全文获取类型
收费全文 | 4729篇 |
免费 | 386篇 |
国内免费 | 1454篇 |
专业分类
安全科学 | 466篇 |
废物处理 | 227篇 |
环保管理 | 348篇 |
综合类 | 2884篇 |
基础理论 | 760篇 |
环境理论 | 2篇 |
污染及防治 | 1095篇 |
评价与监测 | 255篇 |
社会与环境 | 251篇 |
灾害及防治 | 281篇 |
出版年
2024年 | 13篇 |
2023年 | 88篇 |
2022年 | 265篇 |
2021年 | 213篇 |
2020年 | 200篇 |
2019年 | 159篇 |
2018年 | 193篇 |
2017年 | 184篇 |
2016年 | 218篇 |
2015年 | 264篇 |
2014年 | 339篇 |
2013年 | 427篇 |
2012年 | 432篇 |
2011年 | 466篇 |
2010年 | 355篇 |
2009年 | 322篇 |
2008年 | 306篇 |
2007年 | 310篇 |
2006年 | 302篇 |
2005年 | 224篇 |
2004年 | 150篇 |
2003年 | 145篇 |
2002年 | 146篇 |
2001年 | 119篇 |
2000年 | 141篇 |
1999年 | 109篇 |
1998年 | 84篇 |
1997年 | 87篇 |
1996年 | 69篇 |
1995年 | 64篇 |
1994年 | 51篇 |
1993年 | 31篇 |
1992年 | 29篇 |
1991年 | 19篇 |
1990年 | 13篇 |
1989年 | 12篇 |
1988年 | 9篇 |
1987年 | 5篇 |
1986年 | 1篇 |
1985年 | 2篇 |
1984年 | 1篇 |
1981年 | 2篇 |
排序方式: 共有6569条查询结果,搜索用时 406 毫秒
941.
Relationships between net primary productivity and stand age for several forest types and their influence on China's carbon balance 总被引:2,自引:0,他引:2
Affected by natural and anthropogenic disturbances such as forest fires, insect-induced mortality and harvesting, forest stand age plays an important role in determining the distribution of carbon pools and fluxes in a variety of forest ecosystems. An improved understanding of the relationship between net primary productivity (NPP) and stand age (i.e., age-related increase and decline in forest productivity) is essential for the simulation and prediction of the global carbon cycle at annual, decadal, centurial, or even longer temporal scales. In this paper, we developed functions describing the relationship between national mean NPP and stand age using stand age information derived from forest inventory data and NPP simulated by the BEPS (Boreal Ecosystem Productivity Simulator) model in 2001. Due to differences in ecobiophysical characteristics of different forest types, NPP-age equations were developed for five typical forest ecosystems in China (deciduous needleleaf forest (DNF), evergreen needleleaf forest in tropic and subtropical zones (ENF-S), deciduous broadleaf forest (DBF), evergreen broadleaf forest (EBF), and mixed broadleaf forest (MBF)). For DNF, ENF-S, EBF, and MBF, changes in NPP with age were well fitted with a common non-linear function, with R(2) values equal to 0.90, 0.75, 0.66, and 0.67, respectively. In contrast, a second order polynomial was best suitable for simulating the change of NPP for DBF, with an R(2) value of 0.79. The timing and magnitude of the maximum NPP varied with forest types. DNF, EBF, and MBF reached the peak NPP at the age of 54, 40, and 32 years, respectively, while the NPP of ENF-S maximizes at the age of 13 years. The highest NPP of DBF appeared at 122 years. NPP was generally lower in older stands with the exception of DBF, and this particular finding runs counter to the paradigm of age-related decline in forest growth. Evaluation based on measurements of NPP and stand age at the plot-level demonstrates the reliability and applicability of the fitted NPP-age relationships. These relationships were used to replace the normalized NPP-age relationship used in the original InTEC (Integrated Terrestrial Ecosystem Carbon) model, to improve the accuracy of estimated carbon balance for China's forest ecosystems. With the revised NPP-age relationship, the InTEC model simulated a larger carbon source from 1950-1980 and a larger carbon sink from 1985-2001 for China's forests than the original InTEC model did because of the modification to the age-related carbon dynamics in forests. This finding confirms the importance of considering the dynamics of NPP related to forest age in estimating regional and global terrestrial carbon budgets. 相似文献
942.
