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51.
Loreto Donoso Rodrigo Romero Alberto Rondón Emmanuel Fernandez Pedro Oyola Eugenio Sanhueza 《Journal of Atmospheric Chemistry》1996,25(2):201-214
The levels of low molecular weight hydrocarbons were measured at pristine sites and rural locations affected by hydrocarbon emissions from oil and gas producing fields in Venezuela. At the clean sites, lower concentrations of C2 to C6 alkanes were observed, whereas, in comparison with remotes sites, very much higher levels were measured at the polluted sites. Alkenes present relatively high concentrations, with isoprene being the most abundant, all over the study region. The main sources of alkenes are likely to be natural, mainly from vegetation. The levels of alkanes recorded at the clean sites and the alkene levels found everywhere in the region are in agreement with the values reported for other clean sites in the tropics. The increase of ozone production capacity due to the anthropogenic emissions of alkanes from oil and gas fields was estimated. Due to the presence in the atmosphere of important amounts of naturally emitted isoprene, ethene and propene, which makes a substantial contribution to the reactivity of the hydrocarbon mixture, a small increase (<5%) was estimated to occur in the capacity of the ozone production at a regional scale during the rainy season. 相似文献
52.
The proposed existence of magnetic lineations in the Terra Cimmeria and Terra Sirenum regions of Mars was initially explained by Earth-like sea-floor spreading. Here we argue instead that these lineations could have been formed at a convergent plate margin through collision and accretion of terranes. A similar process produced banded magnetic anomalies, similar in geometry and even in size to those in Earth's North American Cordillera. Because only sparse and generally weak anomalies have been detected in the martian northern lowlands, which could constitute an analog to the terrestrial oceanic crust, it is possible that the magnetic field stopped its activity while crustal recycling was still active in Mars. 相似文献
53.
In the first part of this study, results of a computational fluid dynamics simulation over an array of cubes have been validated
against a set of wind-tunnel measurements. In Part II, such numerical results are used to investigate spatially-averaged properties
of the flow and passive tracer dispersion that are of interest for high resolution urban mesoscale modelling (e.g. non resolved
obstacle approaches). The results show that vertical profiles of mean horizontal wind are linear within the canopy and logarithmic
above. The drag coefficient, derived from the numerical results using the classical formula for the drag force, is height
dependent (it decreases with height). However, a modification of the formula is proposed (accounting for subgrid velocity
scales) that makes the drag coefficient constant with height. Results also show that the dispersive fluxes are similar in
magnitude to the turbulent fluxes, and that they play a very important role within the canopy. Vertical profiles of turbulent
length scales (to be used in k–l closure schemes, where k is the turbulent kinetic energy and l a turbulent length scale) are also derived. Finally the distribution of the values around the mean over the reference volumes
are analysed for wind and tracer concentrations. 相似文献
54.
Jos María Viramonte Raúl Alberto Becchio Jos Germn Viramonte Marcio Martins Pimentel Roberto Donato Martino 《Journal of South American Earth Sciences》2007,24(2-4):167-183
New field, petrological, geochemical, and geochronological data (U–Pb and Sm–Nd) for Ordovician rock units in the southeastern Puna, NW Argentina, indicate two lithostratigraphic units at the eastern–northeastern border of salar Centenario: (1) a bimodal volcanosedimentary sequence affected by low- to medium-grade metamorphism, comprising metasediments associated with basic and felsic metavolcanic rocks, dated 485 ± 5 Ma, and (2) a plutonic unit composed of syenogranites to quartz-rich leucogranites with U–Pb zircon ages between 462 ± 7 and 475 ± 5 Ma. Felsic metavolcanic and plutonic rocks are peraluminous and show similar geochemical differentiation trends. They also have similar Sm–Nd isotopic compositions (TDM model ages of 1.54–1.78 Ga; εNd(T) values ranging from −3.2 to −7.5) that suggest a common origin and derivation of the original magmas from older (Meso-Paleoproterozoic?) continental crust. Mafic rocks show εNd(T) ranging from +2.3 to +2.5, indicating a depleted mantle source. The data presented here, combined with those in the literature, suggest Ordovician magmatism mainly recycles preexisting crust with minor additions of juvenile mantle-derived material. 相似文献
55.
