首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   5080篇
  免费   240篇
  国内免费   37篇
安全科学   230篇
废物处理   209篇
环保管理   931篇
综合类   1175篇
基础理论   1299篇
环境理论   9篇
污染及防治   1010篇
评价与监测   227篇
社会与环境   163篇
灾害及防治   104篇
  2021年   48篇
  2020年   33篇
  2019年   68篇
  2018年   135篇
  2017年   116篇
  2016年   138篇
  2015年   137篇
  2014年   160篇
  2013年   485篇
  2012年   214篇
  2011年   267篇
  2010年   217篇
  2009年   213篇
  2008年   226篇
  2007年   251篇
  2006年   255篇
  2005年   192篇
  2004年   158篇
  2003年   154篇
  2002年   166篇
  2001年   95篇
  2000年   88篇
  1999年   65篇
  1998年   62篇
  1997年   65篇
  1996年   51篇
  1995年   67篇
  1994年   65篇
  1993年   39篇
  1992年   39篇
  1991年   29篇
  1990年   31篇
  1986年   23篇
  1985年   22篇
  1984年   24篇
  1983年   27篇
  1980年   22篇
  1979年   28篇
  1978年   23篇
  1965年   25篇
  1964年   22篇
  1963年   32篇
  1962年   24篇
  1960年   27篇
  1959年   29篇
  1957年   26篇
  1955年   31篇
  1954年   36篇
  1953年   21篇
  1951年   26篇
排序方式: 共有5357条查询结果,搜索用时 15 毫秒
21.
Endotoxin exposure is associated with wheeze and asthma morbidity, while early life exposure may reduce risk of allergy and asthma. Unfortunately, it is difficult to compare endotoxin results from different laboratories and environments. We undertook this study to determine if lipopolysaccharide (LPS) extraction efficiency could account for differences among laboratories. We generated and collected aerosols from chicken and swine barns, and corn processing. We randomly allocated side-by-side filter samples to five laboratories for Limulus assay of endotoxin. Lyophilized aliquots of filter extracts were analyzed for 3-hydroxy fatty acids (3-OHFAs) as a marker of LPS using gas chromatography-mass spectrometry. There were significant differences in endotoxin assay and GC-MS (LPS) results between laboratories for all dust types (p < 0.01). Patterns of differences between labs varied by dust type. Relationships between assay and GC/MS results also depended on dust type. The percentages of individual 3-OHFA chain lengths varied across labs (p < 0.0001) suggesting that each lab recovered a different fraction of the LPS available. The presence of large amounts of particle associated LPS and absence of a freezing thawing cycle were associated with lower correlations between LPS and bioactivity, consistent with an absence of Limulus response to cell-bound endotoxin. These data suggest that extraction methods affect endotoxin measurements. The LAL methods may be most suitable when comparing exposures within similar environments; GC-MS offers additional information helpful in optimizing sample treatment and extraction. GC-MS may be of use when comparing across heterogeneous environments and should be considered for inclusion in future studies of human health outcomes.  相似文献   
22.
Despite the numerous benefits of hydropower production, this renewable energy source can have serious negative consequences on the environment. For example, dams act as barriers for the longitudinal migration of organisms and transport of particulate matter. Accelerated siltation processes in the receiving river reduce the vertical connectivity between river and groundwater. Hydropeaks, caused by short-term changes in hydropower operation, result in a negative impact on both habitat and organisms, especially during winter months when natural discharge is low and almost constant. In this study, we report the current deficits present in the River Rhone from two different scientific perspectives – fish ecology and hydrology. Potential rehabilitation solutions in synergy with flood protection measures are discussed. We focus on the effects of hydropeaking in relation to longitudinal and vertical dimensions and discuss local river widening as a potential rehabilitation tool. The fish fauna in the Rhone is characterized by a highly unnatural structure (low diversity, impaired age distribution). A high correlation between fish biomass and monotonous morphology (poor cover availability) was established. Tracer hydrology provided further details about the reduced permeability of the riverbank, revealing a high degree of siltation with K values of about 4.7 × 10?6 m s?1. Improving the hydrologic situation is therefore essential for the successful rehabilitation of the Rhone River. To this end, hydropeaks in the river reaches must be attenuated. This can be realized by a combination of different hard technical and soft operational measures such as retention reservoirs or slower up and down ramping of turbines.  相似文献   
23.
Chemical monitoring of aquatic ecosystems describes the chemical exposures of aquatic biota and measures the success of pollution control. However, meeting water quality criteria cannot assure that aquatic biota are protected from the effects of unexpected chemicals, mixtures and interactions between toxicity and environmental stressors.Biological monitoring is an obvious solution since aquatic biota integrate spatial and temporal variations in exposure to many simultaneous stressors. Top predators, typical of specific ecosystems (e.g. lake trout in cold water oligotrophic lakes) indicate whether environmental criteria have been met. The presence of naturally reproducing, self-sustaining and productive stocks of edible fish demonstrates a high quality environment. If these conditions are not met, there is a clear sign of environmental degradation. Specific changes in population structure and performance may also diagnose which life stage is affected and the nature of the stressor.Unfortunately, environmental managers cannot rely solely on populations, communities or ecosystems to indicate chmical effects. The lag between identifying a problem and finding a cause may destroy the resource that we wish to protect, particularly where chemicals are persistent.A solution to this dilemma is the measurement of primary or secondary responses of individual organisms to chemical exposure. Since toxicity at any level of organization must start with a reaction between a chemical and a biological substrate, these responses are the most sensitive and earliest sign of chemical exposure and effect.Application of this idea requires research on molecular mechanisms of chemical toxicity in aquatic biota and adaptation of existing mammalian diagnostic tools. Since relevance of biochemical responses to populations and ecosystems is not obvious, there is a need to study the links between chemical exposure and responses of individuals, populations and ecosystems.The recognition of chemical problems and cause-effect relationships requires the integration of chemical and biological monitoring, using the principles of epidemiology to test the strength of relationships and to identify specific research needs. The contamination of a reservoir with selenium and impacts on fish populations provide an excellent example of this approach.  相似文献   