943.
944.
我国体制转轨时期,面对当前的腐败,究其原因除了腐败本身具有的顽固性尚未得到根本遇制外,缺少科学的廉政考核体系也是一个重要原因.本文从我国公务员康政概念开始,通过一系列指标体系的测量,充分利用现代科技不断进行制度和方法的创新,不断完善我国公务员康政考核的指标体系. 相似文献
945.
Optimizing model for land use/land cover retrieval from remote sensing imagery based on variable precision rough sets 总被引:2,自引:0,他引:2
The suitable spectral mode in remote sensing is often desirable to facilitate the inversion of ecological environment and landscape. This paper put forward an optimizing model based on variable precision rough sets (VPRS) for the land cover discrimination in wetland inventory. In the case study of Lake Baiyangdian which has important ecological functions to the northern China, this model is established successfully according to the domain-experts knowledge. The procedure is as follows. First step is data collection, including remote-sensing data (e.g., Landsat-5 TM bands), the digitized relief maps, and statistical yearbooks. Second, the remote sensing imagery (RSI) and relief maps are co-registered into the same resolution. Third, a condition set, including various attributes is derived from spectral bands, band math or ratio indices based on previous studies, at the same time, the decision set is derived from true land types after investigation and validation. Then, the remote sensing decision table (RSDT) is constructed by linking condition set with decision set according to the sequential pixels in RSI. Fourth, we create one forward greedy searching algorithm based on VPRS to handle this RSDT. After adjusting parameters such as β and knowledge granularity diameter (KGD), we obtain the stable optimized results. Comparative experiments and evaluation show that the discrimination or retrieval accuracy of VPRS model is satisfying (overall accuracy: 87.32% and KHAT: 0.84) and better than original data. Moreover, data dimension has been decreased dramatically (from 12 to 3) and key attributes found by the model may be useful for specific retrieval in wetland inventories. 相似文献
946.
Feng Qin Shujuan Wang Inna Kim Hallvard F. Svendsen Changhe Chen 《International Journal of Greenhouse Gas Control》2011,5(3):405-412
A reaction calorimeter was used to determine the enthalpies of absorption of CO2 in aqueous ammonia and in aqueous solutions of ammonium carbonate at temperatures of 35–80 °C. The heat of absorption of CO2 with 2.5 wt% aqueous ammonia solution was found to be about 70 kJ/mol CO2, which is lower than that with MEA (around 85 kJ/mol) at 35 and 40 °C. The value decreases with increased loading, but not to as low a value as expected by the carbonate–bicarbonate reaction (26.88 kJ/mol). The enthalpy of absorption of CO2 in aqueous ammonia at 60 and 80 °C decreases with loadings at first, then increases between 0.2 mol CO2/mol NH3 and 0.6 mol CO2/mol NH3, and then decreases again. The behavior of the heat of absorption of CO2 in 10 wt% ammonium carbonate solution was found to be the same as that of aqueous ammonia at loadings above 0.6 mol CO2/mol NH3. The heat of absorption increases with increasing temperature. The heats of absorption are directly related to the extent of the various reactions with CO2 and can be assessed from the species variation in the liquid phase. 相似文献
947.
948.
