6,082 research outputs found
Air/Sea Transfer of Highly Soluble Gases Over Coastal Waters
The deposition of soluble trace gases to the sea surface is not well studied due to a lack of flux measurements over the ocean. Here we report simultaneous air/sea eddy covariance flux measurements of water vapor, sulfur dioxide (SO2), and momentum from a coastal North Atlantic pier. Gas transfer velocities were on average about 20% lower for SO2 than for H2O. This difference is attributed to the difference in molecular diffusivity between the two molecules (DSO2/DH2O = 0.5), in reasonable agreement with bulk parameterizations in air/sea gas models. This study demonstrates that it is possible to observe the effect of molecular diffusivity on air-side resistance to gas transfer. The slope of observed relationship between gas transfer velocity and friction velocity is slightly smaller than predicted by gas transfer models, possibly due to wind/wave interactions that are unaccounted for in current models
Optical Characterization and Distribution of Chromophoric Dissolved Organic Matter (CDOM) in Soil Porewater from a Salt Marsh Ecosystem
To characterize chromophoric dissolved organic matter (CDOM) in marsh porewaters and its contribution as a carbon source, optical properties (absorbance, fluorescence indices, 3-dimensional excitation-emission matrices [EEMs]) of soil porewater and surface water were measured in a southern Californian salt marsh. Absorption coefficients and fluorescence intensities were higher in porewater than in overlying surface waters, consistent with higher CDOM concentration at depth. Humic-type peaks A and C were observed in EEMs in all samples, and peak M was observed in surface waters and shallow porewater to -5 cm depth. Fluorescence:absorbance (flu:abs) ratios and spectral slopes (S) decreased across the surface interface, and emission peak maxima were red-shifted—changes that are consistent with increasing molecular weight (MW) and aromaticity in soil porewater due to humification, and lower-MW, less aromatic material in oxic surface waters from oxidative photochemical and biological processing. At lower depths, bands were observed where intensity, flu:abs ratios and S increased; absorption coefficients decreased; emission maxima for humic-type peaks were blue-shifted; and tryptophan-type protein peaks were observed. These changes are consistent with lower-MW and less aromatic material from enhanced microbial activity. Variations in iron concentrations and sulfate depletion with depth were consistent with these bands having different dominant anaerobic microbial metabolic pathways. Overall, optical property trends suggest that soil porewater is a reservoir of CDOM in the salt marsh, with organic material from terrestrial watershed inputs and in situ production from marsh vegetation stored and processed in sediments
Early stages of ramified growth in quasi-two-dimensional electrochemical deposition
I have measured the early stages of the growth of branched metal aggregates
formed by electrochemical deposition in very thin layers. The growth rate of
spatial Fourier modes is described qualitatively by the results of a linear
stability analysis [D.P. Barkey, R.H. Muller, and C.W. Tobias, J. Electrochem.
Soc. {\bf 136}, 2207 (1989)]. The maximum growth rate is proportional to
where is the current through the electrochemical cell,
the electrolyte concentration, and . Differences
between my results and the theoretical predictions suggest that
electroconvection in the electrolyte has a large influence on the instability
leading to ramified growth.Comment: REVTeX, four ps figure
Optical Characterization of Chromophoric Dissolved Organic Matter (CDOM) and Fe(II) Concentrations in Soil Porewaters Along a Channel-Bank Transect in a Salt Marsh
Chromophoric dissolved organic matter (CDOM) optical properties were measured in surface and porewaters as a function of depth and distance from the channel in a transect up the bank in a southern California salt marsh. Higher absorbance coefficients and fluorescence intensities in porewaters at depth vs. surface waters and shallower porewaters suggest soil porewater is a reservoir of CDOM in the marsh. Higher values were observed at the marsh sites compared to the channel site, suggesting increased production and storage in the marsh sites, and reduced leaching into overlying surface waters, is occurring. Spectral slope ratios decreased with depth, consistent with more aromatic, higher molecular weight material in the deeper porewaters, possibly due to different bacterial processing in the anaerobic vs. aerobic zones. Fe(II) concentrations, indicative of anaerobic bacterial processing, increased significantly at depth to values \u3e 1000 μM, consistent with active anaerobic microbial processing occurring at depth. The transitions to higher reduced iron concentrations correlated with increased absorbance and fluorescence, suggesting processing by anaerobic iron-reducing bacteria in these deeper zones may not mineralize as much carbon as in the shallower aerobic zones. Alternatively, this may be due to reduction of solid iron oxides coated with organic matter releasing both DOM and Fe(II). The ratio of humic-like fluorescence to the absorption coefficient decreased with increasing iron concentration, possibly due to optical interference by iron species. Taken together, the data indicate that marsh sites in the salt marsh act as a reservoir for higher molecular weight, more aromatic organic matter