Graciela Salinas de Salmuni Inés Velasco Mirta Fresina Alberto L. Flores 《GeoJournal》2007,70(4):273-279
Much of the central-western region of Argentina, where San Juan Province is located, experiences arid to semi-arid climatic
conditions with low average annual rainfall accompanied by substantial evapotranspiration. Consequently, a viable crop industry
depends to a large extent upon irrigation from major river systems. Increasing demand for water in the lower basin of the
San Juan River is emphasizing the need for more accurate estimates of water used for irrigation. Since the water demand for
a particular crop is very closely related to crop area, monitoring the area of crop under irrigation is considered a proxy
for the amount of water used. Landsat 5 imagery for the growing season, field data and aerial photographs were used to evaluate
crop area. 相似文献
56.
Alberto Berretti 《Planetary and Space Science》1998,46(11-12)
We review some results about the analytic structure of Lindstedt series for some complex analytic dynamical systems: in particular, we consider Hamiltonian maps (like the standard map and its generalizations), the semi-standard map and Siegel's problem of the linearization of germs of holomorphic diffeomorphisms of (
,0). The analytic structure of those series can be studied numerically using Padé approximants, and one can show the existence of natural boundaries for real, diophantine values of the rotation number; by complexifying the rotation number, we show how these natural boundaries arise from the accumulation of singularities due to resonances, providing a new intuitive insight into the mechanism of the break-down of invariant KAM curves. Moreover, we study the Lindstedt series at resonances, i.e. for rational values of the rotation number, by suitably rescaling to 0 the value of the perturbative parameter, and a simple analytic structure emerges. Finally, we present some proofs for the simplest models and relate these results to renormalization ideas. 相似文献
57.
58.
Luciano Sbriz Margarita R. Aquino Natividad M. Alberto de rodriguez Scott W. Fowler Jose L. Sericano 《Marine pollution bulletin》1998,36(12):971-979
Bivalves (Crassostrea rizhophorae, Isognomon alatus, Codakia orbicularis Linné, and Tellina fausta) and sediment samples collected from 12 locations along the Dominican Republic coastline were used to assess the degree of nearshore contamination by selected chlorinated hydrocarbons and heavy metals. Concentrations of total chlordanes, DDTs and PCBs ranged from 0.51 to 7.47 ng/g, < MDL to 30.9 ng/g and 11.3 to 82.3 ng/g, and from < MDL to 1.73 ng/g, 0.21 to 12.5 ng/g and 0.46 to 41.9 ng/g in bivalves and surface sediments, respectively. The concentration ranges of predominantly anthropogenic Cu, Pb and Zn were 3.08–866 μg/g, 0.08–1.46 μg/g and 22.9–4380 μg/g in bivalve soft parts and 1.01–111 mg/g, 0.420–134 μg/g and 1.96–277 μg/g in surface sediments, respectively. Mercury concentrations ranged from 0.290 to 7.02 μg/g and from 0.096 to 0.565 μg/g in both matrices. The relatively high Hg concentrations found in bivalves from two stations in Bahia Samana, near to a known source of past Hg contamination in the sediments, require a more in-depth survey of the area. In general the concentrations of selected chlorinated hydrocarbons and trace metals, compared with those reported for the same matrices in the Caribbean and west Atlantic coastal areas, show at present the existence of only very localized contamination problems in the Dominican Republic. 相似文献
59.
