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61.
The global distribution of acidifying wet deposition 总被引:2,自引:0,他引:2
The acid-base status of precipitation is a result of a balance between acidifying compounds--mainly oxides of sulfur and nitrogen--and alkaline compounds--mainly ammonia and alkaline material in windblown soil dust. We use current models of the global atmospheric distribution of such compounds to estimate the geographical distribution of pH in precipitation and of the rate of deposition of hydrogen ion or bicarbonate ion. The lowest pH values--mainly due to high concentration of sulfuric acid--occur in eastern parts of North America, Europe, and China. A comparison with observed pH values shows fair agreement in most parts of the world. However, in some areas, e.g. western North America, southwestern Europe, and northern China the estimated pH is too low, indicating that we have underestimated the deposition flux of alkaline material, probably mainly CaCO3. Our neglect of organic acids may have contributed to an overestimate of pH especially in certain tropical areas. To illustrate the potential effects of acidifying deposition on nitrogen saturated terrestrial ecosystems we also calculate the deposition of "potential acidity" that takes into account the microbial transformation of ammonium to nitrate in such ecosystems, resulting in the release of hydrogen ion. Compared to the deposition of acidity, with its maxima over Europe, eastern North America, and southern China, the deposition of potential acidity exhibits an additional maximum in India and Bangladesh and in several other smaller hot spots where the cycling of ammonia is enhanced by a dense cattle population. To the extent that soils in these areas of high potential acidity deposition actually become nitrogen saturated a depletion of base cations and other changes in soil chemistry and biology should be expected. Potential problem areas forfuture soil acidification include several regions with sensitive soils in southern, southeastern, and eastern Asia as well as in central parts of South America. 相似文献
62.
Chris?L.?Barrett Henning?S.?MortveitEmail author Christian?M.?Reidys 《Artificial Life and Robotics》2002,6(4):167-169
We present our work on the special class of dynamical systems referred to as discrete sequential dynamical systems (SDS).
The definition of these systems is motivated by the generic structure of computer simulations. In computer simulations, we
typically find agents or entities with certain properties or states. The entities can retrieve information from other entities,
but usually only from the ones in their own vicinity. Based on these states, they may update their own state. A schedule will
take care of the update order of the entities. One possible interpretation of this is to have each entity as a vertex in a
(dependency) graph where two vertices are connected if the corresponding two entities can communicate. Without loss of generality,
we can associate a binary state to each vertex or entity. Finally, we fix some ordering of the vertices that represents the
update ordering of the entities. The above construction will be put in a strict mathematical context, and leads to the concept
of a sequential dynamical system (SDS).
This work was presented in part at the Sixth International Symposium on Artificial Life and Robotics, Tokyo, January 15–17,
2001 相似文献
63.
Jochen Alber Hongbing Fan Henning Fernau Rolf Niedermeier Fran Rosamond 《Journal of Computer and System Sciences》2005,71(4):385-405
We establish a refined search tree technique for the parameterized DOMINATING SET problem on planar graphs. Here, we are given an undirected graph and we ask for a set of at most k vertices such that every other vertex has at least one neighbor in this set. We describe algorithms with running times O(8kn) and O(8kk+n3), where n is the number of vertices in the graph, based on bounded search trees. We describe a set of polynomial time data-reduction rules for a more general “annotated” problem on black/white graphs that asks for a set of k vertices (black or white) that dominate all the black vertices. An intricate argument based on the Euler formula then establishes an efficient branching strategy for reduced inputs to this problem. In addition, we give a family examples showing that the bound of the branching theorem is optimal with respect to our reduction rules. Our final search tree algorithm is easy to implement; its analysis, however, is involved. 相似文献
64.
Henning GB Millar RW Hill NJ 《Journal of the Optical Society of America. A, Optics, image science, and vision》2000,17(7):1147-1159
Two-alternative forced-choice procedures were used to measure the detectability of bright and dark bars at various locations across luminance patterns that produced Mach bands. Detection performance was significantly affected by both dark and bright Mach bands: poor detection performance was observed at locations near, but not in, the Mach bands; relatively good detection performance at locations within the Mach bands was caused by reliable changes in the width, depth, or symmetry of the bands produced by the signal bars. The changes were apparent with signals of lower luminance than that needed for detection in the plateau regions far from the bands, but, because the cues were not sufficiently reliable to allow errorless performance, unusually shaped psychometric functions were obtained. 相似文献
65.