Assessment of elemental distribution and trace element contamination in surficial wetland sediments, Southern Tibetan Plateau 总被引:2,自引:0,他引:2
Qianggong Zhang Shichang Kang Chaoliu Li Feng Chen Zuzana Boukalova Ivo ?erny 《Environmental monitoring and assessment》2011,173(1-4):301-313
In order to analyze and evaluate different trace metals on surface water of the Changjiang River, concentrations of dissolved trace metals (Cu, Ni, Fe, Co, Sc, Al, Zn, Pb, Cd, Se, As, Cr, and Hg), major elements(Ca and Mg), and nutrient(NO $_{3}^{-})$ were measured. Samples were taken at 76 positions along Changjiang River in flood and dry seasons during 2007?C2008. Spatial distributions identified two main large zones mainly influenced by mineral erosion (sites 1?C22) and anthropogenic action (sites 23?C76), respectively. Principal component analysis (PCA) and hierarchical cluster analysis were used to identify the variance distinguishing the origin of water. Four significant components were extracted by PCA, explaining 74.91% of total variable. Cu, Ni, Fe, Co, Sc, Al, Ca, and Mg were mainly associated with the weathering and erosion of various rocks and minerals, while an anthropogenic source was identified for Cd and As. Although erosion was one source of Pb and Zn, they were also input by atmospheric deposition and industrial pollutions. NO $_{3}^{-}$ and Se were mainly associated with agriculture activities. However, Hg and Cr showed different sources. CA confirmed and completed the results obtained by PCA, classifying the data into two large groups representing different areas. Group 1 referred to the upper reaches which represented samples mainly corresponding to natural background areas. Group 2 referred to the middle and lower reaches including samples under anthropogenic influence. Meanwhile, group 2 was subdivided into three new groups, representing agricultural, industrial, and various artificial pollution sources, respectively. 相似文献
949.
Xu H Bi XH Feng YC Lin FM Jiao L Hong SM Liu WG Zhang XY 《Environmental monitoring and assessment》2011,183(1-4):581-592
To understand the origin and chemical characteristics of precipitation in Hangzhou, rainwater samples were collected from June 2006 to May 2008. All samples were analyzed for pH, electrical conductivity, and major ions (NH??, Ca2?, Mg2?, Na?, K?, SO?2?, NO??, F?, and Cl?). Acidification of precipitation in Hangzhou was serious with volume-weighted mean pH value of 4.5, while frequency of acid rain was 95%. The calculated SO?2?/NO?? ratio in Hangzhou precipitation was 2.87, which indicated that the precipitation of Hangzhou belonged to sulfate-based acid rain. The results of acid neutralization analysis showed that not all the acidity in the precipitation of Hangzhou was neutralized by alkaline constituents. The results of sea salt contribution analysis showed that nearly all SO?2?, Ca2?, and Mg2? and 33.7% of K? were of non-sea origins, while all Na? and Cl? and 66.3% of K? originated from sea sources. The principal component analysis which was used to analyze the sources of various ions indicated that chemical compositions of precipitation in Hangzhou mainly came from terrestrial sources, factory emissions, fuel wood burning, and marine sources. 相似文献
950.
Deming Dong Zhiyong Guo Xiuyi Hua Ying Lan Jinting Zhou Xiaoou Ding Qianqian Qiao 《Environmental Chemistry Letters》2011,9(3):361-367
The sorption by solid materials is one of the most important processes that affect the fate of hydrophobic organic pollutants
in aquatic systems. Current studies focus on the sorption of hydrophobic pollutants by single natural solid material, whereas
few studies compared the sorption behaviors of different solids. Furthermore, natural aquatic environment is a complex system
where hydrophobic pollutants coexist with many other pollutants, but only limited studies reported the effects of coexisting
components on the sorption. Here we studied the sorption of dichlorodiphenyltrichloroethanes (DDTs) by three common solids
in natural water, i.e., biofilms, suspended particles and sediments. The effects of typical coexisting heavy metals on the
sorption were also examined. The results show that the sorption is described by a dual-mode sorption model. Compared with
sediments, more DDTs were sorbed by biofilms and suspended particles. Coexisting Pb, Cd and Cu increased the sorption of DDTs
on sediments while decreasing the sorption on biofilms slightly, with little effect on the sorption on suspended particles.
The different sorption behaviors of the solids were mainly determined by the composition of the solids: more organics were
observed in biofilms and suspended particles. 相似文献