1,721,071 research outputs found
A review of Secondary Organic Aerosol (SOA) formation from isoprene
Full text linkRecent field and laboratory evidence indicates that the oxidation of isoprene, (2-methyl-1,3-butadiene, C[subscript 5]H[subscript 8]) forms secondary organic aerosol (SOA). Global biogenic emissions of isoprene (600 Tg yr[superscript −1]) are sufficiently large that the formation of SOA in even small yields results in substantial production of atmospheric particulate matter, likely having implications for air quality and climate. Here we present a review of field measurements, experimental work, and modeling studies aimed at understanding the mechanisms, yield, and atmospheric importance of isoprene-derived SOA. SOA yields depend on a number of factors, including organic aerosol loading (M[subscript o]), NO[subscript x] level (RO[subscript 2] chemistry), and, because of the importance of multigenerational chemistry, the degree of oxidation. These dependences are not always included in SOA modules used in atmospheric transport models, and instead most yield parameterizations rely on a single set of chamber experiments (carried out over a limited range of conditions); this may lead to very different estimates of the atmospheric importance of isoprene SOA. New yield parameterizations, based on all available laboratory data (M[subscript o]=0–50 μg m[superscript −3]), are presented here, so that SOA formation may be computed as a function of M[subscript o], NO[subscript x] level, and temperature. Current research needs and future research directions are identified.United States. Environmental Protection Agency. Office of Research and Developmen
Characterisation of lightly oxidised organic aerosol formed from the photochemical aging of diesel exhaust particles
No full textThe oxidative aging of the semivolatile fraction of diesel exhaust aerosol is studied in order to better understand the influence of oxidation reactions on particle chemical composition. Exhaust is sampled from an idling diesel truck, sent through a denuder to remove gas-phase species and oxidised by hydroxyl (OH•) radicals in a flow reactor. OH• concentrations are chosen to approximately match the OH• exposures a particle would experience over its atmospheric lifetime. Evolving particle composition is monitored using aerosol mass spectrometry in two different modes, electron impact ionisation (EI) for the measurement of elemental ratios and vacuum ultraviolet (VUV) photoionisation for the measurement of molecular components. Changes to mass spectra in both modes indicate major changes to particle composition over the range of OH• exposures studied. The product aerosol is only lightly oxidised (O/C < 0.3), suggesting an intermediate oxidation state between primary organics and the highly oxidised organic aerosol observed in the atmosphere. These lightly oxidised organics appear to be composed of secondary organic aerosol from semivolatile species, as well as from heterogeneously oxidised particle-phase organics. Key chemical characteristics (elemental ratios, oxidation kinetics and mass spectrometric features) of the reaction system are examined in detail. Similarities between this laboratory-generated aerosol and ‘hydrocarbon-like organic aerosol’ (HOA) reported in ambient studies suggest that HOA might not be entirely primary in origin, as is commonly assumed, but rather might include a significant secondary component.United States. Dept. of Energy (Director, Office of Energy Research, Office of Basic Energy Sciences, and Chemical Sciences Division (contract number DE-AC02-05CH11231)Henry & Camille Dreyfus Foundation (Postdoctral program in Environmental Chemistry
Average chemical properties and potential formation pathways of highly oxidized organic aerosol
Full text linkMeasurements of ambient organic aerosol indicate that a substantial fraction is highly oxidized and low in volatility, but this fraction is generally not reproduced well in either laboratory studies or models. Here we describe a new approach for constraining the viable precursors and formation pathways of highly oxidized organic aerosol, by starting with the oxidized product and considering the possible reverse reactions, using a set of simple chemical rules. The focus of this work is low-volatility oxidized organic aerosol (LV-OOA), determined from factor analysis of aerosol mass spectrometer data. The elemental composition and volatility of the aerosol enable the determination of its position in a three-dimensional chemical space (defined by H/C, O/C, and carbon number) and thus its average chemical formula. Consideration of possible back-reactions then defines the movement taken through this chemical space, constraining potential reaction pathways and precursors. This approach is taken for two highly oxidized aerosol types, an average of LV-OOA factors from ten field campaigns (average formula C[subscript 10.5]H[subscript 13.4]O[subscript 7.3]), and extremely oxidized LV-OOA (from Mexico City, average formula C[subscript 10]H[subscript 12.1]O[subscript 8.4]). Results suggest that potential formation pathways include functionalization reactions that add multiple functional groups per oxidation step, oligomerization of highly oxidized precursors, and, in some cases, fragmentation reactions that involve the loss of small, reduced fragments.National Science Foundation (U.S.) (Grant CHE-1012809)National Science Foundation (U.S.) (Grant AGS-1056225
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
A two-dimensional volatility basis set – Part 2: Diagnostics of organic-aerosol evolution
Full text linkWe discuss the use of a two-dimensional volatility-oxidation space (2-D-VBS) to describe organic-aerosol chemical evolution. The space is built around two coordinates, volatility and the degree of oxidation, both of which can be constrained observationally or specified for known molecules. Earlier work presented the thermodynamics of organics forming the foundation of this 2-D-VBS, allowing us to define the average composition (C, H, and O) of organics, including organic aerosol (OA) based on volatility and oxidation state. Here we discuss how we can analyze experimental data, using the 2-D-VBS to gain fundamental insight into organic-aerosol chemistry. We first present a well-understood "traditional" secondary organic aerosol (SOA) system – SOA from α-pinene + ozone, and then turn to two examples of "non-traditional" SOA formation – SOA from wood smoke and dilute diesel-engine emissions. Finally, we discuss the broader implications of this analysis
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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