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Since 1979 the Laboratory of Geophysics, University of Aarhus, has been developing a new prospecting tool for obtaining information on the topmost 100 m of the earth. The method is an extension of the conventional geoelectric sounding method, but instead of direct current (DC) the AC-geoelectrical sounding method uses alternating current (AC) with frequencies in the range 100 Hz to 20,000 Hz. The use of alternating current adds an inductive contribution to the ordinary galvanic electric field, thus producing two different sorts of information about the underlying earth structure. These two sets of information are, in many cases, of complementary nature, which enables determination of the ground parameters much more accurately than would otherwise be possible from ordinary DC-geoelectrical soundings. Among these cases is the high resistivity equivalence which appears so frequently in Danish Quaternary deposits. 相似文献
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U. LOHMANN W. R. LEAITCH L. BARRIE K. LAW Y. YI D. BERGMANN C. BRIDGEMAN M. CHIN J. CHRISTENSEN R. EASTER J. FEICHTER A. JEUKEN E. KJELLSTRÖM D. KOCH C. LAND P. RASCH G.&#;J. ROELOFS 《Tellus. Series B, Chemical and physical meteorology》2001,53(5):646-672
A comparison of large‐scale models simulating atmospheric sulfate aerosols (COSAM) was conducted to increase our understanding of global distributions of sulfate aerosols and precursors. Earlier model comparisons focused on wet deposition measurements and sulfate aerosol concentrations in source regions at the surface. They found that different models simulated the observed sulfate surface concentrations mostly within a factor of two, but that the simulated column burdens and vertical profiles were very different amongst different models. In the COSAM exercise, one aspect is the comparison of sulfate aerosol and precursor gases above the surface. Vertical profiles of SO2 , SO2− 4 , oxidants and cloud properties were measured by aircraft during the North Atlantic Regional Experiment (NARE) experiment in August/September 1993 off the coast of Nova Scotia and during the Second Eulerian Model Evaluation Field Study (EMEFSII), in central Ontario in March/April 1990. While no single model stands out as being best or worst, the general tendency is that those models simulating the full oxidant chemistry tend to agree best with observations although differences in transport and treatment of clouds are important as well. 相似文献
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Kullman, Leif 1987 03 01: Sequences of Holocene forest history in the Scandes, inferred from megafossil Pinus sylvestris. Boreas , Vol. 16, pp. 21–26. Oslo. ISSN 0300–9483.
Altogether 5414 C-datings of megafossil Pinus sylvestris L. from above the present tree-limit in the southern Swedish Scandes were evaluated. The samples were discussed in relation to the present-day tree-limit of pine. Pine reached almost 200 m higher than at present shortly after deglaciation and remained at very high altitudes until at least 4,000–3,500 14 C years B.P. The conclusion was that the precise biological and climatological interpretation of these regionally sampled data is still obscure. It is impossible to judge whether clusters of megafossil pine represent periods relatively favourable for pine growth and reproduction or merely periods advantageous for production of dead wood and its long-term preservation. In terms of climate, these alternatives reasonably implicate widely different conditions. 相似文献
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Methane flux from northern wetlands and tundra 总被引：2，自引：0，他引：2