Seawater-air Interface Research Articles

Transmission characteristics of linear Gaussian array beams propagating through seawater to air

We examine the transmission characteristics of linear Gaussian array beams (LGABs) propagating through seawater to air considering oceanic and atmospheric turbulences. We derive the expressions of the mean-squared beam width and Rayleigh range of LGABs for the case of both coherent and incoherent combinations using stochastic wave theory and the extended Huygens–Fresnel principle. In addition, the properties of mean-squared beam width versus propagation distance are studied. Furthermore, the characteristics of the Rayleigh range versus the ocean turbulence parameters and beam number as well as the interval distance are investigated. In the seawater–air interface, the Rayleigh range of LGABs is up to three times higher than that of the underwater circumstance. Moreover, the Rayleigh range increases linearly with the increase of interval distance in a turbulent ocean and turbulent atmosphere. It can be concluded that the Rayleigh range is more affected by the rate of dissipation of turbulent kinetic energy and the rate of dissipation of mean-square temperature than by the parameter related to temperature and salinity.

Bubble mobility in seawater during free-rise, bouncing, and coalescence with the seawater-air interface

The way bubbles in seawater interact with the seawater-air interfaces plays an important role in many naturally occurring processes. Here we use high-speed video imaging to investigate how millimeter-sized bubbles free-rise, bounce, and coalesce with the seawater-air interface in seawater samples collected from the Red Sea coastal area. The samples salinity and organics content are typical for the open oceans and seas. To evaluate the seawater-air interface mobility, which is one of the fundamental properties determining the bubble hydrodynamic interactions, we measure the free-rise velocity of the bubbles and their bouncing trajectories from the interface. Our experiments demonstrate that both the bubble interface and the pool seawater-air interface are fully mobile, indicating that the presence of high concentration of electrolytes and organics in the seawater does not affect the interfaces mobility during free-rise and bouncing for these bubble sizes. In contrast, the presence of electrolytes in the seawater is shown to increase the characteristic time for the final bubble coalescence with the interface from milliseconds to seconds. Whereas the delay of the bubble coalescence by the high concentration of the electrolyte is a well-known phenomenon, here we evaluate this effect for single bubbles in actual seawater samples to separate the effects of the electrolyte and organic components.

Carbon sinks/sources in the Yellow and East China Seas—Air-sea interface exchange, dissolution in seawater, and burial in sediments

The sinks/sources of carbon in the Yellow Sea (YS) and East China Sea (ECS), which are important continental shelf seas in China, could exert a great influence on coastal ecosystem dynamics and the regional climate change process. The CO2 exchange process across the seawater-air interface, dissolved and particulate carbon in seawater, and carbon burial in sediments were studied to understand the sinks/sources of carbon in the continental shelf seas of China. The YS and the ECS generally have different patterns of seasonal air-sea CO2 exchange. In the YS, regions west of 124°E can absorb CO2 from the atmosphere during spring and winter, and release CO2 to the atmosphere during summer and autumn. The entire YS is considered as a CO2 source throughout the year with respect to the atmosphere, but there are still uncertainties regarding the exact air-sea CO2 exchange flux. Surface temperature and phytoplankton production were the key controlling factors of the air-sea CO2 exchange flux in the offshore region and nearshore region of the YS, respectively. The ECS can absorb CO2 during spring, summer, and winter and release CO2 to the atmosphere during autumn. The annual average exchange rate in the ECS was −4.2±3.2 mmol m−2 d−1 and it served as an obvious sink for atmospheric CO2 with an air-sea exchange flux of 13.7×106 t. The controlling factors of the air-sea CO2 exchange in the ECS varied significantly seasonally. Storage of dissolved inorganic carbon (DIC) and dissolved organic carbon (DOC) in the YS and the ECS were 425×106 t and 1364×106 t, and 28.2×106 t and 54.1×106 t, respectively. Long-term observation showed that the DOC content in the YS had a decreasing trend, indicating that the “practical carbon sink” in the YS was decreasing. The total amount of particulate organic carbon (POC) stored in the YS and ECS was 10.6×106 t, which was comparable to the air-sea CO2 flux in these two continental shelf seas. The amounts of carbon sequestered by phytoplankton in the YS and the ECS were 60.42×106 t and 153.41×106 t, respectively. Artificial breeding of macroalgae could effectively enhance blue carbon sequestration, which could fix 0.36×106–0.45×106 t of carbon annually. Organic carbon (OC) buried in the sediments of the YS was estimated to be 4.75×106 t, and OC of marine origin was 3.03×106 t, accounting for 5.0% of the TOC fixed by phytoplankton primary production. In the ECS, the corresponding depositional flux of OC in the sediment was estimated to be 7.4×106 t yr−1 , and the marine-origin OC was 5.5×106 t, accounting for 5.4% of the phytoplankton primary production. Due to the relatively high average depositional flux of OC in the sediment, the YS and ECS have considerable potential to store a vast amount of “blue carbon.”