The paper presents an analysis of 17 long annual maximum series (AMS) of flood flows for Swiss Alpine basins, aimed at checking the presence of changes in the frequency regime of annual maxima. We apply Pettitt's change point test, the nonparametric sign test and Sen's test on trends. We also apply a parametric goodness‐of‐fit test for assessing the suitability of distributions estimated on the basis of annual maxima collected up to a certain year for describing the frequency regime of later observations. For a number of series the tests yield consistent indications for significant changes in the frequency regime of annual maxima and increasing trends in the intensity of annual maximum discharges. In most cases, these changes cannot be explained by anthropogenic causes only (e.g. streamflow regulation, construction of dams). Instead, we observe a statistically significant relationship between the year of change and the elevation of the catchment outlet. This evidence is consistent with the findings of recent studies that explain increasing discharges in alpine catchments with an increase in the temperature controlling the portion of mountain catchments above the freezing point. Finally, we analyse the differences in return periods (RPs) estimated for a given flood flow on the basis of recent and past observations. For a large number of the study AMS, we observe that, on average, the 100‐year flood for past observations corresponds to a RP of approximately 10 to 30 years on the basis of more recent observation. From a complementary perspective, we also notice that estimated RP‐year flood (i.e. flood quantile (FQ) associated with RP) increases on average by approximately 20% for the study area, irrespectively of the RP. Practical implications of the observed changes are illustrated and discussed in the paper. Copyright © 2011 John Wiley & Sons, Ltd. 相似文献
60.
Alberto Escapa 《Celestial Mechanics and Dynamical Astronomy》2011,110(2):99-142
We explore the evolution of the angular velocity of an elastic Earth model, within the Hamiltonian formalism. The evolution
of the rotation state of the Earth is caused by the tidal deformation exerted by the Moon and the Sun. It can be demonstrated
that the tidal perturbation to spin depends not only upon the instantaneous orientation of the Earth, but also upon its instantaneous
angular velocity. Parameterizing the orientation of the Earth figure axis with the three Euler angles, and introducing the
canonical momenta conjugated to these, one can then show that the tidal perturbation depends both upon the angles and the
momenta. This circumstance complicates the integration of the rotational motion. Specifically, when the integration is carried
out in terms of the canonical Andoyer variables (which are the rotational analogues to the orbital Delaunay variables), one
should keep in mind the following subtlety: under the said kind of perturbations, the functional dependence of the angular
velocity upon the Andoyer elements differs from the unperturbed dependence (Efroimsky in Proceedings of Journées 2004: Systèmes
de référence spatio-temporels. l’Observatoire de Paris, pp 74–81, 2005; Efroimsky and Escapa in Celest. Mech. Dyn. Astron. 98:251–283, 2007). This happens because, under angular velocity dependent perturbations, the requirement for the Andoyer elements to be canonical
comes into a contradiction with the requirement for these elements to be osculating, a situation that parallels a similar
antinomy in orbital dynamics. Under the said perturbations, the expression for the angular velocity acquires an additional
contribution, the so called convective term. Hence, the time variation induced on the angular velocity by the tidal deformation
contains two parts. The first one comes from the direct terms, caused by the action of the elastic perturbation on the torque-free
expressions of the angular velocity. The second one arises from the convective terms. We compute the variations of the angular
velocity through the approach developed in Getino and Ferrándiz (Celest. Mech. Dyn. Astron. 61:117–180, 1995), but considering the contribution of the convective terms. Specifically, we derive analytical formulas that determine the
elastic perturbations of the directional angles of the angular velocity with respect to a non-rotating reference system, and
also of its Cartesian components relative to the Tisserand reference system of the Earth. The perturbation of the directional
angles of the angular velocity turns out to be different from the evolution law found in Kubo (Celest. Mech. Dyn. Astron.
105:261–274, 2009), where it was stated that the evolution of the angular velocity vector mimics that of the figure axis. We investigate comprehensively
the source of this discrepancy, concluding that the difference between our results and those obtained in Ibid. stems from an oversimplification made by Kubo when computing the direct terms. Namely, in his computations Kubo disregarded
the motion of the tide raising bodies with respect to a non-rotating reference system when compared with the Earth rotational
motion. We demonstrate that, from a numerical perspective, the convective part provides the principal contribution to the
variation of the directional angles and of length of day. In the case of the x and y components in the Tisserand system, the convective contribution is of the same order of magnitude as the direct one. Finally,
we show that the approximation employed in Kubo (Ibid.) leads to significant numerical differences at the level of a hundred micro-arcsecond. 相似文献