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   16947篇
  免费   152篇
  国内免费   145篇
安全科学   343篇
废物处理   692篇
环保管理   2005篇
综合类   3725篇
基础理论   3804篇
环境理论   6篇
污染及防治   4340篇
评价与监测   1211篇
社会与环境   1070篇
灾害及防治   48篇
  2022年   181篇
  2021年   182篇
  2019年   127篇
  2018年   237篇
  2017年   225篇
  2016年   360篇
  2015年   263篇
  2014年   402篇
  2013年   1221篇
  2012年   453篇
  2011年   667篇
  2010年   516篇
  2009年   593篇
  2008年   694篇
  2007年   713篇
  2006年   643篇
  2005年   533篇
  2004年   556篇
  2003年   543篇
  2002年   506篇
  2001年   633篇
  2000年   399篇
  1999年   301篇
  1998年   188篇
  1997年   210篇
  1996年   226篇
  1995年   231篇
  1994年   232篇
  1993年   221篇
  1992年   201篇
  1991年   210篇
  1990年   198篇
  1989年   173篇
  1988年   170篇
  1987年   162篇
  1986年   157篇
  1985年   150篇
  1984年   170篇
  1983年   168篇
  1982年   176篇
  1981年   144篇
  1980年   134篇
  1979年   124篇
  1978年   138篇
  1977年   116篇
  1976年   109篇
  1975年   110篇
  1974年   120篇
  1971年   99篇
  1967年   102篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
911.
A portable, battery-operated, electro-optical dust analyzer operating on the principle of light scattering from individual dust particles in air drawn through a high-intensity light field has been developed for monitoring dust levels where external power may be unavailable. The instrument counts and size-discriminates airborne particles over two size ranges upwards of 0.3 microns with immediate readout. A dilution mechanism permits analysis of particulate concentrations as high as 2400 particles/cm3 on each channel. The self-contained Ni-Cd batteries are rechargeable. Total weight, including batteries and all components, is approximately 14 Ib. Total power used, mainly for pumping a filtered air sheath at 300 cm/sec and for the light source, is less than 34 watts. Another unique feature is the flow system designed for turbulence suppression.

Electronic calibration in terms of number of particles/cm3 for each size range is accomplished in the laboratory by means of a pulse generator with respect to which the readout indicator is calibrated for count. The input voltages to the pulse-height discriminators are appropriately attenuated for various pulse amplitudes simulating the output of the light-sensitive pickup, a photomultiplier tube.Pulse height as a function of particle size was measured for polystyrene latex beads ranging from 0.3 to 2 microns in diameter. Good correlation is obtained with results from commercial analyzers.

Field surveys have been undertaken with the CAES prototype analyzer at various sites in central Pennsylvania and at numerous sampling points within the cities of Pittsburgh and Philadelphia. Particulate concentrations as high as 1400 particles/ cm3 have been measured at a coal cleaning plant.  相似文献   
912.
Studies of roof monitor emissions are conducted for two reasons: (1) to obtain design data necessary to engineer control systems to meet existing regulations, and (2) to determine projected control costs which can influence present day proposals for process change. Heated wire anemometers and rotating vane anemometers have been selected for velocity measurements, and high-volume air samplers have been selected to collect the particulate sample. Evaluation of other types of velocity sensing devices are described in the paper.

Roof monitor studies must be preceded by a preliminary survey to allow the project engineer to determine the test sampling locations and specific methodology necessary for the given study, and to allow the engineer to determine the type of safety equipment, scaffolds, and power requirements necessary to complete the study.

Field tests are conducted by operating a number of high-volume air samplers simultaneously while at the same time measuring the velocity of the gas through the monitor opening. Curves of particulate concentration and velocity as a function of monitor length are constructed. The concentration and velocity curves are then integrated together and the resultant curve is multiplied by the monitor width to determine a curve of mass emission rate as a function of monitor length. The total mass emission rate is represented by the area under the mass emission rate curve.

Procedures for calibrating the anemometers and correcting for the effect of power fluctuation on the high-volume sampler operation are described. Data evaluation procedures and discussion of test error are also described.

A study can cost between 10 and 20 thousand dollars, including the cost of manpower and the cost of scaffolds, power, cables, etc. It can take four months or more to conduct a study, from the preliminary survey phase through the report phase.  相似文献   
913.
Honorary Memberships are conferred by the Air Pollution Control Association on persons of widely recognized eminence in some part of the field of air pollution control or who have rendered especially meritorious service to the Association. Honorary Memberships were awarded this year to Dr. Leonard Greenburg and Mr. Milton Reizenstein, both past presidents of APCA.  相似文献   
914.
Verification of forecasts of COH levels at the Downtown Sampling Station using our objective forecast method developed in conjunction with the Allegheny County Bureau of Air Pollution Control gave excellent results. The next step was to expand the area of forecast coverage.

COH data taken from July 1968 through June 1969 from six sampling stations located roughly in a large concentric arc around the Downtown Station were studied. Three of the stations are located in river valleys near large industrial plants and the other three are located on higher terrain in suburban areas.

