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   25649篇
  免费   232篇
  国内免费   241篇
安全科学   665篇
废物处理   892篇
环保管理   3321篇
综合类   5987篇
基础理论   6100篇
环境理论   9篇
污染及防治   6391篇
评价与监测   1568篇
社会与环境   1058篇
灾害及防治   131篇
  2022年   173篇
  2018年   308篇
  2017年   312篇
  2016年   458篇
  2015年   369篇
  2014年   576篇
  2013年   1882篇
  2012年   709篇
  2011年   954篇
  2010年   741篇
  2009年   851篇
  2008年   949篇
  2007年   1005篇
  2006年   903篇
  2005年   766篇
  2004年   748篇
  2003年   779篇
  2002年   692篇
  2001年   961篇
  2000年   640篇
  1999年   443篇
  1998年   280篇
  1997年   298篇
  1996年   329篇
  1995年   344篇
  1994年   356篇
  1993年   290篇
  1992年   326篇
  1991年   335篇
  1990年   349篇
  1989年   319篇
  1988年   299篇
  1987年   248篇
  1986年   251篇
  1985年   270篇
  1984年   247篇
  1983年   260篇
  1982年   263篇
  1981年   266篇
  1980年   222篇
  1979年   223篇
  1978年   226篇
  1977年   195篇
  1976年   213篇
  1975年   178篇
  1974年   227篇
  1973年   195篇
  1972年   198篇
  1971年   173篇
  1967年   201篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
91.
92.
93.
Living veligers of the Cassoidea have been observed to use a mantle appendage to form and resorb periostracum. Anatomical and histological examinations of a ranellid (Cymatium sp.) larva collected from the Red Sea in 1987 revealed the structure and location of the pallial appendage. The mantle edges of juvenile or adult species of the Cassoidea do not show a comparable specialization. It is demonstrated that cassoid larval conch characters are sufficient to prove the existence of a pallial appendage without anatomical confirmation. A mantle appendage is not known from teleplanic (long-living planktic) veligers of other gastropod superfamilies. In cases where the larval strategies of the latter are known they are totally different. Therefore it is suggested that the adaptation of cassoid larvae to pelagic life is unique among gastropods representing an autapomorphic character of the superfamily.  相似文献   
94.
This paper examines the relationship between survivor reactions to a downsizing and retention subsequent to a downsizing. We hypothesize that survivors who experience the downsizing as distributively, procedurally, and interactionally just and who see top management as trustworthy will feel more attached to the organization because each reduces the threat inherent in downsizing. In addition, we hypothesize that survivors who feel empowered will also feel more attached to the organization because they feel better able to cope with the downsizing. We further hypothesize that those survivors who feel more attached to the organization following the downsizing will be more likely to remain with the organization in the coming year. The theoretical model is tested on a sample of aerospace employees who survived an organizational downsizing. The trustworthiness of management, distributive justice, procedural justice, and three dimensions of empowerment are found to facilitate more organizational attachment. Higher levels of attachment are found, in turn, to facilitate less voluntary turnover in the year following the downsizing. Copyright © 2002 John Wiley & Sons, Ltd.  相似文献   
95.
96.
97.
Stormwater runoff is now a major contributor to the pollution of coastal waters in the United States. Public agencies are responding by requiring stormwater monitoring to satisfy the National Pollutant Discharge Elimination System stormwater permit. However, studies to understand the utility of the current programs or to improve their usefulness have not yet been performed. In this paper, we evaluate the land-use-based program, the industrial stormwater permit program, and beach water-quality monitoring in the County of Los Angeles, California, to determine if the results will be helpful to planners and regulators in abating stormwater pollution. The utility of the program has been assessed based on the programs' ability to accurately estimate the emissions for different classes of land use. The land-use program appears successful, while the industrial monitoring program does not. Beach water-quality monitoring suffers from a lack of real-time monitoring techniques. We also provide suggested improvements, such as sampling method and time, and parameter selection.  相似文献   
98.
The amount of NO2 and NO produced by the machine smoking of cigarettes was determined for 15 commercial Canadian brands. Average yield of NO was 1.44 μmoles or about 13% of the average reported for American cigarettes. Levels of NO2 were less than 12% of NO and were probably due to the oxidation of NO. In order to assess the contribution of tobacco smoke to levels of NO in ambient air, 5 brands of cigarettes were smoked in 27 cubic meter controlled environment room. Ventilation conditions were either 2.5 or 5.0 air changes per hour (ACH) and each experiment was replicated 3 times for a total of 30 experiments. Ventilation rates of 0.3 and 1.5 ACH were also selected in a second series of experiments in which only one brand of cigarette was smoked. Least squares estimates for the effective ventilation rates were obtained in the usual manner after linearizing the decay portion of the NO time curve. In each of the experiments, the regression explained at least 95% of the variation in the levels of NO with time. Loss of NO due to factors other than ventilation appeared to be constant within experimental error and averaged 2.22 ACH. Equilibrium values for NO were grossly underestimated when results from currently accepted proecedures for smoke analysis were used in modeling the growth and decay of NO. Goodness-of-fit was improved when equilibrium values were estimated based on observed levels in ambient air. This approach may be more suitable for evaluating the potential contribution of cigarette smoke to levels of indoor air pollutants.  相似文献   
99.
100.
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