1,720,974 research outputs found
Particle dry deposition to water surfaces: Processes and consequences
No full textAlgal blooms (increased abundance of phytoplankton) are an increasingly common phenomenon which has been causally linked to increased fluxes of nutrient (particularly nitrogenous) compounds to aquatic ecosystems. These blooms have implications for water quality and human health in addition to ecosystem productivity, health and ecological diversity. Anthropogenic emissions of nitrogen to the atmosphere are estimated to be comparable to, or greater than, biogenic emissions but are considerably more concentrated in space. Although fluvial pathways typically dominate the annually averaged nitrogen flux to coastal waters, atmosphere-surface exchange represents a significant component of the total flux and may be particularly critical during the summertime when both the riverine input and ambient nutrient concentrations are often at a minimum. In this chapter, we present an overview of the physical and chemical processes which dictate the quantity (and direction) of atmosphere-surface fluxes of trace chemicals to (and above) water surfaces with particular emphasis on the role of particles.Dry deposition (transfer to the surface in the absence of precipitation) of particles is determined by meteorological conditions, atmospheric concentrations, surface type/ condition and the specific chemical and physical properties of the particle. Dry deposition can be conceptualized as a three-step process: (1) the gas or particle is moved toward the surface by thermally or mechanically driven eddies; (2) it is transferred by diffusion across a thin layer close to the surface where turbulence is absent; and (3) the gas or particle is captured by the surface. In the case of larger particles a second parallel pathway exists; particles are drawn towards the surface by gravity, Atmospheric particles determine dry deposition fluxes not only by serving as a conduit for transfer but also because of their action as sources or sinks of trace gases. The example given here is the transfer of nitric acid to sea salt particles as a result of heterogeneous chemistry acting as a competing sink to surface removal. To illustrate the importance of current uncertainties in our understanding of dry deposition processes and to highlight the role of some of the key parameters in determining the transfer rate (the deposition velocity) a simple model of particle dry deposition is presented. The model describes the calculation of the rate at which a particle of a given size and chemical composition will be moved towards the surface under given environmental conditions. Observational and experimental techniques for measuring dry deposition fluxes are also reviewed. The techniques used for gases are largely reliant on use of highly temporally resolved sampling (e.g., concentrations sampled 10 times per second) or highly accurate and precise measurements of concentrations, either in the vertical to resolve the gradient to or from the surface or conditionally sampled by the direction of transfer (to or from the surface). These stringent measurement requirements represent significant barriers to application to measurement of particle dry deposition fluxes although, as discussed, innovative solutions are now becoming available.In the final section, we examine meteorological controls on deposition to the coastal zone. This region of the world's oceans and seas is most significantly impacted by human activities. More than half of the world's population lives within 100 km of a coast and hence the overwhelming majority of anthropogenic fluxes to aquatic systems occur in the coastal zone. We discuss the particular challenges that arise from efforts to simulate and measure fluxes close to the coastline. These arise in part from the complexity of atmospheric flow in this region where energy and chemical fluxes are highly inhomogeneous in space and time and thermally generated atmospheric circulations are commonplace. (C) 2000 Elsevier Science Ltd. All rights reserved
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
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