COH levels of Low, Moderate, High, and Very High (where Low = 0–0.9, Moderate = 1.0–1.9, High = 2.0–3.0, and Very High = >3.0) were used.

The total number of occurrences of High and Very High COHS at the Downtown Station was 10.7% of the total data sample. This was more than twice the frequency of any of the outlying stations.

An evaluation of the statistical frequencies indicate the Downtown Station can safely be used as a “control station” in forecasting Low or Moderate COH levels for the six surrounding stations. However, the occurrence of High or Very High COH levels at the six outlying stations do not occur with a high enough statistical consistency to be predicted by using the forecast value of the Downtown Station.

The valley stations exhibit the characteristics of separate sub air basins particularly during a period in which morning inversions occur for two or more consecutive days and the average daily wind speed does not exceed 4 m/sec.

The results show it is possible to forecast only Low or Moderate COH levels for each of the six outlying stations by using statistical correlations derived from the Downtown Station. Separate objective forecasts which relate the pollution buildup in each valley sub air basin must be used when High or Very High COHS are forecast at the Downtown Station.  相似文献   
915.
Gross average automotive exhaust emissions data collected by the Atlantic Richfield Clean Air Caravan during the summer of 1970 showed only slight geographical variations when the specific makes were ignored. When considering specific makes, significant differences were found on an average emissions basis. Vehicle population—emission distributions showed wide variations in the 50% population levels and in the percent of vehicles with emissions greater than specified values. Hydrocarbon (HC) and carbon monoxide (CO) data are given on a gross basis for the 1970, 1968-69, 1966-67, and pre-1966 model year group vehicles. Average HC and CO emissions and vehicle population-Idle emissions distribution curves are included for specific make vehicles in selected areas.  相似文献   
916.
Pollutants sampled during the burning of 30 lb ponderosa pine fuel beds yielded emission factors for CO, hydrocarbon gases, and par-ticulate matter of 146, 8.4, and 9.1 lb/ton of fuel, respectively. When similar beds were treated with diammonium phosphate flame retardant, these factors increased to 166, 11.7, and 19.3 lb/ton of fuel, respectively.

Gas chromatographic analysis of hydrocarbon gases showed that 15-40% of this material was composed of methane and eth-ylene. Ethane and acetylene were the next most abundant materials, with photochemically important materials constituting minor portions of this gaseous component. Fuel beds treated with flame retardant produced more oiefins, and this production lasted throughout the smoldering phase of burning.

These tests showed that the smoldering phase of combustion is of major importance to air pollutant production during slash burning. The initial 80% of the fuel burned accounted for only 20-30% of HC and CO emissions. This suggests that a rapid mop-up of slash burns could substantially reduce air pollutant production.  相似文献   
917.
The initial state implementation plans concentrated on attainment of the ambient air quality standards in the relatively polluted areas of the country. Many of these plans must now be modified to ensure that the ambient standards will be maintained for the foreseeable future, and to ensure that significant deterioration of air quality in clean areas of the country is prevented.

The existing implementation plans currently contain many measures which are applicable to the maintenance and deterioration efforts, but additional measures must also be developed. Many of these additional measures will involve future planning activities—most common of which will be land use planning activities.

The point is made that, after existing sources have reduced their emissions to the lowest practical level, further air pollution control can only be accomplished by implementing rational planning procedures for management of any new sources of air pollution. This will require extensive cooperation among the air pollution control community; regional, state, and local planning agencies; state and local governments; and the general public in order to ensure that future land use plans include appropriate air quality considerations.  相似文献   
918.
Reference methods for the determination of mercury emissions from stationary sources typically include collection of mercury by solutions which are acidic and oxidizing. In the presence of high levels of SO2 the oxidizing capacity of these absorbing solutions will be degraded and the collection efficiency for mercury compromised. This seriously limits the usefulness of the reference methods as they apply to the mining and smelting industries. In the present work peroxide is used to remove SO2 and acidic permanganate is used to collect mercury. At a mean sampling rate of 10 L/min concentrations of at least 12 mg/m3 mercury can be satisfactorily collected in the presence of up to 20,000 ppm SO2.  相似文献   
919.
Methylcyclopentadienyl manganese tricarbonyl (MMT) has been marketed as a combustion improver for fuel oil and turbine fuel. Use concentrations for this purpose are about 0.025 g manganese/gal in fuel oil and 0.08 to 0.5 g/gal in turbine fuels. In addition, it has been used to a small extent in gasoline.  相似文献   
920.
At the Clairton Coke Works of the United States Steel Corporation (the world’s largest coke plant), with coal charges of about 30,000 net tons per day, a system designated as “stage charging” has been developed for charging coal into the coke oven chamber without emissions to the atmosphere. The principles of the system are based on controlled sequential flow of the coal from the charging hoppers, during which the oven chamber is maintained under a very slight negative pressure condition by use of steam aspiration in the offtakes for on-the-main charging. The techniques are basic but require knowledge of the coal characteristics, adequate and properly maintained aspirating systems, including clean offtake piping, control of coal volume in the individual hoppers, and last, and probably most important, adherence by the operating crew to specific charging and leveling practices.  相似文献   
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