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Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe 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
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In situ measurement of inelastic light scattering in natural waters
No full textVariation in the shape of solar absorption (Fraunhofer) lines are used to study the inelastic scattering in natural waters. In addition, oxygen absorption lines near 689nm are used to study the solar stimulated chlorophyll fluorescence. The prototype Oceanic Fraunhofer Line Discriminator (OFLD) has been further developed and improved by using a well protected fiber optic - wire conductor cable and underwater electronic housing. A Monte-Carlo code and a simple code have been modified to simulate the Raman scattering, DOM fluorescence and chlorophyll fluorescence. A series of in situ measurements have been conducted in clear ocean waters in the Florida Straits, in the turbid waters of Florida Bay, and in the vicinity of a coral reef in the Dry Tortugas. By comparing the reduced data with the model simulation results, the Raman scattering coefficient, b\sb{\rm r} with an excitation wavelength at 488nm, has been verified to be 2.6 10\sp{-4}m\sp{-1} (Marshall and Smith, 1990), as opposed to 14.4 10\sp{-4}m\sp{-1} (Slusher and Derr, 1975). The wavelength dependence of b\sb{\rm r} cannot be accurately determined from the data set as the reported values (\lambda\sb{\rm m}\sp{-4} to \lambda\sb{\rm m}\sp{-5}) have an insignificant effect in the natural underwater light field. Generally, in clear water, the percentage of inelastic scattered light in the total light field at 510nm. At low concentrations (a\sb{\rm y}(\lambda = 380nm) less than 0.1m\sp{-1}), DOM fluorescence plays a small role in the inelastic light field. However, chlorophyll fluorescence is much stronger than Raman scattering at 685nm. In shallow waters where a sea bottom affects the ambient light field, inelastic light is negligible for the whole visible band. Since Raman scattering is now well characterized, the new OFLD can be used to measure the solar stimulated in situ fluorescence. As a result, the fluorescence signals of various bottom surfaces, from coral to macrophytes, have been measured and have been found to vary with time possibly due to nonphotochemical quenching and photoinhibition.</p
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Comparison of lab based and solar reflectance based calibration of aureole camera
No full textAerosols play an important role in the Earth's radiative transfer and the remote sensing of the environment. One major difficulty in precisely quantifying the aerosol effects is the uncertainties in aerosol characterization. An aureole camera can provide important ground measurement for characterizing the aerosol size distribution and optical properties. Careful calibration of the aureole camera and correct radiometric interpretation of acquired images are essential to the quality of the retrieved aerosol properties. In my dissertation, I carefully removed many hard-to-notice uncertainties related to lab based calibration. Most importantly, the often ignored point spread function effect of the aureole camera optical system that can cause more than a 3% error is identified and removed. The measured point spread function spans 7 orders of magnitude of intensity and covers distances up to 250 pixels (or 14°) away from the image of a point source. The effect of the point spread function on the radiometric interpretation of the optical system is simulated and experimentally proven with a specially designed small field-of-view device. The correct interpretation and calibration procedure of the aureole camera is thus detailed. With all the calibration and interpretation problems cleared, the traditional lamp based calibration and a Solar Reflectance Based Calibration were performed and compared. The Solar Reflectance Based Calibration removes the uncertainties of individual lamps. I found that the two calibration methods agree within 4%, which is comparable to the uncertainties reported by other authors [Schmid and Wehrli, 1995; Schmid et al., 1998].A preliminary aerosol retrieval scheme based on aureole and aerosol optical depth measurements is also described.</p
Measurement of inelastic scattering in the ocean
No full textA new technique to measure the inelastic scattering in the ocean due to the natural light field is developed. By measuring the in situ irradiance field in the vicinity of the solar Fraunhofer lines at very high spectral resolution (0.008 nm), the total irradiance can be separated into inelastic and elastic components. The variation of the Fraunhofer lines with depth was calculated by using a Monte Carlo simulation method and a simple model. In the calculations, Raman scattering and fluorescence of dissolved organic matters were considered, chlorophyll a was not taken into account since it does not fluoresce at a relevant Fraunhofer line. An instrument was developed to measure the inelastic scattering in situ in the ocean. Field measurements obtained with this instrument are presented. It was found that the Fraunhofer lines in the long wavelength region of the solar spectrum (656, 589 nm) are filled quickly while those in the blue (518, 486 nm) remain almost constant. It is concluded that the contribution of inelastic scattering to the light field distribution in the ocean is dominant in the long wavelength region at depth.</p