Henning GB 《Journal of the Optical Society of America. A, Optics, image science, and vision》2004,21(4):486-490
A modification and extension of Kortum and Geisler's model [Vision Res. 35, 1595 (1995)] of early visual non-linearities that incorporates an expansive nonlinearity (consistent with neurophysiological findings [Vision Res. 35, 2725 (1995)], a normalization based on a local average retinal illumination, similar to Mach's proposal [F. Ratliff, Mach Bands: Quantitative Studies on Neural Networks in the Retina (Holden-Day, San Francisco, Calif, 1965)], and a subsequent compression suggested by Henning et al. [J. Opt. Soc. Am A 17, 1147 (2000)] captures a range of hitherto unexplained interactions between a sinusoidal grating of low spatial frequency and a contrast-modulated grating 2 octaves higher in spatial frequency. 相似文献
66.
Henning GB Hoddinott KT Wilson-Smith ZJ Hill NJ 《Journal of the Optical Society of America. A, Optics, image science, and vision》2004,21(8):1379-1387
Difficulties arise in measuring masking by Mach bands because very-low-contrast signals distort the bands. [J. Opt. Soc. Am. A 17, 1147 (2000).] Adding narrow luminance increments (bright bars) in the dark Mach band widens the dark band; adding decrements (dark bars) narrows the dark band, and conversely in the bright bands. Randomizing signal polarity prevents observers from using the distortion of the Mach bands as a cue to the presence of the signal. We measured (two-alternative-forced-choice) Mach bands' masking of randomly selected bright (incremental) or dark (decremental) bars. Detection was worse in both dark and bright Mach bands than on the neighboring plateaus. Separate analysis of trials containing only one polarity signal revealed 9-cycle/deg oscillations in performance as a function of location. Oscillations in the two polarities were approximately 180 degrees out of phase. 相似文献
67.
68.
Rohrlack T Christoffersen K Hansen PE Zhang W Czarnecki O Henning M Fastner J Erhard M Neilan BA Kaebernick M 《Journal of chemical ecology》2003,29(8):1757-1770
This paper describes the purification and characterization of microviridin J, a newly discovered metabolite of Microcystis that causes a lethal molting disruption in Daphnia spp., upon ingestion of living cyanobacterial cells. Microviridin J consists of an acetylated chain of 13 amino acids arranged in three rings and two side chains. Unlike other known isoforms of microviridin, microviridin J contains arginine that imparts a unique solution conformation characterized by proximal hydrophobic interactions between Arg and other regions of the molecule. This eventually results in the formation and stabilization of an additional ring system. Microviridin J potently inhibits porcine trypsin, bovine chymotrypsin, and daphnid trypsin-like proteases. The activity against trypsin is most likely due to Arg and its distinctive conformational interactions. Overall, the data presented for microviridin J emphasize once again the ability of cyanobacteria to produce numerous and potent environmental toxins. 相似文献
69.
Laser-induced-incandescence (LII) signal decays are measured in sooting premixed atmospheric and low-pressure flames. Soot particle temperatures are obtained from LII signals measured at two wavelengths. Soot particle size distributions P(r) and flame temperatures T are measured spatially resolved by independent techniques. Heat and mass transfer kinetics of the LII process are determined from measured soot particle temperatures, flame temperatures, and particle sizes. Uncertainties of current LII models are attributed to processes during the absorption of the laser pulse. Implications for LII experiments are made in order to obtain primary soot particle sizes. Soot particle size distributions and flame temperatures are assessed from measured particle temperature decays by use of multi-D nonlinear regression. 相似文献
70.
The quenching of the fluorescence of five aromatic hydrocarbons by three halogenated organics and by molecular oxygen has been measured. Both fluorescence intensity and fluorescence lifetime measurements have been employed to validate results and interpretation; linear Stern-Volmer analyses are shown to apply throughout. The fluorescence quenching rate constant of molecular oxygen for the five aromatic hydrocarbons is essentially equivalent to the diffusion rate constant independent of the fluorophore excitation energy. The halogenated organic-fluorophore rate constants vary by a factor of 965 and are shown to correlate roughly with the energy difference between the quencher and fluorophore excited electronic states in accord with a standard model of quantum two-level mixing. The value of the coupling interaction energy is approximately 2500 cm(-1). 相似文献