全文获取类型
收费全文 | 11598篇 |
免费 | 341篇 |
国内免费 | 804篇 |
学科分类
环境安全 | 12743篇 |
出版年
2024年 | 18篇 |
2023年 | 78篇 |
2022年 | 193篇 |
2021年 | 158篇 |
2020年 | 167篇 |
2019年 | 140篇 |
2018年 | 1611篇 |
2017年 | 1522篇 |
2016年 | 1323篇 |
2015年 | 292篇 |
2014年 | 171篇 |
2013年 | 224篇 |
2012年 | 640篇 |
2011年 | 1519篇 |
2010年 | 832篇 |
2009年 | 708篇 |
2008年 | 1005篇 |
2007年 | 1340篇 |
2006年 | 130篇 |
2005年 | 101篇 |
2004年 | 83篇 |
2003年 | 96篇 |
2002年 | 138篇 |
2001年 | 59篇 |
2000年 | 42篇 |
1999年 | 23篇 |
1998年 | 34篇 |
1997年 | 11篇 |
1996年 | 15篇 |
1995年 | 14篇 |
1994年 | 9篇 |
1993年 | 4篇 |
1992年 | 7篇 |
1991年 | 6篇 |
1990年 | 2篇 |
1989年 | 1篇 |
1988年 | 3篇 |
1987年 | 2篇 |
1986年 | 1篇 |
1984年 | 11篇 |
1983年 | 8篇 |
1935年 | 2篇 |
排序方式: 共有10000条查询结果,搜索用时 890 毫秒
991.
992.
993.
994.
995.
996.
Bioproduction strategies for rare hexose sugars 总被引:1,自引:0,他引:1
Izumori K 《Die Naturwissenschaften》2002,89(3):120-124
A new strategy for the bioproduction of all ketohexoses was developed using hexitols as intermediates. Biocatalysts used to employ the strategy were D-tagatose 3-epimerase, which epimerizes ketohexoses at the C-3 position, and oxidoreductases, which catalyze oxidation-reduction reactions between ketohexoses and the corresponding hexitols. Arranging all the ketohexoses and hexitols in a symmetric ring and connecting them with 20 biochemical reactions, I was able to construct a design for the bioproduction of all the rare ketohexoses. Various aldose isomerases transform ketohexoses into the corresponding aldohexoses, so the strategy is useful for the bioproduction of all the rare hexose sugars. Furthermore, the design revealed that there are four routes to the L-hexose world from the D-hexose one. 相似文献
997.
Ziegler CK 《Environmental management》2002,29(3):409-427
The stability of cohesive sediment deposits during a rare storm is a critical component in the evaluation of remedial options
at a contaminated sediment site. Estimating scour depths during a rare storm, and the resulting contaminant concentrations
in the surficial layer of the bed, is necessary for comparing the efficacy of various remedial alternatives. Evaluation of
sediment stability is accomplished using sediment transport analyses that employ quantitative procedures. Qualitative analyses
or conceptual models can be useful for developing and validating quantitative analysis tools; however, qualitative techniques
alone generally are insufficient for conducting defensible remedial alternative evaluations. The level of analysis used for
a specific site depends on data availability, required level of accuracy, and time and budget constraints. A tier 1 analysis
involves the use of approximate equations to produce order-of-magnitude estimates of scour depths during a rare storm. The
second tier of this analysis scheme employs the development and application of a sediment transport model to evaluate bed
stability. State-of-the-science sediment transport models have been effectively used as management tools for evaluating remedial
options at several contaminated sediment sites. It should not be presumed that rare storm events cause catastrophic impacts
at the site under review. Two case studies demonstrate that a rare storm is not necessarily catastrophic; significant increases
in surficial bed concentrations caused by reexposure of elevated concentrations buried at depth in the bed will not necessarily
occur during a rare storm. However, it is important to note that sediment stability is site-specific. 相似文献
998.
Under the United States Clean Air Act Amendments of 1977, a class I designation safeguards wilderness areas from the negative
effects of new sources of air pollution. We monitored streamwater chemistry in the class I Lye Brook Wilderness in southwestern
Vermont from May 1994 through August 1995. Stream samples were collected biweekly at nine sampling locations throughout the
wilderness and were analyzed for major cations and anions, dissolved organic carbon, pH, and acid-neutralizing capacity. Eight
of nine sites sampled had mean annual acid neutralizing capacity values below zero. During the study period, decreases in
streamwater acid neutralizing capacity values were caused primarily by SO4
2−. At some sites, however, NO3
− and naturally occurring, weak organic acids were seasonally important. During high discharge, the low pH and high concentrations
of inorganic monomeric Al were at levels that are toxic to acid-sensitive aquatic species. Watershed mass balances were calculated
to determine annual gains or losses for measured ions. These budgets indicate that S inputs and outputs were nearly equal,
there was a net loss of base cations, and a net gain in N. How long these watersheds can continue to assimilate additional
N inputs is unknown. 相似文献
999.
Jing Wu Shaw L. Yu Rui Zou 《Journal of the American Water Resources Association》2006,42(5):1193-1204
ABSTRACT: Watershed management strategies generally involve controlling nonpoint source pollution by implementing various best management practices (BMPs). Currently, stormwater management programs in most states use a performance‐based approach to implement onsite BMPs. This approach fails to link the onsite BMP performance directly to receiving water quality benefits, and it does not take into account the combined treatment effects of all the stormwater management practices within a watershed. To address these issues, this paper proposes a water quality‐based BMP planning approach for effective nonpoint source pollution control at a watershed scale. A coupled modeling system consisting of a watershed model (HSPF) and a receiving water quality model (CE‐QUAL‐W2) was developed to establish the linkage between BMP performance and receiving water quality targets. A Monte Carlo simulation approach was utilized to develop alternative BMP strategies at a watershed level. The developed methodology was applied to the Swift Creek Reservoir watershed in Virginia, and the results show that the proposed approach allows for the development of BMP strategies that lead to full compliance with water quality requirements. 相似文献
1000.
In this study, we analyzed the sustainability and determinant factors of waste reduction through variable charging schemes.
We drew three main conclusions from the study. (1) Waste reduction was achieved by variable charging and was sustained over
an average of at least 10 years. The results regarding the sustainability of the price effect were inconclusive. (2) The main
factors contributing to the combustible waste reduction achieved through variable charging were two-tier pricing and the price
of waste bags. The effect of two-tier pricing programs was positive. (3) Waste paper recycling and variable charging reinforce
each other to promote waste reduction.
Received: September 3, 2001 / Accepted: January 13, 2002 相似文献