Gas exchange coefficient

Kremer, J.N., Reischauer, A., and D Avanzo, C. (2003a) Estuary-specific variation in the air-water gas exchange coefficient for oxygen. Estuaries 26, 829-836. 

Chemicals may be removed from some aquatic environments by volatilization. The intrinsic potential for volatilization is determined by the Henry s Law constant (H) of the substance. Volatilization from the aquatic environment is highly dependent on the environmental conditions of the specific water body in question, such as the water depth, the gas exchange coefficients (depending on wind speed and water flow) and stratification of the water body. Because volatilization only represents removal of a chemical from water phase, the Henry s Law constant caimot be used for assessment of degradation in relation to aquatic hazard classification of substances. Substances that are gases at ambient temperature may however for example be considered further in this regard (see also Pedersen et al, 1995). 

Figure 8 A global map of the annual gas exchange coefficient for carbon dioxide for the year 2001. The map was derived from a combination of wind speeds derived from the QSCAT satellite sensor and the W92 parametrization of Note that has been corrected for the solubility of CO2 in this figure (reproduced by permission of Dr. J. Boutin, Laboratorie d Oceanography Dynamique et de Climatologie (LODYC), Universite Pierre et Marie...

Boutin J., Etcheto J., Merlivat L., and Rangama Y. (2002) Influence of gas exchange coefficient parameterisation on seasonal and regional variability of CO2 air-sea fluxes. Geophys. Res. Lett, article no. 1182. 

The most accurate determination of the gas exchange coefficient requires that careful field experiments be conducted. Although techniques for estimating kw from measurable hydraulic attributes of a water body also exist, they are less accurate due to the state of incomplete understanding of the air-water gas exchange process and the fact that multiple factors may control the exchange rate for any particular chemical (Fig. 2-13). Two different models... 

Under this theory, for water-side control, the gas exchange coefficient, few, is equal to the quotient DW/5W. If the atmospheric concentration of a chemical is essentially zero, Eq. [2-29] simplifies to... 

In the absence of tracer data, estimates of gas exchange coefficients in streams can be made from a number of empirical equations, which typically depend on a combination of the stream mean velocity and depth (V and d, respectively). Some equations contain other parameters, such as shear velocity, width, and Froude number (u, w, and N, respectively) of the stream. The Froude number is equal to (V/ fgd), and is the ratio of stream velocity to the travel speed of a shallow-water surface wave. By convention, the empirical equations given for streams are usually for a reaeration coefficient, which is the gas exchange coefficient for oxygen divided by the average stream depth. Examples of empirical equations for reaeration coefficients are shown in Table 2-5. 

The dissolved concentration of trichloroethylene (TCE, C2C13H) in a lake is 1 ppb. Given a dimensionless Henry s law constant, H, of 0.4, and a measured gas exchange coefficient of 3 X 10 cm/sec in water, for propane (C3H8), what is the flux density of TCE from the lake ... 

In the expressions for the gas exchange coefficient employed previously, it is evident that the air-water gas exchange flux density is proportional to the difference between a chemical concentration in the water (Cw) and the corresponding equilibrium concentration (Cw H) in air. Consequently, the difference between actual and equilibrium concentration in the water tends to decay exponentially, as expected for any first-order process. In many situations, exponential decay may provide a useful model of a volatile chemical concentration in a surface water. A classic example is degassing of a dissolved gas from a stream if the gas is present at concentration C0 upstream, atmospheric concentration of the gas is negligible, and flow is steady and uniform along the stream, then the gas concentration in the stream is given by... 

A helium tracer study shows that a certain stream whose depth is 1.6 m has a gas exchange coefficient for helium of 1400 cm/hr. Stream velocity... 

At a certain point, toluene seeps at a rate of 9.0 mg/sec from the ground and dissolves into a stream with a discharge of 3 liter/sec and a pH of 7.5. The stream is 0.2 m deep. You have previously estimated the gas exchange coefficient (piston velocity) for propane (C3H8) under current discharge conditions to be 19 cm/hr, and can safely assume that both propane and toluene volatilization are controlled by the water side. ... 

Here 50, denotes the observed deviation of the atmospheric O, concentration from a standard. The atmospheric tracer APO is dominated primarily by oceanic gas exchanges in addition to a relatively small contribution from fossil fuel not accounted for by the terrestrial stoichiometric factor (i.e., the fossil fuel component scaled by the factor Observations of the seasonal variation of APO in conjuction with surface-water oxy gen measurements have been used to constrain the large-scale magnitude of the air-sea gas exchange coefficient (Keeling et al, 1998) and of marine productivity (Six and Maier-Reimer, 1996 Balkan.ski et al, 1999). Mean annual gradients of APO have also been shown to provide powerful constraints on biogeochemical air-sea fluxes computed by ocean-circulation models with an embedded ocean carbon cycle (Stephens et al, 1998). 

Measurement of radon (Rn) deficiencies in the upper water column can be used to determine gas exchange coefficients and laminar layer thickness, which can then be applied to other gases using eqn [3]. In this method, gaseous Rn, produced by radioactive decay of "Ra, is assumed to be in secular equilibrium within the water column. Rn is relatively short-lived and has an atmospheric concentration of essentially zero. Therefore, in nearsurface waters, a Rn deficiency is observed, due to flux of Rn across the air/water interface. The flux of Rn across the air/water interface can be determined by the depth-integrated difference in measured Rn and that which should occur based on the "Ra inventory. From this flux, the liquid laminar layer thickness, z, can be calculated. 

Other volatile tracers have been used in estuaries to determine gas exchange coefficients. These tracers, such as chlorofluorocarbons (CFCs) or sulfur hexafluoride (SFg), are synthetic compounds with no known natural source. Unlike Rn, these gases are stable in solution. These tracers may be added to the aquatic system and the decrease of the gas due to flux across the air/water interface monitored over time. In some estuaries, point sources of these compounds may exist and the decrease of the tracer with distance downstream may be used to determine k or z values for the estuary. 

From the experiments, the averaged CO2 concentration in the bubble < 002,b and the equivalent bubble diameter have been determined and fitted as a function of time. It should be noted that this is a simplification used to be able to compute a gas exchange coefficient and compare it with... 

The dependency of the gas exchange coefficient on the bubble diameter is shown in Fig. 4.62 in which a faster gas interchange rate is obtained from a smaller bubble. The experimental findings have been compared with the correlation developed by Davidson and Harrison and show a good agreement. The differences between experiment and model have a maximum... 

Although the film thickness cannot be measured directly, the ratio Dw/can be experimentally determined. This ratio is the gas exchange coefficient denoted k in Eq. (2.27). If the atmospheric concentration of a chemical can be neglected, Eq. (2.28) can be written as... 

Assume Ca is zero for both chemicals. Because H for TCE is much greater than 0.01, assume that TCE volatilization is water-side controlled. (The same can be assumed for CsHg, given that values of H for the related alkanes methane, ethane, and octane are 27,20, and 121, respectively, as shown in Table 1.3.) Using Eq. (2.32), the gas exchange coefficient for TCE is approximately equal to the gas exchange coefficient for propane multiplied by the inverse of the ratio of the square roots of the molecular weights ... 

In the absence of tracer data, estimates of gas exchange coefficients in rivers and streams can be made from a number of empirical equations, which typically depend on a combination of velocity and depth. Some equations contain other parameters, such as shear velocity, width, or the Froude number, which is given by ... 

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