Gas exchange of OCPs across the air–water interface at the creek adjoining Mumbai harbour, India

2001 ◽  
Vol 3 (6) ◽  
pp. 635-638 ◽  
Author(s):  
G. G. Pandit ◽  
S. K. Sahu
Keyword(s):  
Gas Exchange ◽  
Water Interface ◽  
Air Water Interface

scholarly journals Continuous measurement of air–water gas exchange by underwater eddy covariance

2017 ◽  
Vol 14 (23) ◽  
pp. 5595-5606 ◽  
Author(s):  
Peter Berg ◽  
Michael L. Pace
Keyword(s):  
Heat Flux ◽  
Gas Exchange ◽  
Exchange Rates ◽  
Eddy Covariance ◽  
High Speed ◽  
Heat Fluxes ◽  
Water Interface ◽  
O2 Concentration ◽  
Air Water Interface ◽  
Water Gas

Abstract. Exchange of gases, such as O2, CO2, and CH4, over the air–water interface is an important component in aquatic ecosystem studies, but exchange rates are typically measured or estimated with substantial uncertainties. This diminishes the precision of common ecosystem assessments associated with gas exchanges such as primary production, respiration, and greenhouse gas emission. Here, we used the aquatic eddy covariance technique – originally developed for benthic O2 flux measurements – right below the air–water interface (∼ 4 cm) to determine gas exchange rates and coefficients. Using an acoustic Doppler velocimeter and a fast-responding dual O2–temperature sensor mounted on a floating platform the 3-D water velocity, O2 concentration, and temperature were measured at high-speed (64 Hz). By combining these data, concurrent vertical fluxes of O2 and heat across the air–water interface were derived, and gas exchange coefficients were calculated from the former. Proof-of-concept deployments at different river sites gave standard gas exchange coefficients (k600) in the range of published values. A 40 h long deployment revealed a distinct diurnal pattern in air–water exchange of O2 that was controlled largely by physical processes (e.g., diurnal variations in air temperature and associated air–water heat fluxes) and not by biological activity (primary production and respiration). This physical control of gas exchange can be prevalent in lotic systems and adds uncertainty to assessments of biological activity that are based on measured water column O2 concentration changes. For example, in the 40 h deployment, there was near-constant river flow and insignificant winds – two main drivers of lotic gas exchange – but we found gas exchange coefficients that varied by several fold. This was presumably caused by the formation and erosion of vertical temperature–density gradients in the surface water driven by the heat flux into or out of the river that affected the turbulent mixing. This effect is unaccounted for in widely used empirical correlations for gas exchange coefficients and is another source of uncertainty in gas exchange estimates. The aquatic eddy covariance technique allows studies of air–water gas exchange processes and their controls at an unparalleled level of detail. A finding related to the new approach is that heat fluxes at the air–water interface can, contrary to those typically found in the benthic environment, be substantial and require correction of O2 sensor readings using high-speed parallel temperature measurements. Fast-responding O2 sensors are inherently sensitive to temperature changes, and if this correction is omitted, temperature fluctuations associated with the turbulent heat flux will mistakenly be recorded as O2 fluctuations and bias the O2 eddy flux calculation.

scholarly journals Gas Exchange and Bubble-Induced Supersaturation in a Wind-Wave Tank

2004 ◽  
Vol 21 (12) ◽  
pp. 1925-1935 ◽  
Author(s):  
Peter Bowyer ◽  
David Woolf
Keyword(s):  
Steady State ◽  
Gas Exchange ◽  
Wind Wave ◽  
Relative Strength ◽  
Water Interface ◽  
Wave Tank ◽  
Gas Concentration ◽  
Gas Fluxes ◽  
Air Water Interface ◽  
Series Of Experiments

Abstract Gas exchange and bubble-induced supersaturation were measured in a wind-wave tank using total gas saturation meters. The water in the tank was subjected to bubbling using a large number of frits at a depth of 0.6 m. A simple linear model of bubble-mediated gas exchange implies that this should force an equilibrium supersaturation of 3%. This is confirmed by experiment, but a small additional steady-state supersaturation is also forced by warming. The total steady-state supersaturation is approached asymptotically. When the bubblers were switched off, the total gas pressure approached a new steady state at much lower supersaturation, at a rate that depended on the state of the wind and waves in the tank. The rates of approach on the various equilibria enabled the gas flux across the surface of the bubbles or across the air–water interface to be calculated. In addition a series of experiments was conducted where the water was subjected to bubbling in the presence of wind or wind and paddle waves: in this case gas invasion from the bubbles was balanced by gas evasion near or at the surface resulting in an equilibrium at <3% and enabling the relative strength of the invasion and evasion to be estimated. Gas concentrations could be measured in a rapid, automated manner using simple apparatus. To derive gas fluxes, corrections for changes in water temperature and fluctuations in air pressure are necessary, and these are quantified. In addition, transient fluctuations in gas concentration at the start of bubbling periods allowed mixing within the tank to be observed.

scholarly journals Continuous measurement of air-water gas exchange by underwater eddy covariance