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Measurement of inelastic scattering in the ocean
No full textA new technique to measure the inelastic scattering in the ocean due to the natural light field is developed. By measuring the in situ irradiance field in the vicinity of the solar Fraunhofer lines at very high spectral resolution (0.008 nm), the total irradiance can be separated into inelastic and elastic components. The variation of the Fraunhofer lines with depth was calculated by using a Monte Carlo simulation method and a simple model. In the calculations, Raman scattering and fluorescence of dissolved organic matters were considered, chlorophyll a was not taken into account since it does not fluoresce at a relevant Fraunhofer line. An instrument was developed to measure the inelastic scattering in situ in the ocean. Field measurements obtained with this instrument are presented. It was found that the Fraunhofer lines in the long wavelength region of the solar spectrum (656, 589 nm) are filled quickly while those in the blue (518, 486 nm) remain almost constant. It is concluded that the contribution of inelastic scattering to the light field distribution in the ocean is dominant in the long wavelength region at depth.</p
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Bi-directional reflectance studies of prepared compact particulate surfaces
No full textControlled laboratory BRDF and transmission measurements on layers of polymer and glass spheres have been carried out to investigate the connection between single particle optics and the optics of a packed surface. The measurements show that despite being closely packed, significant features of single scattering, such as the rainbow peaks, are preserved even in aggregated sphere layers. The measurements have been compared to 5 radiative transfer model predictions: the Hapke's model and its improved version, the Lumme-Bowell model, Mishchenko et al.'s BRF algorithm and DISORT. It has been found that strict numerical RTE models predict the measurements well in some regions, but have errors in both forward and backward scattering directions. The discrepancies have been attributed to the non-ideal factors such as internal inhomogeneity and surface roughness and may be corrected using Lumme-Bowell's roughness correction factor for oblique incident light. The inadequacy of the semi-empirical models can be partly attributed to the exclusion of a diffraction contribution in the models.In-situ BRDF measurements on submerged sediments with grain sizes ranging from 300 mum to over 1000 mum have been carried out. For normally illuminated small grain size samples the BRDF was nearly Lambertian, but samples with larger grain sizes are less Lambertian, with the BRDF decreasing with increasing view angles. Under oblique incident angles the samples become increasingly non-Lambertian; the dominant feature in the BRDF is enhanced backscattering. An empirical model is presented for each sediment type which represents the data within the standard deviation of the sample variation. This model is well behaved at angles out to 90°, and thus can be incorporated into the radiative transfer models to improve the light field predictions in shallow water.The BRDF of both dry and wet ooid sand layers with different particle size distributions and layer thicknesses on a reflecting mirror have been measured to determine the sensible depth in the optical region. The hemispherical reflectance (albedo) was evaluated from the BRDF data to quantify the subtle BRDF changes caused by the layer thickness. It was found that the depth that influences BRDF measurements is just a few particle diameters which is in qualitative agreement with radiative transfer calculations.</p
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Measurements of aerosol optical properties over the ocean using sunphotometry and lidar
No full textAerosols suspended in the atmosphere scatter and absorb downwelling and upwelling radiation and also serve as cloud-condensation nuclei. This interaction perturbs the radiation balance in the atmosphere and causes changes in regional and global climates. Aerosols also effect the remote sensing of the earth's surface and atmosphere as the signals measured by the satellite sensors must pass through the aerosol layers. It is necessary to determine the optical properties of the aerosols in order to better understand the atmospheric system, provide accurate input to climate models, and to correct remotely sensed data. The primary aerosol optical properties are the aerosol optical depth (AOD) and its spectral dependence characterized by the Angstrom exponent, the aerosol phase function, the aerosol single scattering albedo, the vertical structure of the aerosol backscatter and extinction coefficients, and the aerosol backscatter-extinction ratio. There is little information on the optical properties of marine aerosols because measurements over the ocean are difficult to perform. In order to determine the optical properties of the marine aerosols, a series of measurements were performed using sunphotometers, automated shadowband radiometers, and a micro-pulse lidar system.The AOD and the Angstrom exponent in the visible and near-infared were determined using data taken in support of the Atmosphere/Ocean Chemistry Experiment (AEROCE) in Miami, Bermuda, and Barbados from August 1993 to December 1995. The AOD measurements were made using hand-held sunphotometers and automated shadowband radiometers. The resulting AOD data were correlated with surface