全文获取类型
收费全文 | 1918篇 |
免费 | 60篇 |
国内免费 | 18篇 |
专业分类
测绘学 | 47篇 |
大气科学 | 126篇 |
地球物理 | 544篇 |
地质学 | 614篇 |
海洋学 | 210篇 |
天文学 | 307篇 |
综合类 | 7篇 |
自然地理 | 141篇 |
出版年
2021年 | 22篇 |
2020年 | 26篇 |
2019年 | 27篇 |
2018年 | 45篇 |
2017年 | 33篇 |
2016年 | 59篇 |
2015年 | 43篇 |
2014年 | 53篇 |
2013年 | 86篇 |
2012年 | 50篇 |
2011年 | 67篇 |
2010年 | 82篇 |
2009年 | 84篇 |
2008年 | 73篇 |
2007年 | 94篇 |
2006年 | 68篇 |
2005年 | 63篇 |
2004年 | 65篇 |
2003年 | 60篇 |
2002年 | 50篇 |
2001年 | 30篇 |
2000年 | 39篇 |
1999年 | 30篇 |
1998年 | 32篇 |
1997年 | 29篇 |
1996年 | 30篇 |
1995年 | 34篇 |
1994年 | 33篇 |
1993年 | 14篇 |
1992年 | 32篇 |
1991年 | 32篇 |
1990年 | 42篇 |
1989年 | 26篇 |
1988年 | 26篇 |
1987年 | 18篇 |
1986年 | 19篇 |
1985年 | 32篇 |
1984年 | 40篇 |
1983年 | 28篇 |
1982年 | 22篇 |
1981年 | 32篇 |
1980年 | 24篇 |
1979年 | 22篇 |
1978年 | 22篇 |
1977年 | 16篇 |
1976年 | 12篇 |
1975年 | 13篇 |
1974年 | 15篇 |
1973年 | 16篇 |
1972年 | 12篇 |
排序方式: 共有1996条查询结果,搜索用时 15 毫秒
221.
Natural background and pollution levels of some heavy metals in soils from the area of Dayton,Ohio 总被引:2,自引:0,他引:2
In a reconnaissance study, trace amounts of Cd, Cu, Pb, and Zn were determined in “A-zone” soil from 22 locations in the Dayton,
Ohio, area. Soil samples were collected at high-volume air monitoring sites in urban, suburban, and rural areas. Measurable
amounts of the elements were found in all of the samples. Positive correlations occur between each metal and particulate matter,
with correlation coefficients of aboutr=0.70, at the 99% confidence level. Natural background values for the elements were measured in soil from rural areas. Theoretical
background values, which are very close to those measured, were calculated from the individual regression equations. Natural
background levels do not exceed 1.00 ppm Cd, 15 ppm Cu, 25 ppm Pb, and 55 ppm Zn. The heavy metal contents of most soils in
the area exceed background by factors of up to 3.0 for Cd, up to 4.5 for Cu, up to 11 for Pb, and up to 4.5 for Zn.
Significant positive correlations among the metals suggest a common source (or sources) for at least some, if not most, of
the heavy metals. For the most part, the highest metal values are found in soils near coal-burning plants. Fly ash from a
local plant contains substantial amounts of the elements. The high lead values are largely due to vehicular exhaust. However,
there is evidence that the metals can also come from the normal deterioration of vehicles.
It appears that airborne pollution is an important source of heavy metals in Dayton area soils. 相似文献
222.
Charles E. Weaver 《Sedimentary Geology》1980,27(4):301-313
Fine-grained sediments and rocks containing a high percentage of phyllosilicate minerals have never been satisfactorily classified. Rocks names are poorly defined. The definition of many terms such as clay, clay mineral and shale are logically inconsistent. The major problems are caused by the assumption that there is a fixed relation between grain size and mineralogy and that structure is a basic parameter.Shallow-buried recent and ancient marine sediments with a high content of phyllosilicates commonly contain 60–80% clay and 20–40% silt. Phyllosilicates are present in approximately the same amounts as the clay-size material. Many fine-grained sediments that have been deeply buried commonly contain 30–40% clay; the phyllosilicate mineral content is in the range of 60–80%.In most fine-grained sediments, as deposited, the majority of the material is in the clay size. With burial and diagenesis, much of the clay-sized material grows to silt size. Thus, claystones are converted to siltstones.The term physil, an abbreviated form of phyllosilicate, is proposed to describe all sheet silicate minerals regardless of grain size. Physilites are rocks with a high content of physils. A simple classification is proposed in which size, mineralogy and structure are treated as independent variables. 相似文献
223.