24.
Aerosol constituents (elemental carbon, organic carbon, soluble ions including organic acids, and selected trace metals) were investigated from samples of a field campaign taking place at Bhola Island in the Bay of Bengal (Bangladesh). The campaign took place in the pre-monsoon season (May 2001) using low volume samplers. Carbonaceous material comprised the majority of the analysed components. The average concentrations of EC and OC were 2.8 and 4.6 microg m(-3), respectively. Oxalic acid was the most abundant dicarboxylic acid (average 268 ng m(-3)) followed by malonic and malic acid. The contribution of carboxylic acids-carbon to organic carbon was 2.0%. Average concentrations observed for sulfate and nitrate were 3.7 microg m(-3) and 1.5 microg m(-3). Two different types of aerosol were identified at the rural background site on Bhola Island during southerly synoptic flow by means of trajectory analysis: air masses were transported from the Bay of Bengal to the sampling site in all cases. However, during "Period 1" they experienced longer residence times over the Indian Ocean, while the "Period 2" trajectories came along the Indian coast or passed over the Indian continent. During Period 1 the concentration levels of soluble ions were a factor of 4-6 lower than during Period 2. The concentrations of EC, OC and K differed less than a factor of 1.5 between the two periods. The Period 1 aerosol showed similarities to the haze layers observed during winter-monsoon conditions south of India during the INDOEX experiment. Based on EC/TC and K/EC ratios we find that around 80% of the carbonaceous aerosol from Period 1 in Bhola is from fossil fuel and only around 50% from Period 2. Absolute concentrations of carbonaceous species, soluble ions and trace metals indicate that the background site on Bhola Island is affected by emissions from urbanized regions of Southeast Asia.  相似文献   
25.
An electrical device capable of continuously measuring micronsized changes in stem diameter of woody and herbaceous plants is described. Fumigation of sunflowers with automotive exhaust was used to test the ability of the device to detect short-term plant responses.  相似文献   
26.
Extreme pollutant values are of great interest in water quality monitoring because of their frequent toxicological significance. The principal barrier to the detection of these values, however, is the cost of extensive and comprehensive monitoring. This paper demonstrates an efficient method to determine the maximum sample measurement from a finite set of sequential samples without explicitly testing them all. It is assumed that the process of sample measurement is distinct from collection and has higher costs. It is further assumed that the measurements have high positive autocorrelation.A methodology is presented based on a common industrial testing procedure referred to as composite sampling—the physical pooling or compositing of a set of sequential samples before measurement. A method known as primary first order compositing (PFOC) was found to be superior to the traditional technique of random sampling, particularly if small composite sizes are utilized.The authors are, respectively, operations research analyst, Vancouver, B. C.; Associate Professor and Chairman, Policy Analysis Division, Faculty of Commerce and Business Administration, University of British Columbia; and Associate Professor, Management Science Division, Faculty of Commerce, U.B.C.  相似文献   
27.
The Impact of Landsat Satellite Monitoring on Conservation Biology   总被引:1,自引:0,他引:1  
Landsat 7s recent malfunctioning will result in significant gaps in long-term satellite monitoring of Earth, affecting not only the research of the Earth science community but also conservation users of these data. To determine whether or how important Landsat monitoring is for conservation and natural resource management, we reviewed the Landsat programs history with special emphasis on the development of user groups. We also conducted a bibliographic search to determine the extent to which conservation research has been based on Landsat data. Conservation biologists were not an early user group of Landsat data because a) biologists lacked technical capacity – computers and software – to analyze these data; b) Landsats 1980s commercialization rendered images too costly for biologists budgets; and c) the broad-scale disciplines of conservation biology and landscape ecology did not develop until the mid-to-late 1980s. All these conditions had changed by the 1990s and Landsat imagery became an important tool for conservation biology. Satellite monitoring and Landsat continuity are mandated by the Land Remote Sensing Act of 1992. This legislation leaves open commercial options. However, past experiments with commercial operations were neither viable nor economical, and severely reduced the quality of monitoring, archiving and data access for academia and the public. Future satellite monitoring programs are essential for conservation and natural resource management, must provide continuity with Landsat, and should be government operated.  相似文献   
28.
Book reviews     
Cost Benefit Analysis of Environmental Change

Per‐Olov Johansson

Cambridge, Cambridge University Press, 1993, 232 pp., £13.95

Computerised Environmental Modelling: A Practical Introduction using Excel

J. Hardisty, D. M. Taylor & S. E. Metcalfe

Chichester, Wiley, 1993, 204 pp., £14.95

Keeping Pace with Science and Engineering: Case Studies in Environmental Regulation

Washington DC, US National Academy of Engineering, 1993

World Without End: Economics, Environment and Sustainable Development

D. W. Pearce & J. J. Warford

Oxford University Press, New York, 1993, 440 pp., £35.95  相似文献   

29.
A principle of natural self-organization   总被引:20,自引:0,他引:20  
The Science of Nature - This paper is the first part of a trilogy, which comprises a detailed study of a special type of functional organization and demonstrates its relevance with respect to the...  相似文献   
30.
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号