全文获取类型
收费全文 | 974篇 |
免费 | 49篇 |
国内免费 | 5篇 |
专业分类
测绘学 | 31篇 |
大气科学 | 88篇 |
地球物理 | 240篇 |
地质学 | 314篇 |
海洋学 | 128篇 |
天文学 | 91篇 |
综合类 | 2篇 |
自然地理 | 134篇 |
出版年
2023年 | 2篇 |
2022年 | 6篇 |
2021年 | 19篇 |
2020年 | 23篇 |
2019年 | 23篇 |
2018年 | 33篇 |
2017年 | 40篇 |
2016年 | 40篇 |
2015年 | 32篇 |
2014年 | 46篇 |
2013年 | 66篇 |
2012年 | 49篇 |
2011年 | 88篇 |
2010年 | 53篇 |
2009年 | 52篇 |
2008年 | 46篇 |
2007年 | 53篇 |
2006年 | 36篇 |
2005年 | 39篇 |
2004年 | 43篇 |
2003年 | 40篇 |
2002年 | 21篇 |
2001年 | 14篇 |
2000年 | 14篇 |
1999年 | 16篇 |
1998年 | 7篇 |
1997年 | 8篇 |
1996年 | 8篇 |
1995年 | 8篇 |
1994年 | 10篇 |
1993年 | 5篇 |
1992年 | 11篇 |
1991年 | 6篇 |
1990年 | 4篇 |
1989年 | 4篇 |
1988年 | 10篇 |
1987年 | 4篇 |
1986年 | 2篇 |
1985年 | 3篇 |
1984年 | 4篇 |
1983年 | 4篇 |
1982年 | 4篇 |
1980年 | 2篇 |
1979年 | 6篇 |
1977年 | 3篇 |
1975年 | 2篇 |
1974年 | 2篇 |
1973年 | 3篇 |
1969年 | 3篇 |
1967年 | 2篇 |
排序方式: 共有1028条查询结果,搜索用时 31 毫秒
971.
Effects of land use on ground water quality in the Anoka Sand Plain Aquifer of Minnesota 总被引:6,自引:0,他引:6
We began a study, in 1996, to compare ground water quality under irrigated and nonirrigated agriculture, sewered and nonsewered residential developments, industrial, and nondeveloped land uses. Twenty-three monitoring wells were completed in the upper meter of an unconfined sand aquifer. Between 1997 and 2000, sampling occurred quarterly for major ions, trace inorganic chemicals, volatile organic compounds (VOCs), herbicides, and herbicide degradates. On single occasions, we collected samples for polynuclear aromatic hydrocarbons (PAHs), perchlorate, and coliform bacteria. We observed significant differences in water chemistry beneath different land uses. Concentrations of several trace inorganic chemicals were greatest under sewered urban areas. VOC detection frequencies were 100% in commercial areas, 52% in sewered residential areas, and <10% for other land uses. Median nitrate concentrations were greatest under irrigated agriculture (15,350 microg/L) and nonsewered residential areas (6080 microg/L). Herbicides and degradates of acetanilide and triazine herbicides were detected in 86% of samples from irrigated agricultural areas, 68% of samples from nonirrigated areas, and <10% of samples from other land uses. Degradates accounted for 96% of the reported herbicide mass. We did not observe seasonal differences in water chemistry, but observed trends in water chemistry when land use changes occurred. Our results show land use is the dominant factor affecting shallow ground water quality. Trend monitoring programs should focus on areas where land use is changing, while resource managers and planners must consider potential impacts of land use changes on ground water quality. 相似文献
972.
973.
974.
Jennifer Romon-MartinMaria A. Barucci Catherine de BerghAlain Doressoundiram Nuno Peixinho François Poulet 《Icarus》2002,160(1):59-65
Near-infrared spectra of Centaur 8405 Asbolus have been obtained, covering the whole rotational period. Complementary photometry and visible spectroscopy have also been performed. The aim of this project was to confirm or refute the water ice detection on one side of Asbolus reported by Kern et al. (2000 Astrophys. J.542, L155-L159). We found no absorption feature at any rotational phase of Asbolus. Our study is the first in-depth investigation of a centaur over a complete rotational period. Our results seem also to indicate that a change in the slope from 0.8 to 1.0 μm may occur in Asbolus spectrum. 相似文献
975.
