全文获取类型
收费全文 | 879篇 |
免费 | 143篇 |
国内免费 | 435篇 |
专业分类
安全科学 | 135篇 |
废物处理 | 17篇 |
环保管理 | 57篇 |
综合类 | 752篇 |
基础理论 | 302篇 |
污染及防治 | 96篇 |
评价与监测 | 43篇 |
社会与环境 | 45篇 |
灾害及防治 | 10篇 |
出版年
2024年 | 5篇 |
2023年 | 15篇 |
2022年 | 42篇 |
2021年 | 51篇 |
2020年 | 47篇 |
2019年 | 50篇 |
2018年 | 50篇 |
2017年 | 69篇 |
2016年 | 82篇 |
2015年 | 73篇 |
2014年 | 73篇 |
2013年 | 119篇 |
2012年 | 80篇 |
2011年 | 61篇 |
2010年 | 60篇 |
2009年 | 77篇 |
2008年 | 72篇 |
2007年 | 99篇 |
2006年 | 56篇 |
2005年 | 44篇 |
2004年 | 44篇 |
2003年 | 25篇 |
2002年 | 16篇 |
2001年 | 12篇 |
2000年 | 31篇 |
1999年 | 13篇 |
1998年 | 12篇 |
1997年 | 12篇 |
1996年 | 10篇 |
1995年 | 14篇 |
1994年 | 12篇 |
1993年 | 7篇 |
1992年 | 3篇 |
1991年 | 5篇 |
1990年 | 3篇 |
1989年 | 1篇 |
1987年 | 1篇 |
1986年 | 2篇 |
1985年 | 1篇 |
1984年 | 2篇 |
1982年 | 1篇 |
1980年 | 1篇 |
1979年 | 2篇 |
1972年 | 1篇 |
1971年 | 1篇 |
排序方式: 共有1457条查询结果,搜索用时 708 毫秒
751.
A good estimate of the design effect is critical for calculating the most efficient sample size for cluster surveys. We reviewed the design effects for seven nutrition and health outcomes from nine population-based cluster surveys conducted in emergency settings. Most of the design effects for outcomes in children, and one-half of the design effects for crude mortality, were below two. A reassessment of mortality data from Kosovo and Badghis, Afghanistan revealed that, given the same number of clusters, changing sample size had a relatively small impact on the precision of the estimate of mortality. We concluded that, in most surveys, assuming a design effect of 1.5 for acute malnutrition in children and two or less for crude mortality would produce a more efficient sample size. In addition, enhancing the sample size in cluster surveys without increasing the number of clusters may not result in substantial improvements in precision. 相似文献
752.
753.
二氧化锰颗粒对Hela细胞DNA损伤的尺度依赖性毒作用(英文) 总被引:1,自引:0,他引:1
为了观察二氧化锰颗粒物所致的尺度依赖性DNA损伤作用,将纳米尺度二氧化锰颗粒物(Nano-MnO2)和常规尺度二氧化锰颗粒物(Nor-MnO2)所致的DNA损伤进行了对比研究.将Hela细胞分别暴露于不同浓度(0、100、200、400μg·mL-1)的Nano-MnO2和Nor-MnO2中,染毒24h,采用彗星实验检测Hela细胞的DNA损伤水平.结果表明:与对照组相比,Nano-MnO2和Nor-MnO2均可使彗尾DNA百分比(TailDNA%)和尾矩(TailMoment)显著增加(p<0.01);而在同一浓度水平上,Nano-MnO2所致的DNA损伤则比Nor-MnO2所致的DNA损伤更为严重(p<0.01).结果提示:二氧化锰颗粒对Hela细胞DNA损伤具有尺度依赖性毒作用,纳米尺度比常规尺度二氧化锰颗粒毒作用更强烈. 相似文献
754.
A two-year study on temporal variations in the ground water heterotrophic bacterial cell sizes of free living bacteria(FLB)and particle bound bacteria(PBB)from the agricultural,domestic and industrial areas was carried out from Februar y2005 to January 2007.The overall mean cell length of FLB and PBB was similar in all the ground water studied.However,the season wise grouped data revealed significant seasonal changes in cell length of FLB and PBB,as smaller bacteria were noticed during rainy season in the ground water in agricultural area in both the years,and only in the second year of study in domestic and industrial areas.Generally,it was noticed that there were summer maximum and rainy minimum values of the cell length of PBB in the ground water in agricultural,domestic and industrial areas in the second year of study.The Pearson's correlations showed the presence of 8(in agricultural area),5(in domestic)and 3(in industrial) significant correlations with environmental(Physico-chemical)parameters,respectively.The regression analysis revealed that as much as 12%of variation in the mean length of FL Bwas due to NO3( )in agricultural area and 9%due to total solids( )indomestic area.However,the 8% variation in bacterial cell size of FLB was due to Mg( )in industrial area.Whereas,13%variation in mean length of PBB was due to S04( )in agncultural area and 10%due to total anions of strong acid(TASA)( )in domestic area.Furthermore,10% of variation Was due to PO4( )in industrial area.Thus,the statistical analysis revealed that several environmental variables were potentially responsible for some of the temporal variations in aquatic heterotrophic bacterial cell size,suggesting probably the stressed environment in these ecosystems. 相似文献
755.
