus TOUR (CONTOUR) was selected byNASA for a new start as part of the Discovery line. In this paper, we review the status of the mission, the mission timeline and the instruments to be flown. Detail is given of the science goals and how they are to be accomplished. 相似文献
999.
Gavin Ramsay Mark Cropper Kinwah Wu K. O. Mason Pasi Hakala 《Monthly notices of the Royal Astronomical Society》2000,311(1):75-84
We have detected the optical counterpart of the proposed double degenerate polar RX J1914+24. The I -band light curve is modulated on the 9.5-min period seen in X-rays. There is no evidence for any other periods. No significant modulation is seen in J . The infrared colours of RX J1914+24 are not consistent with a main-sequence dwarf secondary star. Our ASCA spectrum of RX J1914+24 is typical of a heavily absorbed polar and our ASCA light curve also shows only the 9.5-min period. We find that the folded I band and X-ray light curves are out of phase. We attribute the I -band flux to the irradiated face of the donor star. The long-term X-ray light curve shows a variation in the observed flux of up to an order of magnitude. These observations strengthen the view that RX J1914+24 is indeed the first double degenerate polar to be detected. In this light, we discuss the synchronizing mechanisms in such a close binary and other system parameters. 相似文献
1000.
Mark J. Dekkers Hilde F. Passier & Martin A. A. Schoonen 《Geophysical Journal International》2000,141(3):809-819
The magnetic behaviour of hydrothermally synthesized greigite was analysed in the temperature range from 4 K to 700 °C. Below room temperature, hysteresis parameters were determined as a function of temperature, with emphasis on the temperature range below 50 K. Saturation magnetization and initial susceptibility were studied above room temperature, along with X-ray diffraction analysis of material heated to various temperatures. The magnetic behaviour of synthetic greigite on heating is determined by chemical alteration rather than by magnetic unblocking. Heating in air yields more discriminative behaviour than heating in argon. When heated in air, the amount of oxygen available for reaction with greigite determines the products and magnetic behaviour. In systems open to contact with air, haematite is the final reaction product. When the contact with air is restricted, magnetite is the final reaction product. When air is excluded, pyrrhotite and magnetite are the final reaction products; the amount of magnetite formed is determined by the purity of the starting greigite and the degree of its surficial oxidation. The saturation magnetization of synthetic greigite is virtually independent of temperature from room temperature down to 4 K. The saturation remanent magnetization increases slowly by 20–30 per cent on cooling from room temperature to 4 K. A broad maximum is observed at ~10 K which may be diagnostic of greigite. The coercive and remanent coercive force both increase smoothly with decreasing temperature to 4 K. The coercive force increases from ~50 mT at room temperature to approximately 100–120 mT at 4 K, and the remanent coercive force increases from approximately 50–80 mT at room temperature to approximately 110–180 mT at 4 K. 相似文献