摘要:

The bubble transport of natural surfactant materials from seawater to the air‐water interface and to the air is investigated experimentally by a radiotracer test system. We have studied the rates at which four lipid materials of differing molecular structure are scavenged from artificial seawater and ejected into the atmosphere by air bubbles of various size distributions at differing airflow rates and bubbling times. We have found that the bubble transport to the air for all materials studied is linearly proportional to the quantity of material transported to the surface microlayer, and the constant of proportionality appears to be independent of the type of material but is a function of the airflow rate and bubble size. Small bubbles were found generally to be more effective in transporting surface‐active organic compounds from seawater to the

ISSN:0148-0227    

DOI:10.1029/91JC00954

年代:1992

数据来源: WILEY

摘要:

Remote sensing of oceanic and atmospheric quantities, such as the waveheight spectrum and wind velocity, depend largely on the interaction of electromagnetic radar waves with ocean surface waves in the short gravity and capillary range. The propagation of such ripple waves upon the ocean surface is strongly affected by the presence of surface films which constitute the ocean microlayer. In order to understand the impact of surface films on oceanic and atmospheric remote sensing, it is necessary to have an accurate and, if possible, convenient model relating film properties with the propagation and damping of ripple waves. This study examines two different methods of obtaining the damping coefficient of short gravity and capillary waves. One method, developed by Bock and Mann, utilizes numerical techniques based on the modified Levich characteristic equation to accurately yield complex roots governing the propagation of Laplace transverse and Marangoni longitudinal waves on the air‐sea interface. The other method, developed by Cini and Lombardini, involves finding an approximate analytical solution of the Levich equations using a perturbational technique. A comparison of the two methods yields tolerance ranges of various parameters, including frequency, modulus of surface dilational viscosity, and modulus of surface dilational elasticity, over which the two methods agree to 5–10%. To make our comparison concrete, we apply both methods to a specific surface film sample collected by Garret. Both theoretical results are compared to experimental resu

ISSN:0148-0227    

DOI:10.1029/91JC01503

年代:1992

数据来源: WILEY

摘要:

The results of an open sea experiment devised to investigate the feasibility of detecting and monitoring oil slicks by sonar through their effect on the acoustic backscatter signal are presented. The analysis, carried out over an angular range 10° to 70° from the horizontal, reveals a reduction in backscatter from the slick. The reduction in the mean backscattershows an angular dependence which reaches a maximum in the range 45° to 65°. A gradual decrease ofas wind speed decreased and as the slick developed was also observed. Comparison of the acoustic data with Kuband radar reflection from an oil covered surface under similar wind speed conditions shows that differential radar backscatter from oil slicks may exceed that of sonar by a fac

ISSN:0148-0227    

DOI:10.1029/91JC01479

年代:1992

数据来源: WILEY

摘要:

The generation and growth of surface water waves by the wind and including modified boundary conditions appropriate for the presence of surfactants at the surface are reconsidered using linear instability theory to describe the process. The new features of our work are the inclusion of both surfactants and a wind drift layer. We allow a coupled mean flow in both the air and water; the depth‐dependent mean flow is considered to be wind‐induced. The complex wave frequencies are determined numerically. Uncontrolled numerical errors associated with a direct numerical solution of the Orr‐Sommerfeld equation for the coupled mean flow are avoided by solving an associated equation obtained by a Riccati transformation of the original equation, allowing a straightforward and efficient determination of the eigenvalues of the linear stability problem. A different aspect of our analysis is the change of surface boundary conditions due to contamination of the surface, notably, the change of tangential stress at the surface due to surface tension gradients induced by the film (known as the Marangoni effect). Comparisons are made with previous work on both film‐free and contaminated surfaces. The dominant effect of introducing film boundary conditions is a large change in the wave number of the most unstable wave with an attendant reduction in the growth rate. Implications for wind wave growth on the ocean are di

ISSN:0148-0227    

DOI:10.1029/91JC01876

年代:1992

数据来源: WILEY

摘要:

Surface‐active films are found in all parts of the ocean. They are easily detected visually on the ocean surface when the wind speed is less than 5 m/s (10 knot). They are generally of biological origin and consist mainly of fulvic and humic acids, carbohydrates, proteins, and lipids. Their molecules have hydrophilic (partially water soluble) heads and hydrophobic (insoluble carbon chain) tails. Surface‐active films that have been concentrated at the edges of the centerline wake generated by the passage of a surface ship strongly influence the propagation of the short gravity and capillary waves which interact with electromagnetic waves at both radar and visible wavelengths. An accurate method for detecting the presence of surface films and measuring the fine‐scale surface film pressure and surface elasticity distributions on a water surface was developed at the Naval Research Laboratory (NRL). These measurements, together with model‐scale flow visualization experiments, have allowed us to develop a better understanding of the role these ship‐generated surface‐active film distributions play in the formation of the dark centerline and “railroad track” or dark lines which are aligned at some narrow angle to the ship's track in synthetic aperture radar (SAR) image

ISSN:0148-0227    

DOI:10.1029/91JC01875

年代:1992

数据来源: WILEY

摘要:

Reflected optical second‐harmonic generation (SHG) was applied for the first time to in situ study of the air‐sea interface. SHG signals were detected from the first several molecular layers of the ocean surface during the SAR X Band Ocean Nonlinearities (SAXON) Chesapeake Light Tower experiment in 1988, and during experiments on Nantucket Sound at the Woods Hole Oceanographic Institution in 1989. The SHG response of the ocean surface was observed to correlate with increased slick activity and surface tension measurements of surface water organic content. The SHG response was similar for naturally occurring slicks and for artificially created slicks of several known materials. The SHG signal intensity was also used to estimate the second‐order nonlinear optical susceptibility of the ocean surface. It was determined that the SHG nonlinear laser technique is a useful noninvasive probe for in situ studies of ocean surface chem