Electromagnetic field radiated in air from a horizontal/vertical magnetic dipole in sea

The propagation characteristics of electromagnetic (EM) waves across the seawater–air interface influence seawater to air communication. The type of underwater antenna determines EM wave propagation performance. However, studies rarely compared the performance of horizontal and vertical magnetic dipoles (HMD and VMD). In this study, the EM fields in air due to HMD/VMD immersed in sea were derived to provide a basis for selecting a suitable underwater antenna for EM wave propagation in undersea–air communication. Analytic results were obtained from the model of a three-layered conducting media. Associated expressions in air were numerically calculated using the fast Fourier Transform algorithm to present the propagation curves and the distribution patterns of the EM fields. A proof of concept measurement system was constructed, and sea experiments were conducted and compared with theoretical studies. The maximum transmitting–receiving distance in the experiment was achieved by the HMD. The theoretical and experimental results indicate that the HMD antenna is superior to the VMD antenna and that the suggested band of EM wave is feasible for transmission across the seawater–air interface.

Boron isotopic fractionation during seawater evaporation

Laboratory experiments were undertaken to investigate the behaviour of boron at the seawater–air interface during seawater evaporation. Seawaters adjusted to different pH values were heated to 27, 33 and 40 °C inside a quartz evaporator. The vapor was collected with a quartz condenser using a cold trap. A natural seawater–vapor process in quiet air conditions and excluding the sea-spray component was realized using this arrangement. The results indicate an enrichment of 11B in the condensate. This implies that in the natural environment, rainfall with δ 11B values lower than that of seawater has been affected either by continental boron sources or by the boron extracted from seawater under airflow conditions. While the net flux of boron carried away from the ocean by air masses due to seawater evaporation is significant compared to other fractionation-associated processes removing boron from the ocean, the effect of isotopic fractionation as boron enters the vapor phase on the isotopic composition of boron in the ocean over time is small.

A preliminary experimental study of the boron concentration in vapor and the isotopic fractionation of boron between seawater and vapor during evaporation of seawater

A laboratory experiment was undertaken to investigate the behaviour of boron at the seawater-air interface under air flow conditions. Dried air at 25 and 35℃ was passed over or bubbled through seawater at the same temperature. A combination of ice-chilled condensers and KOH impregnated cellulose fibre filters was used to collect boron from the reacted air. When air stripped of boron was passed over the seawater, boron was found in the reacted air, and its concentration was higher in the higher temperature test. In the tests where air was bubbled through seawater the concentration of boron in the reacted air was directly proportional to the air flow rate. In this situation the boron in the reacted air was mainly introduced as a spray of microdroplets. Isotopic analysis of the collected boron in the non-bubbled tests yields fractionation factors which demonstrate that the lighter isotope, 10 B, is enriched in the reacted air. The size of the fractionation changes with temperature, ruling out a purely kinetic effect.

Carbon cycling in a large coastal embayment, affected by wind-driven upwelling:short-time-scale variability and spatial differences