FT-IR Study of the Kinetics and Products of the Reactions of Dimethylsulphide, Dimethylsulphoxide and Dimethylsulphone with Br and BrO 总被引:3,自引:0,他引:3
Kinetics and products of the gas-phase reactions of dimethylsulphide (DMS), dimethylsulphoxide (DMSO) and dimethylsulphone (DMSO2) with Br atoms and BrO radicals in air have beeninvestigated using on-line Fourier Transform Infrared Spectroscopy (FT-IR) as analytical technique at 740 ± 5 Torr total pressure and at 296 ± 3 K in a480 L reaction chamber. Using a relative rate method for determining the rate constants; the following values (expressed in cm3molecule–1 s–1) were found: kDMS+Br = (4.9 ±1.0) ×10–14, kDMSO + Br < 6 × 10–14,kDMSO
2 + Br 1 × 10–15,kDMSO + BrO = (1.0 ± 0.3) × 10–14 andkDMSO
2 + BrO 3 × 10–15 (allvalues are given with one on the experimental data). DMSO, SO2, COS, CH3SBr andCH3SO2Br were identified as the main sulphur containing products of the oxidation of DMS by Br atoms. From the reaction between DMSO and Br atoms, DMSO2and CH3SO2Br were the only sulphur containing products thatwere identified. DMSO, DMSO2 and SO2 were identified as themain sulphur containing products of the reaction between DMS and BrO.DMSO2 was found to be the only product of the reaction between DMSO and BrO. For the reactions of DMSO2 with Br and BrO no products were identified because the reactions were too slow.The implications of these results for atmospheric chemistry are discussed. 相似文献
976.
977.
Jennifer L. Lewicki Deborah Bergfeld Carlo Cardellini Giovanni Chiodini Domenico Granieri Nick Varley Cynthia Werner 《Bulletin of Volcanology》2005,68(1):76-90
We present a comparative study of soil CO2 flux () measured by five groups (Groups 1–5) at the IAVCEI-CCVG Eighth Workshop on Volcanic Gases on Masaya volcano, Nicaragua. Groups 1–5 measured using the accumulation chamber method at 5-m spacing within a 900 m2 grid during a morning (AM) period. These measurements were repeated by Groups 1–3 during an afternoon (PM) period. Measured ranged from 218 to 14,719 g m−2 day−1. The variability of the five measurements made at each grid point ranged from ±5 to 167%. However, the arithmetic means of fluxes measured over the entire grid and associated total CO2 emission rate estimates varied between groups by only ±22%. All three groups that made PM measurements reported an 8–19% increase in total emissions over the AM results. Based on a comparison of measurements made during AM and PM times, we argue that this change is due in large part to natural temporal variability of gas flow, rather than to measurement error. In order to estimate the mean and associated CO2 emission rate of one data set and to map the spatial distribution, we compared six geostatistical methods: arithmetic and minimum variance unbiased estimator means of uninterpolated data, and arithmetic means of data interpolated by the multiquadric radial basis function, ordinary kriging, multi-Gaussian kriging, and sequential Gaussian simulation methods. While the total CO2 emission rates estimated using the different techniques only varied by ±4.4%, the maps showed important differences. We suggest that the sequential Gaussian simulation method yields the most realistic representation of the spatial distribution of , but a variety of geostatistical methods are appropriate to estimate the total CO2 emission rate from a study area, which is a primary goal in volcano monitoring research.Editorial responsibility: H Shinohara 相似文献
978.
979.
980.
Jennifer Lawson Peter T Doran Fabien Kenig David J Des marais John C Priscu 《Aquatic Geochemistry》2004,10(3-4):269-301
The perennially ice-covered lakes in the McMurdo Dry Valleys, Antarctica, are part of the coldest and driest ecosystem on earth. To understand lacustrine carbon and nitrogen cycling in this end-member ecosystem, and to define paleolimnological proxies for ice-covered lakes, we measured the stable carbon and nitrogen isotopic composition of particulate organic matter (POM) and benthic organic matter (BOM) within the lakes of Taylor Valley. The 13C compositions of seasonally ice-free edges of the lakes (moats) are enriched relative to under-ice organic matter. Thus, the organic carbon isotopic composition of buried sediments may be a proxy for sample position within the lake. In the moats, 13C values are governed by limited CO2diffusion across benthic cyanobacterial cell membranes. During a high glacial melt (2001–2002) season, both 13CPOM and 13CBOM in the moats were more depleted than during previous low melt years. We propose that this occurred in response to higher [CO2](aq) and/or reduced growth rates resulting from turbidity-induced light limitation. Though moats and under-ice environments are usually poorly connected, during the 2001–2002 season, the enrichment of the 13CPOM values at 6 m depth in the stream-proximal sites relative to deep-profile sites implies enhanced connectivity between these environments. The 13C compositions of BOM and POM profiles in Lake Hoare and Lake Fryxell indicate that these lakes are dominated by benthic productivity. In contrast, in Lake Bonney, the similarity of the 13C values of BOM and POM indicates the pelagic component dominance in the carbon cycle. 相似文献