ISSN:0148-0227    

DOI:10.1029/91JC02125

年代:1992

数据来源: WILEY

摘要:

Microbial biomass and utilization (incorporation plus respiration) of dissolved free amino acids (DFAA) were studied in sea surface films or microlayers in nearshore and offshore waters off the coast of southern California during 1988 (September) and 1989 (October). Microbial biomass (bacterial numbers via cell counts and total microbial biomass via adenosine triphosphate concentrations), DFAA, and dissolved combined amino acids (DCAA) were always enriched in films compared with subsurface waters, with enrichment factors (ratio of microlayer value compared to 10‐cm control value) often exceeding those measured previously in microlayers from diverse Atlantic and Pacific coastal and island environments. Higher enrichment factors were generally obtained for offshore stations than for nearshore sites. Glutamic acid utilization was greater in microlayers than in control waters at 9 of 13 stations. At those sites where higher utilization occurred in the 10‐cm waters, the microbial populations were highly active and had been subjected to substantial wind mixing prior to sampling. Glutamic acid concentrations were increased in subsurface waters at these sites relative to other stations, apparently by the input of dissolved film constituents. Calculated turnover times were longer for films than for 10‐cm waters, in large pan because of the substantially larger glutamic acid pools in the films. Individual amino acids were not enriched in films compared with 10‐cm waters to the same degree in both the DFAA and DCAA. The relative levels of the individual amino acids in the free and combined pools may have been influenced by their differential production or preferential util

ISSN:0148-0227    

DOI:10.1029/91JC02614

年代:1992

数据来源: WILEY

摘要:

Surface‐active organic matter from natural marine microlayers has been isolated by solid phase adsorption using reversed phase (C18) cartridges. Surfactants collected from a suite of microlayer films in the California Bight and the Gulf of Maine and isolated by this technique were respread on clean seawater to reconstitute surface films which exhibited surface pressure‐area (π‐A) isotherms and static (Gibbs) surface elasticities closely approximating those measured for films formed by diffusion in the original fresh microlayer samples. Microlayer‐subsurface (10 cm depth) carbon enrichment factors were estimated to range from 3.9 to 16. The estimated mean recovery of microlayer dissolved organic carbon (DOC) was 20 ± 5%. The select hydrophobic DOC fraction isolated was the dominant surfactant class responsible for microlayer film characteristics as measured quasi‐statically, even though other relatively hydrophilic DOC fractions which may have been enriched in the microlayer were not absorbed and passed through the isolation procedure. This technique allows collection of marine microlayer films for systematic ex situ study and correlation of their surface physical and chemical properties. For the first time, the π‐Aisotherms for microlayer films can be expressed on a specific area basis and, in cases where molecular weights can be estimated, according to mean

ISSN:0148-0227    

DOI:10.1029/91JC02724

年代:1992

数据来源: WILEY

摘要:

Heterogeneity in the chemical composition of sea surface films due to variation in sources of surfactant materials and in physical dynamics during film formation may lead to spatial and temporal variability in interfacial physical properties. Alternatively, slick physical properties may be postulated to be largely averaged and therefore similar across a variety of oceanic regimes. In order to assess interslick variability in regimes of different productivity, surface pressure‐area (π‐A) isotherms have been measured for a suite of microlayers sampled at stations in productive coastal and mesotrophic waters of the southern California Bight. Surface‐active organics isolated from the microlayers by solid phase extraction on C18Sep‐Pak cartridges were respread on clean seawater, and the π‐Aisotherms of the reconstituted films were determined. Several bulk chemical parameters including dry weight, UV absorbance, and carbon, nitrogen, and lipid content have been examined in an attempt to scale the isotherms; the latter were shown not to collapse to a single well‐constrained envelope when compared on a specific area basis. However, for several parameters there was a clear separation of the slicked microlayer isotherms into two distinct film types composed primarily of inshore and offshore samples, respectively. Area scaling and virial equation modeling also clustered the isotherms into two groups with different elastic properties suggesting the presence of distinct “end‐member” components. Surfactants from an unslicked microlayer appeared to produce films that exhibited much lower elastic moduli than those from slicked microlayers. The observed interslick variability in π‐Acharacteristics was larger than the uncertainties inherent in the methodology, and it was concluded that the variability was related to significant differences in chemical composition of films. The inferred compositional differences must be taken into account in order to correctly define interfacial boundary conditions in modeling of

ISSN:0148-0227    

DOI:10.1029/91JC02723

年代:1992

数据来源: WILEY

摘要:

The suppression of wind‐generated ripples by natural films was investigated in an oval‐shaped wind‐wave tank filled with seawater. Experiments were performed over the water surface with and without natural films; the former were produced through aging and the latter was assured through overflowing. Time series of the water surface slope were obtained with a computer‐interfaced matrix camera system to determine the slope spectra. The inception of waves by wind was much delayed over the slick surface, while at higher winds there were insignificant differences between the mean square slopes over clean and slick surfaces. At low winds, suppression of ripples by natural films was observed from the intrinsic wave frequency of about 7 Hz and up, with the maximum suppression at about 19 Hz. No obvious spectral dip was observed to account for the Marangoni effect observed by others over artificial slick s

ISSN:0148-0227    

DOI:10.1029/91JC02525

年代:1992

数据来源: WILEY