Carbon species fluxes and net budgets were studied with a 2-D box model in the Ria de Arousa (Spain), a large indentation in the NW Iberian upwelling system. The embayment acted as a CO2 source to the atmosphere during the upwelling season (average CO2 flux across the seawater-air interface, Fo, = + l 6 mg C m-' d.'), despite the elevated net community production (NCP) rates (average NCP = 0.84 g C m-' d-'). The high pCOz levels in source Eastern Narth Atlantic Central Water (ENACW) and the reduced residence times within the Ria (-6 d) seem to be the reasons for the observed Fcol The CO2 increase associated with CaCO, fixation by the intensive culture of mussels on hanging ropes did not affect Fco, significantly. High Fco, values were observed during strong upwelling, w h c h was 3 times larger than the average during the upwelling season. Under these conditions the CO2-rich source ENACW reached the surface and this coincided with low phytoplankton production (NCP = -0.01 g C m-2 d.'). Source ENACW was enriched ~ n t h i n the Ria with carbon released from the sediments. Fco, was reduced to Y of ~ t s value compared to the average upwelling season under conditions of moderate upwehng. Phytoplankton grew rapidly (average NCP = 1.49 g C m-2 d-') at the expense of the gentle injection of new nutrients at the base of the pycnocline. Negligible FCOl values were observed during upwelling relaxations, when ENACW into the Ria was replaced by the warm and CO2-equilibrated shelf surface waters. Fco, reached a maximum during a strong autumn downwelling event because of the concurrence of warm shelf surface water flow into the Ria with strong carbon release from the sediments. CO2 fluxes per unit area always increased coastwards, because of the progressive CO2 ennchinent of the source ENACW as it enters the embayment and the decrease of residence times.

Transport of surface-active organic materials from seawater to the air-water interface by an ascending current field

Surface slicks at sea are characterized by the occurence of capillary-wave damping materials at the air-water interface. Physical mechanisms causing such slicks are believed to include internal waves, Langmuir circulations, and other phenomena involving vertical or horizontal transport and compression of surfactants We report the rates of transport and deposition of four naturally occurring surface-active organic materials to the seawater-air interface by an ascending current field. Our results indicate that the susceptibility of these materials to transport by current follows the same relative order as their susceptibility by bubbles (although the latter rate is dramatically greater). The current transport results obtained can be explained by standard surface adsorption kinetics. Our data also indicate that subtle variations in the chemical structure and polarity of different materials may be major determining factors influencing their transport to the sea surface by currents.

Meteoric water-basalt interactions. II: A field study in N.E. Iceland

The compositions of rain, snow, melt, spring and geothermal waters from the rift zone of N.E. Iceland can be explained by seaspray addition, chemical fractionation at the seawater-air interface, burning of fossil fuel, farming activities, purification by partial melting of snow and ice, dissolution of basalts and buffering by alteration minerals. The dissolution of the rocks appears to be incongruent. During solute acquisition, spring compositions move through the stability fields of kaolinite and smectite to the laumontite and illite fields. All but four of the springs are undersaturated with respect to calcite. Silica concentrations are compatible with the solubility of basaltic glass. The reactions reflected in the spring waters appear to have taken place sealed off from atmospheric CO 2 after initial saturation. The geothermal waters which are recharged by the spring waters are depleted in Mg and Ca but enriched in carbon and sulfur with respect to dissolution of primary rocks. Expressions are derived relating dissolution rates of rocks, age of groundwaters, physical properties of groundwaters and mass transfer. The characteristic rock particle radii in the cold water aquifers range from 0.2 to 2 cm and the characteristic crack openings are of the order 0.04 to 0.4 cm. Using laboratory studies on the Icelandic lavas as a guide, the residence times of the cold waters in the aquifers can be estimated at 60 days to 4 years. The average active surface area of the aquifers enclosing 1000 g of spring water is of the order of 0.6 to 6 m 2 and these 1000 g of water have reacted with 0.1 to 1 g of basaltic rocks. The same mass of thermal water has interacted with 100 to 300 g of unaltered rocks.

Properties of the seawater-air interface: Dragčević Dj. and V. Pravdić, 1986. 3. The electrification and energy transfer through surface films. Limnol. Oceanogr., 31(3):525–532

Properties of the seawater-air interface: Dragčević Dj. and V. Pravdić, 1986. 3. The electrification and energy transfer through surface films. Limnol. Oceanogr., 31(3):525–532

Properties of the seawater-air interface. 2. Rates of surface film formation under steady state conditions : Dragčević, D. and V. Pravdić, 1981 Limnol. Oceanogr., 26(3):492–499

Properties of the seawater-air interface. 2. Rates of surface film formation under steady state conditions : Dragčević, D. and V. Pravdić, 1981 Limnol. Oceanogr., 26(3):492–499