A new flocculation model using variable fractal dimension is proposed and validated with several experimental data and an
existing model. The proposed model consists of two processes: aggregation and breakup due to flow turbulence. For aggregation
process, the aggregate structure is considered to have the characteristic of self-similarity, the main concept of fractal
theory. Under this assumption, a variable fractal dimension instead of a fixed one adopted by previous studies is utilized
here for general cohesive sediment transport. For breakup, similar concept is adopted in a more empirical manner because breakup
is too abrupt to entirely apply the concept of variable fractal dimension. By a linear combination of the formulations for
aggregation and breakup processes, a flocculation model which can describe the temporal evolution of floc size is obtained.
Flocculation model using variable fractal dimension is capable of predicting equilibrium floc size when compared with several
experimental data sets using different types of mud provided that empirical coefficients are calibrated. Through model-data
comparison with Manning and Dyer (Marine Geology 160:147–170, 1999), it is also clear that some of the empirical coefficients
may depend on sediment concentration. Model results for the temporal evolution of floc size are less satisfactory, despite
model results shows a more smooth “S-curve” for the temporal evolution of floc size as compared with the previous model using
fixed fractal dimension. The proposed model is limited to mono-size of primary particle and dilute flow condition. These other
features shall be investigated as future work. 相似文献
756.
Griffith DA 《Environmental geochemistry and health》2008,30(6):495-509
Properly sampling soils and mapping soil contamination in urban environments requires that impacts of spatial autocorrelation
be taken into account. As spatial autocorrelation increases in an urban landscape, the amount of duplicate information contained
in georeferenced data also increases, whether an entire population or some type of random sample drawn from that population
is being analyzed, resulting in conventional power and sample size calculation formulae yielding incorrect sample size numbers
vis-à-vis model-based inference. Griffith (in Annals, Association of American Geographers, 95, 740–760, 2005) exploits spatial statistical model specifications to formulate equations for estimating the necessary sample
size needed to obtain some predetermined level of precision for an analysis of georeferenced data when implementing a tessellation
stratified random sampling design, labeling this approach model-informed, since a model of latent spatial autocorrelation
is required. This paper addresses issues of efficiency associated with these model-based results. It summarizes findings from
a data collection exercise (soil samples collected from across Syracuse, NY), as well as from a set of resampling and from
a set of simulation experiments following experimental design principles spelled out by Overton and Stehman (in Communications in Statistics: Theory and Methods, 22, 2641–2660). Guidelines are suggested concerning appropriate sample size (i.e., how many) and sampling network (i.e., where).
相似文献
Daniel A. GriffithEmail: |
757.
Range Size and Extinction Risk in Forest Birds 总被引:2,自引:0,他引:2
Abstract: Small geographical range size is the single best predictor of threat of extinction in terrestrial species. Knowing how small a species' range has to be before authorities consider it threatened with extinction would allow prediction of a species' risk from continued deforestation and warming climates and provide a baseline for conservation and management strategies aspiring to mitigate these threats. To determine the threshold at which forest-dependent bird species become threatened with extinction, we compared the range sizes of threatened and nonthreatened species. In doing so, we present a simple, repeatable, and practical protocol to quantify range size. We started with species' ranges published in field guides or comparable sources. We then trimmed these ranges, that is, we included only those parts of the ranges that met the species' requirements of elevation and types of forest preferred. Finally, we further trimmed the ranges to the amount of forest cover that remains. This protocol generated an estimate of the remaining suitable range for each species. We compared these range estimates with those from the World Conservation Union Red List. We used the smaller of the two estimates to determine the threshold, 11,000 km2 , below which birds should be considered threatened. Species considered threatened that have larger ranges than this qualified under other (nonspatial) red list criteria. We identified a suite of species (18) that have not yet qualified as threatened but that have perilously small ranges—about 11% of the nonthreatened birds we analyzed. These birds are likely at risk of extinction and reevaluation of their status is urgently needed. 相似文献
758.
Pahokee泥炭腐殖酸的相对分子量分布特征及其对菲的吸附影响 总被引:8,自引:0,他引:8
使用切面流超滤方法,将Pahokee泥炭中分离出的腐殖酸分为具有不同相对分子质量的8个级分(UF1:<1k;UF2:1~3k;UF3:3~5k;UF4:5~10k;UF5:10~30k;UF6:30~50k;UF7:50~100k:UF8:>300k),表明该泥炭中的腐殖酸主要分布于几个高相对分子量级分中(UF5,UF6和UF8),可占所提取的腐殖酸质量的83 5%.体积排阻色谱的流出曲线说明可以通过超滤手段将腐殖酸这种复杂的、非均匀性的大分子物质分成相对均一的、窄相对分子量分布的分子.而两种方法所测相对分子量的差异可能与两个操作系统间不同的操作条件有关.各超滤级分的腐殖酸对菲的吸附实验说明相对分子量是影响腐殖酸吸附的一个重要参数;随相对分子量的增加,腐殖酸对菲的吸附及其非线性特征均增强. 相似文献
759.
760.
为探究进气口宽度对火旋风燃烧特性的影响,利用自然驱动的四面边墙夹缝式装置对不同进气口宽度使用150,200,250 mm 3种直径进行实验研究。由燃料质量随时间变化曲线得出准稳态阶段火旋风的质量燃烧速率;利用摄像机记录火焰的连续图像并获得火焰高度。结果表明:针对实验室小型火旋风(150~250 mm),在进气口宽度为35 mm左右时,火旋风的燃烧速率最大,即火旋风的燃烧强度存在1个最佳的进气口宽度;当火焰高度达到最高时,环量也存在1个临界值。 相似文献