2017 ◽  
Author(s):  
Peter Berg ◽  
Michael L. Pace
Keyword(s):  
Gas Exchange ◽  
Exchange Rates ◽  
Eddy Covariance ◽  
High Speed ◽  
Heat Fluxes ◽  
Water Interface ◽  
O2 Concentration ◽  
Air Water Interface ◽  
Ecosystem Assessments ◽  
Water Gas

Abstract. Abstract. Exchange of gasses, such as O2, CO2, and CH4, over the air-water interface is an important component in aquatic ecosystem studies, but exchange rates are typically measured or estimated with substantial uncertainties. This diminishes the precision of common ecosystem assessments associated with gas exchanges such as primary production, respiration, and greenhouse gas emission. Here, we use the aquatic eddy covariance technique – originally developed for benthic O2 flux measurements – right below the air-water interface (~ 5 cm) to determine gas exchange rates and coefficients. Using an Acoustic Doppler Velocimeter and a fast-responding dual O2-temperature sensor mounted on a floating platform, the 3D water velocity, O2 concentration, and temperature are measured at high-speed (64 Hz). By combining these data, concurrent vertical fluxes of O2 and heat across the air-water interface are derived, and from the former, gas exchange coefficients. Proof-of-concept deployments at different river sites gave standard gas exchange coefficients (k600) in the range of published values. A 40 h long deployment revealed a distinct diurnal pattern in air-water exchange of O2 that was controlled largely by physical processes (e.g., diurnal variations in air temperature and associated air-water heat fluxes) and not by biological activity (primary production and respiration). This physical control of gas exchange is prevalent in lotic systems and adds uncertainty to common ecosystem assessments of biological activity relying on water column O2 concentration recordings. For example, in the 40 h deployment, there was close-to constant river flow and insignificant winds – two main drivers of lotic gas exchange – but we found gas exchange coefficients that varied by several fold. This was presumably caused by vertical temperature-density gradient formation and erosion in the surface water driven by the heat flux into or out of the river that controlled the turbulent mixing. This effect is unaccounted for in widely used empirical correlations for gas exchange coefficients and is another source of uncertainty in gas exchange estimates. The aquatic eddy covariance technique allows studies of air-water gas exchange processes and their controls at an unparalleled level of detail. A finding related to the new approach is that heat fluxes at the air-water interface can, contrary to those typically found in the benthic environment, be substantial and require correction of O2 sensor readings using high-speed parallel temperature measurements. Fast-responding O2 sensors are inherently sensitive to temperature changes, and if this correction is omitted, temperature fluctuations associated with the turbulent heat flux will mistakenly be recorded as O2 fluctuations and bias the O2 eddy flux calculation.

scholarly journals A numerical scheme to calculate temperature and salinity dependent air-water transfer velocities for any gas

2010 ◽  
Vol 7 (1) ◽  
pp. 251-290 ◽  
Author(s):  
M. T. Johnson
Keyword(s):  
Thin Film ◽  
Gas Exchange ◽  
Numerical Scheme ◽  
Full Range ◽  
General Interest ◽  
Water Interface ◽  
Henry's Law ◽  
Transfer Velocity ◽  
Henry’S Law ◽  
Air Water Interface

Abstract. The transfer velocity determines the rate of exchange of a gas across the air-water interface for a given deviation from Henry's law equilibrium between the two phases. In the thin film model of gas exchange, which is commonly used for calculating gas exchange rates from measured concentrations of trace gases in the atmosphere and ocean/freshwaters, the overall transfer is controlled by diffusion-mediated films on either side of the air-water interface. Calculating the total transfer velocity (i.e. including the influence from both molecular layers) requires the Henry's law constant and the Schmidt number of the gas in question, the latter being the ratio of the viscosity of the medium and the molecular diffusivity of the gas in the medium. All of these properties are both temperature and (on the water side) salinity dependent and extensive calculation is required to estimate these properties where not otherwise available. The aim of this work is to standardize the application of the thin film approach to flux calculation from measured and modelled data, to improve comparability, and to provide a numerical framework into which future parameter improvements can be integrated. A detailed numerical scheme is presented for the calculation of the gas and liquid phase transfer velocities (ka and kw respectively) and the total transfer velocity, K. The scheme requires only basic physical chemistry data for any gas of interest and calculates K over the full range of temperatures, salinities and wind-speeds observed in and over the ocean. Improved relationships for the wind-speed dependence of ka and for the salinity-dependence of the gas solubility (Henry's law) are derived. Comparison with alternative schemes and methods for calculating air-sea flux parameters shows good agreement in general but significant improvements under certain conditions. The scheme is provided as a downloadable program in the supplementary material, along with input files containing molecular weight, solubility and structural data for 80 gases of general interest, enabling calculation of the total transfer velocity over ranges of temperature and salinity for each gas.

Trace gas exchange at the air/water interface: Measurements of mass accommodation coefficients

1990 ◽  
Vol 10 (4) ◽  
pp. 427-449 ◽  
Author(s):  
W. Kirchner ◽  
F. Welter ◽  
A. Bongartz ◽  
J. Kames ◽  
S. Schweighoefer ◽  
...  
Keyword(s):  
Gas Exchange ◽  
Trace Gas ◽  
Water Interface ◽  
Air Water Interface ◽  
Accommodation Coefficients
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