Jaidong Ko Nancy E. Brown Alexandra Navrotsky Charles T. Prewitt Tibor Gasparik 《Physics and Chemistry of Minerals》1989,16(8):727-733
The phase boundary between MnTiO3 I (ilmenite structure) and MnTiO3 II (lithium niobate structure) has been determined by analysis of quench products from reversal experiments in a cubic anvil
apparatus at 1073–1673 K and 43–75 kbar using mixtures of MnTiO3 I and II as starting materials. Tight brackets of the boundary give P(kbar)=121.2−0.045 T(K). Thermodynamic analysis of this boundary gives ΔHo=5300±1000 J·mol−1, ΔSo = 1.98 ±1J·K−1· mol−1. The enthalpy of transformation obtained directly by transposed-temperature-drop calorimetry is 8359 ±2575 J·mol−1. Possible topologies of the phase relations among the ilmenite, lithium niobate, and perovskite polymorphs are constrained
using the above data and the observed (reversible with hysteresis) transformation of II to III at 298 K and 20–30 kbar (Ross
et al. 1989). The observed II–III transition is likely to lie on a metastable extension of the II–III boundary into the ilmenite
field. However the reversed I–II boundary, with its negative dP/ dT does represent stable equilibrium between ilmenite and lithium niobate, as opposed to the lithium niobate being a quench
product of perovskite. We suggest a topology in which the perovskite occurs stably at low T and high P with a triple point (I, II, III) at or below 1073 K near 70 kbar. The I–II boundary would have a negative P-T slope while the II–III and I–III boundaries would be positive, implying that entropy decreases in the order lithium niobate,
ilmenite, perovskite. The inferred positive slope of the ilmenite-perovskite transition in MnTiO3 is different from the negative slopes in silicates and germanates. These thermochemical parameters are discussed in terms
of crystal structure and lattice vibrations. 相似文献
224.
225.
226.
Climatic Change - The severity and frequency of climate change hazards are increasing around the world. Because the impacts are most acutely felt in local communities, it is critical to improve... 相似文献
227.
Climate Dynamics - This study derived twelve Extreme Rainfall Indices (ERIs) such as the Maximum Dry Spell (MDS) and Maximum Wet Spell (MWS) from daily rainfall observed over the period... 相似文献
228.
229.
We compute the interannual fluctuations of the surface heat budget of the North Atlantic using the trimmed monthly summaries of the Comprehensive Ocean-Atmosphere DataSet (COADS) for the period 1950–1979. The presence of long-period trends in the heat budget imply large variations of the northward cross-equatorial heat transport over the years. To assess the reliability of these variations, we compare the COADS climate signal to that derived from the ocean weather stations (OWSs) of the North Atlantic. The sea surface temperature, air temperature and sea level pressure show good correlation between the anomaly time series derived from the merchant ship monthly summaries of COADS, and those derived from OWS monthly summaries, except for northernmost locations during winter. In contrast, the sensible and latent heat parameters, which require simultaneous measurements of various variables, have merchant ships and ocean weather stations anomaly time series that are poorly correlated. Only in heavily travelled latitudes and during winter, when the air-sea heat exchange anomalies are large, are the merchant ship measurements able to reproduce the interannual fluctuations of the heat fluxes. The long-period trends in the surface heat budget of North Atlantic equatorward of 40° N implied by COADS thus appear unrepresentative of true climate trends. The COADS trends result from a gradual increase in the magnitude of the reported winds over the years due probably to variations in the ratio of measured to estimated winds, as well as from long period fluctuations in the near surface vertical temperature and humidity gradients.
Offprint requests to: R Michaud 相似文献
230.
Man-Ting Cheng Wei-Chun Chou Chia-Pin Chio Shih-Chieh Hsu Yi-Ru Su Pei-Hsuan Kuo Ben-Jei Tsuang Shuen-Hsin Lin Charles C.-K. Chou 《Journal of Atmospheric Chemistry》2008,61(2):155-173
A study has been carried out on water soluble ions, trace elements, as well as PM2.5 and PM2.5–10 elemental and organic carbon samples collected daily from Central Taiwan over a one year period in 2005. A source apportionment
study was performed, employing a Gaussian trajectory transfer coefficient model (GTx) to the results from 141 sets of PM2.5 and PM2.5–10 samples. Two different types of PM10 episodes, local pollution (LOP) and Asian dust storm (ADS) were observed in this study. The results revealed that relative
high concentrations of secondary aerosols (NO3−, SO42− and NH4+) and the elements Cu, Zn, Cd, Pb and As were observed in PM2.5 during LOP periods. However, sea salt species (Na+ and Cl−) and crustal elements (e.g., Al, Fe, Mg, K, Ca and Ti) of PM2.5–10 showed a sharp increase during ADS periods. Anthropogenic source metals, Cu, Zn, Cd, Pb and As, as well as coarse nitrate
also increased with ADS episodes. Moreover, reconstruction of aerosol compositions revealed that soil of PM2.5–10 elevated approximately 12–14% in ADS periods than LOP and Clear periods. A significantly high ratio of non-sea salt sulfate
to elemental carbon (NSS-SO42−/EC) of PM2.5–10 during ADS periods was associated with higher concentrations of non-sea-salt sulfates from the industrial regions of China.
Source apportionment analysis showed that 39% of PM10, 25% of PM2.5, 50% of PM2.5–10, 42% of sulfate and 30% of nitrate were attributable to the long range transport during ADS periods, respectively. 相似文献