measurements of aerosol concentrations to analyze seasonal trends in the AOD and to determine characteristic AOD and Angstrom exponent patterns for key marine aerosol species. Measurements of the vertical structure of the aerosol optical properties were performed using a micro-pulse lidar system. Measurements were made on Tenerife, Canary Islands during the Aerosol Characterization Experiment 2 (ACE-2) in June and July of 1997 and on a SeaWiFS initialization cruise (MOCE-4) around Hawaii in January and February of 1998. The data were used to develop vertical profiles of the optical properties of the key marine aerosols.Introductory and background theoretical information, descriptions of each type of instrument along with corresponding calibration and error correction procedures, and the conclusions drawn from each experiment are presented. The results show that the spectral dependence of the AOD over the oceans can be accurately described by the Angstrom exponent. Also, seasonal variations in the AOD are caused by seasonal changes in the concentrations of key marine aerosol species and each species was found to have unique AOD and Angstrom exponent characteristics. Results from work with the lidar have shown that the micro-pulse lidar system can operate successfully in the field and that accurate optical profiles can be calculated. Also, specific characteristics of the aerosol layers present during ACE-2 and MOCE-4 are presented. Finally, future applications of the result obtained in this study are discussed in the concluding chapter.</p
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Lidar measurements of marine aerosols with improved analysis techniques
No full textAerosols have been receiving increasing attention for two main reasons. First, their effects on the global radiation budget, both direct and indirect, are now understood to be substantial, and the difficulty in measuring their properties and distribution is the single largest source of uncertainty in calculations of global warming and climate change. Second, the interference of the atmosphere in satellite-based measurements of Earth surface properties is large and the variability is dominated by aerosols. In order to better understand the present and future states of the environment it is important to be able to measure the spatial and temporal distribution and optical properties of aerosols. My work has focused upon measuring the vertical distribution of aerosols over the oceans. In this dissertation I describe several new techniques for the analysis of lidar data and their application to data collected during ACE-Asia (Asian Pacific Regional Aerosol Characterization Experiment). The intensive field phase of this experiment was conducted in the spring of 2001 in and around the Sea of Japan. The field phase included a variety of instruments on land, sea and air measuring physical, optical and chemical properties of aerosols during a period of especially high dust levels in the atmosphere. The lidar system described in this dissertation was located aboard the NOAA Research Vessel Ronald H. Brown between March 14 and April 20, 2001. The lidar ACE-Asia data products include aerosol layer heights and thicknesses, aerosol optical depths, aerosol volume extinction and backscatter coefficients at 523 nm wavelength vs. altitude and corresponding extinction-to-backscatter ratios ("lidar ratios"). Improvements in the reliability (reduced uncertainty) of these data products is demonstrated based on improved inversion techniques. These data are also correlated with other data products collected on the cruise.</p
Comparison of lab based and solar reflectance based calibration of aureole camera
No full textAerosols play an important role in the Earth's radiative transfer and the remote sensing of the environment. One major difficulty in precisely quantifying the aerosol effects is the uncertainties in aerosol characterization. An aureole camera can provide important ground measurement for characterizing the aerosol size distribution and optical properties. Careful calibration of the aureole camera and correct radiometric interpretation of acquired images are essential to the quality of the retrieved aerosol properties. In my dissertation, I carefully removed many hard-to-notice uncertainties related to lab based calibration. Most importantly, the often ignored point spread function effect of the aureole camera optical system that can cause more than a 3% error is identified and removed. The measured point spread function spans 7 orders of magnitude of intensity and covers distances up to 250 pixels (or 14°) away from the image of a point source. The effect of the point spread function on the radiometric interpretation of the optical system is simulated and experimentally proven with a specially designed small field-of-view device. The correct interpretation and calibration procedure of the aureole camera is thus detailed. With all the calibration and interpretation problems cleared, the traditional lamp based calibration and a Solar Reflectance Based Calibration were performed and compared. The Solar Reflectance Based Calibration removes the uncertainties of individual lamps. I found that the two calibration methods agree within 4%, which is comparable to the uncertainties reported by other authors [Schmid and Wehrli, 1995; Schmid et al., 1998].A preliminary aerosol retrieval scheme based on aureole and aerosol optical depth measurements is also described.</p
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