Statistical Properties of Radio Emission from the Palomar Seyfert Galaxies
We have carried out an analysis of the radio and optical properties of a
statistical sample of 45 Seyfert galaxies from the Palomar spectroscopic survey
of nearby galaxies. We find that the space density of bright galaxies (-22 mag
<= M_{B_T} <= -18 mag) showing Seyfert activity is (1.25 +/- 0.38) X 10^{-3}
Mpc^{-3}, considerably higher than found in other Seyfert samples. Host galaxy
types, radio spectra, and radio source sizes are uncorrelated with Seyfert
type, as predicted by the unified schemes for active galaxies. Approximately
half of the detected galaxies have flat or inverted radio spectra, more than
expected based on previous samples. Surprisingly, Seyfert 1 galaxies are found
to have somewhat stronger radio sources than Seyfert 2 galaxies at 6 and 20 cm,
particularly among the galaxies with the weakest nuclear activity. We suggest
that this difference can be accommodated in the unified schemes if a minimum
level of Seyfert activity is required for a radio source to emerge from the
vicinity of the active nucleus. Below this level, Seyfert radio sources might
be suppressed by free-free absorption associated with the nuclear torus or a
compact narrow-line region, thus accounting for both the weakness of the radio
emission and the preponderance of flat spectra. Alternatively, the flat spectra
and weak radio sources might indicate that the weak active nuclei are fed by
advection-dominated accretion disks.Comment: 18 pages using emulateapj5, 13 embedded figures, accepted by Ap
B0850+054: a new gravitational lens system from CLASS
We report the discovery of a new gravitational lens system from the CLASS
survey. Radio observations with the VLA, the WSRT and MERLIN show that the
radio source B0850+054 is comprised of two compact components with identical
spectra, a separation of 0.7 arcsec and a flux density ratio of 6:1. VLBA
observations at 5 GHz reveal structures that are consistent with the
gravitational lens hypothesis. The brighter of the two images is resolved into
a linear string of at least six sub-components whilst the weaker image is
radially stretched towards the lens galaxy. UKIRT K-band imaging detects an
18.7 mag extended object, but the resolution of the observations is not
sufficient to resolve the lensed images and the lens galaxy. Mass modelling has
not been possible with the present data and the acquisition of high-resolution
optical data is a priority for this system.Comment: 5 pages, 4 figures, accepted for publication in MNRA
Principle of Maximum Entropy Applied to Rayleigh-B\'enard Convection
A statistical-mechanical investigation is performed on Rayleigh-B\'enard
convection of a dilute classical gas starting from the Boltzmann equation. We
first present a microscopic derivation of basic hydrodynamic equations and an
expression of entropy appropriate for the convection. This includes an
alternative justification for the Oberbeck-Boussinesq approximation. We then
calculate entropy change through the convective transition choosing mechanical
quantities as independent variables. Above the critical Rayleigh number, the
system is found to evolve from the heat-conducting uniform state towards the
convective roll state with monotonic increase of entropy on the average. Thus,
the principle of maximum entropy proposed for nonequilibrium steady states in a
preceding paper is indeed obeyed in this prototype example. The principle also
provides a natural explanation for the enhancement of the Nusselt number in
convection.Comment: 13 pages, 4 figures; typos corrected; Eq. (66a) corrected to remove a
double counting for ; Figs. 1-4 replace
Nearest pattern interaction and global pattern formation
We studied the effect of nearest pattern interaction on a globally pattern
formation in a 2-dimensional space, where patterns are to grow initially from a
noise in the presence of periodic supply of energy. Although our approach is
general, we found that this study is relevant in particular to the pattern
formation on a periodically vibrated granular layer, as it gives a unified
perspective of the experimentally observed pattern dynamics such as oscillon
and stripe formations, skew-varicose and crossroll instabilities, and also a
kink formation and decoration
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